plc 2012

PLC Chapter 1:

Introduction to Programmable Controllers

1.1 Definitions of PLC

The programmable (logic) controller (PLC) is an electronic device for machine or process control. The PLC receives signals via inputs, processes them according to the instructions of a program, and transfers signals to the outputs. The program is created using programming software which is able to link inputs and outputs in any required sequence, to measure time, or even carry out arithmetic operations. 1.2 PLC Components and Principles of Operation

A typical PLC can be divided into five components. These components consist of the processor unit, memory, power supply, input/output section (interface) and the programming device. Some manufacturers refer to the processor as a C.P.U. or central processing unit. The components are shown in Figure 1-1.

Figure 1-1. Programmable controller block diagram.

The input/ output (I/O) system is physically connected to the field devices that are encountered in the machine or that are used in the control of a process (Figure 1-2). These field devices may be discrete or analog input/output devices, such as limit switches, pressure transducers, push buttons, motor starters, solenoids, etc. The discrete input modules are available in wide range of voltages for various applications. Some more common voltage modules are 120V AC, 240V AC, 24VDC. The I/O interfaces provide the connection between the CPU and the information

PLC-2012 Eng. Mohammad Al-Arni

providers (inputs) and controllable devices (outputs). Optically coupled input and output modules are used as they provide isolation of processor circuit from the real word input and output devices which may be energized on higher level voltages (Figure 1-3 and Figure 1-4).

Figure 1-2. Typical input / output modules (a) input module (b) output module

(a) Simplified DC discrete input module circuit with indication light

(b) Simplified AC discrete input module circuit with indication light

Figure 1-3. Optically coupled discrete input modules


(a) Simplified DC discrete output module circuit

(a) Simplified AC discrete output module circuit

Figure 1-4. Optically coupled discrete output modules

Although not generally considered a part of the controller, the programming

device, usually a personal computer or a manufacturers miniprogrammer unit, is required to enter the control program into memory. The programming device must be connected to the controller when entering or monitoring the control program.

The operation of a programmable controller is relatively simple. During its

operation, the CPU completes three processes: (1) it reads, or accepts, the input data from the field devices via the input interfaces, (2) it executes, or performs, the control program stored in the memory system, and (3) it writes, or updates, the output devices via the output interfaces. This process of sequentially reading the inputs, executing the program in memory, and updating the outputs is known as scanning. Figure 1-5. illustrates a graphic representation of a scan.

Figure 1-5. Illustration of a scan.


The total time for one complete program scan is a function of processor speed, I/O modules used, and length of user program. Typically, hundreds of complete scans can take place in 1 second. Horizontal Scanning Order (rung scanning,)

The processor examines input and output instructions from the first command, top left in the program, horizontally, rung by rung. Vertical Scanning Order (column scanning)

The processor examines input and output instructions from the first command, vertically, column by column and page by page. Pages are executed in sequence. 1.3 PLC Size

There are five classes of PLC according to number of inputs , number of outputs, cost, and physical size:

1. Nano: 2. Micro 3. Small 4. Medium 5. large

Some PLCs are integrated into a single unit (Picocontroller, Micrologix), whereas others are modular (PLC5, SLC500). Integrated PLCs are sometimes called brick PLCs because of their small size. These PLCs have embedded I/O (i.e. the I/O is a part of the same unit as the controller itself). Modular PLCs have extended I/O. Figure 1-6. show a PLC size examples

Figure 1-6. Allen-Bradley PLCs examples

1.4 Advantages and disadvantages of PLCs

In general, PLC architecture is modular and flexible, allowing hardware and software elements to expand as the application requirements change. In the event that an application outgrows the limitations of the programmable controller, the unit can be easily replaced with a unit having greater memory and I/O capacity, and the old


hardware can be reused for a smaller application. A PLC system provides many benefits to control solutions, from reliability and repeatability to programmability.

Table 1-1 lists some of the many features and benefits obtained with a programmable controller.

Table 1.1

PLC Disadvantages Fixed Program Applications.

Some applications are single-function applications. It does not pay to use a PLC that includes multiple programming capabilities if they are not needed. Their operational sequence is seldom or never changed, so the reprogramming available with the PLC would not be necessary.

Fail-Safe Operation. In relay systems, the stop button electrically disconnects the circuit; if the power fails, the system stops. This, of course, can be programmed into the PLC; however, in some PLC programs, you may have to apply an input voltage to cause a device to stop. These systems may not be fail-safe.

1.5 Typical area of PLC applications

Since its inception, the PLC has been successfully applied in virtually every segment of industry, including steel mills, paper plants, food-processing plants, chemical plants, and power plants. PLCs perform a great variety of control tasks, from repetitive ON/OFF control of simple machines to sophisticated manufacturing and process control.


Chapter 2: Logic Concepts

Operations performed by digital equipment, such as programmable controllers,

are based on three fundamental logic functionsAND, OR, and NOT. These functions combine binary variables to form statements. Each function has a rule that determines the statement outcome (TRUE or FALSE) and a symbol that represents it. For the purpose of this discussion, the result of a statement is called an output (Y), and the conditions of the statement are called inputs (A and B).

Logic Functions 2.1 THE AND FUNCTION

An AND function can have an unlimited number of inputs, but it can have only one output. Figure 2-1 shows a two-input AND gate and its electrical circuit representation, based on all possible input combinations. The letters A and B represent inputs to the controller. This mapping of outputs according to predefined inputs is called a truth table.

Figure 2-1. Two-input AND gate.

The boolean expression of AND gate is:

Y = AB

We can representation AND gate by a ladder logic as:

AND Ladder logic representation

THE OR FUNCTION

As with the AND function, an OR gate function can have an unlimited number of inputs but only one output. Figure 2-2 shows an OR function and its electrical circuit representation the resulting output Y, based on all possible input combinations.


Figure 2-2. Two-input OR gate.

The boolean expression of OR gate is:

Y=A+B We can representation OR gate by a ladder logic as:

OR Ladder logic representation

THE NOT FUNCTION

The NOT function, unlike the AND and OR functions, can have only one input.

It is seldom used alone, but rather in conjunction with an AND or an OR gate. Figure 2-3 shows the NOT operation and its electrical-circuit representation. Note that an A with a bar on top represents NOT A.

Figure 2-3. NOT gate.

The boolean expression of NOT gate is: Y=

We can representation OR gate by a ladder logic as:

NOT Ladder logic representation

At first glance, it is not as easy to visualize the application of the NOT function

as it is the AND and OR functions. However, a closer examination of the NOT function shows it to be simple and quite useful. At this point, it is helpful to recall three points that we have discussed:

1. Assigning a 1 or 0 to a condition is arbitrary. 2. A 1 is normally associated with TRUE, HIGH, ON, etc. 3. A 0 is normally associated with FALSE, LOW, OFF, etc.


Examining statements 2 and 3 shows that logic 1 is normally expected to activate some device (e.g., if Y = 1, then motor runs), and logic 0 is normally expected to deactivate some device (e.g., if Y = 0, then motor stops). If these conventions were reversed, such that logic 0 was expected to activate some device (e.g., if Y = 0, then motor runs) and logic 1 was expected to deactivate some device (e.g., Y = 1, then motor stops), the NOT function would then have a useful application. THE EXCLUSIVE OR FUNCTION (XOR)

The EXCLUSIVE OR function, can have only two input and one output.

Figure 2-4. XOR gate and its truth table.

XOR Electrical-circuit and ladder logic representation.

The boolean expression of XOR gate is:

THE EXCLUSIVE NOR FUNCTION (XNOR) The EXCLUSIVE NOR function, can have only two input and one output.

Figure 2-5. XNOR gate and its truth table.


XNOR Electrical-circuit and ladder logic representation

The boolean expression of XOR gate is:

EXAMPLE 2-1 Show the logic gate, truth table, and circuit representation for a solenoid valve (V1) that will be open (Energized) if selector switch S1 is ON (closed) and if level switch L1 is NOT ON (Not closed, liquid has not reached level).

SOLUTION

Note: In this example, the level switch L1 is normally open, but it closes when the liquid level reaches L1. The ladder circuit requires an auxiliary control relay (CR1) to implement the not normally open L1 signal. When L1 closes (ON), CR1 is energized, thus opening the normally closed CR1-1 contacts and deactivating V1. S1 is ON when the system operation is enabled.


Principles of Boolean Algebra and Logic 2.2

An understanding of the Boolean techniques for writing shorthand expressions for complex logical statements can be useful when creating a control program of Boolean statements or conventional ladder diagrams. Figure 2-4 summarizes the basic Boolean operators as they relate to the basic digital logic functions AND, OR, and NOT. These operators use capital letters to represent the wire label of an input signal, a multiplication sign () to represent the AND operation, and an addition sign (+) to represent the OR operation. A bar over a letter represents the NOT operation.

Figure 2-4. Boolean algebra as related to the AND, OR, and NOT functions.


Table 2-1. Logic operations using Boolean algebra.

EXAMPLES 1-Boolean Equation:

Ladder Logic for Equation

2- Boolean Equation:

Ladder Logic for Equation

3- Boolean Equation:


The circuit and equivalent ladder logic.

4- Given the controller equation;

The circuit is given below, and equivalent ladder logic is shown

The PLC does not allow for programming vertical contacts Figure 2-6.. In the real world, one could wire the circuit as shown in the figure, but programming restrictions would not allow the PLC to be programmed in this manner, the user must reprogram the rung with forward power flow to all contact elements. The next example illustrates the solution to the vertical contact rung in Figure 2-6.


Figure 2-6. Reverse power flow at contact D.

EXAMPLE 2-2

Solve the logic rung shown in Figure 2-6. so that no reverse power flow condition exists. The reverse condition is not part of the required logic for the output to be energized. SOLUTION

The forward power flow of the logic determines output Y. Lets implement it using logic concepts. The output Y is defined, using forward paths only, as:

which can be minimized, using Boolean algebras distributed rule.

Figure 2-7 shows the implementation of this logic gate, while Figure 2-8 gives the ladder-equivalent solution.

Figure 2-7. Logic solution for Example 2-3.

Figure 2-8. Ladder diagram implementation for Example 2-3.


Chapter 3: ProgrammingPLC

3.1 Programming Devices

Although the way to enter the control program into the PLC has changed since the first PLCs came onto the market, PLC manufacturers have always maintained an easy human interface for program entry. This means that users do not have to spend much time learning how to enter a program, but rather they can spend their time programming and solving the control problem. Most PLCs are programmed using very similar instructions. The only difference may be the mechanics associated with entering the program into the PLC, which may vary from manufacturer to manufacturer. This involves both the type of instruction used by each particular PLC and the methodology for entering the instruction using a programming device. The two basic types of programming devices are: Miniprogrammers Personal Computers

Miniprogrammers, also known as handheld or manual programmers, are an

inexpensive and portable way to program small PLCs (up to 128 I/O). Physically, these devices resemble handheld calculators, but they have a larger display and a somewhat different keyboard. The type of display is usually LED (light-emitting diode) or dot matrix LCD (liquid crystal display), and the keyboard consists of numeric keys, programming instruction keys, and special function keys. Instead of handheld units, some controllers have built-in miniprogrammers. In some instances, these built-in programmers are detachable from the PLC. Even though they are used mainly for editing and inputting control programs, miniprogrammers can also be useful tools for starting up, changing, and monitoring the control logic. Figure 3-1 shows a typical miniprogrammer along with a small PLC, in which miniprogrammers are generally used.

Most miniprogrammers are designed so that they are compatible with two or more controllers in a product family. The miniprogrammer is most often used with the smallest member of the PLC family or, in some cases, with the next larger member, which is normally programmed using a personal computer with special PLC programming software. With this programming option, small changes or monitoring required by the larger controller can be accomplished without carrying a personal computer to the PLC location.

Some miniprogrammers offer removable memory cards or modules, which store a complete program that can be reloaded at any time into any member of the PLC family. This type of storage is useful in applications where the control program of one machine needs to be duplicated and easily transferred to other machines.

Figure 3-1. A typical miniprogrammer and a small PLC.


Personal computers Common usage of the personal computer (PC) in our daily lives has led to the

practical elimination of dedicated PLC programming devices. Due to the personal computers general-purpose architecture and standard operating system, most PLC manufacturers and other independent suppliers provide the necessary PC software to implement ladder program entry, editing, documentation, and real-time monitoring of the PLCs control program. The large screens of PCs can show one or more ladder rungs of the control program during programming or monitoring operation.

Personal computers are the programming devices of choice not so much because of their PLC programming capabilities, but because PCs are usually already present at the location where the user is performing the programming.

The different types of desktop, laptop, and portable PCs give the programmer flexibility they can be used as programming devices, but they can also be used in applications other than PLC programming. For instance, a personal computer can be used to program a PLC, but it may also be connected to the PLCs local area network to gather and store, on a hard disk, process information that could be vital for future product enhancements.

3.2 Types of PLC Languages

The five types of programming languages used in PLCs are:

1- Ladder (Logic) Diagram (LAD or LD)- Relay logic diagram based programming 2- Function Block Diagrams (FBD)- A graphical dataflow programming method 3- Instruction List (Statement List )(IL) -This is effectively mnemonic programming 4- Structured Text (ST)- A BASIC like programming language 5- Sequential Function Charts (SFC)- A graphical method for structuring programs

1- Ladder Diagram (LAD)

A very commonly used method of programming PLCs is based on the use of ladder diagrams. Writing a program is then equivalent to drawing a switching circuit. The ladder diagram consists of two vertical lines representing the power rails. Circuits are connected as horizontal lines, i.e. the rungs of the ladder, between these two verticals.

Figure 3-2. Ladder Diagram.

In drawing a ladder diagram Figure 3-2., certain conventions are adopted:

1- The vertical lines of the diagram represent the power rails between which circuits are connected. The power flow is taken to be from the left-hand vertical across a rung.

2- Each rung on the ladder defines one operation in the control process. 3- A ladder diagram is read from left to right and from top to bottom. PLC-2012 Eng. Mohammad Al-Arni

4- Each rung must start with an input or inputs and must end with at least one output. The term input is used for a control action, such as closing the contacts of a switch, used as an input to the PLC. The term output is used for a device connected to the output of a PLC, e.g. a contactor.

5- A particular device can appear in more than one rung of a ladder. For example, we might have a relay which switches-on one or more devices. The same letters and/or numbers are used to label the device in each situation.

6- The inputs and outputs are all identified by their addresses or notation used depending on the PLC manufacturer. This is the address of the input or output in the memory of the PLC.

2- Function Block Diagram (FBD)

The term function block diagram (FBD) is used for PLC programs described in terms of graphical blocks. It is described as being a graphical language for depicting signal and data flows through blocks, these being reusable software elements. A function block is a program instruction unit which, when executed, yields one or more output values. Thus a block is represented in the manner shown in the figure below with the function name written in the box.

Figure 3-2. function block diagram.

3- Instruction Lists ( IL) Instruction lists (IL) is a programming method, which can be considered to be

the entering of a ladder program using text. Instruction list gives programs which consist of a series of instructions, each instruction being on a new line. An instruction consists of an operator followed by one of more operands, i.e. the subjects of the operator. In terms of ladder diagrams an operator may be regarded as a ladder element. Each instruction may either use or change the value stored in a memory register. For this, mnemonic codes are used, each code corresponding to an operator/ladder element. The codes used differ to some extent from manufacturer to manufacturer, though a standard IEC 61131 has been proposed and is being widely adopted. Table 3.1 shows some of the codes used by manufacturers, and the proposed standard. 4- Structured Text (ST)

If you know how to program in any high level language, such as Basic or C, you will be comfortable with Structured Text (ST) programming. The language is composed of written statements separated by semicolons. The statements use predefined statements and program subroutines to change variables. The variables can be explicitly defined values, internally stored variables, or inputs and outputs. An example program is shown in Figure 3-3.

Figure 3-3. ST program example.


Table 3.1 Instruction code mnemonics

5- Sequential Function Charts (SFC) Sequential Function Charts (SFCs) are a graphical technique for writing

concurrent control programs. For the application shown in Figure 3-4, the PLC will execute action 2 only after step 1 receives a valid input and transition 1 occurs (i.e., the limit switch LS_Reach triggers). After the PLC finishes action 2, it will wait for transition 2 (IF Temp_1100) to occur and then move to step 3.

Figure 3-4. SFC program example.

3.3 Examples

AND Gate

FBD


Standard IL LD I0.0 (*Load I0.0*) AND I0.1 (*AND I0.1*) ST Q0.0 (* Store result in Q0.0, i.e. output to Q0.0*)

Siemens

OR Gate

LAD

FBD

Siemens IL

NOT Gate

LAD

FBD

Siemens IL


Siemens (Simatic Step7-300)

&

&>=1

I0.0

I0.2

I0.2

I0.1

Q0.0

&

&

>=1

I0.3

I0.4

I0.3

I0.4

Q2.0

>=1&

I0.0

I0.1

&I0.1

I0.2


XOR gate: (a) Mitsubishi, (b) Siemens LAD

(a) Mitsubishi (b) Siemens

In such a situation Mitsubishi uses an ORB instruction to indicate OR together parallel branches. ORB (OR branches/blocks together) FBD

Two branched AND gates: (a) Mitsubishi, (b) Siemens


3.4 Addresses Used in PLC's

Each symbol on a rung will have a reference number, which is the address in memory where the current status (1 or 0) for the referenced input is stored. When a field signal is connected to an input or an output interface, its address will be related to the terminal where the signal wire is connected. The address for a given input/output can be used throughout the program as many times as required by the control logic. This PLC feature is an advantage when compared to relay-type hardware, where additional contacts often mean additional hardware.


Chapter 4: PLC Basic Instructions

4.1 Examine if Closed (Examine if ON) (XIC)

When an input device completes its circuit the input terminal wired to the device indicates an on state. This on state is reflected in memory for the corresponding bit. When the processor finds an XIC instruction having the same address, it determines that the input device is on or closed and sets the instruction logic to true. When the input device no longer completes its circuit, the processor sets the logic for this instruction to false.

If the rung containing this instruction also contains an output instruction, the output instruction is enabled when the XIC instruction is True (input closed); a non-retentive output instruction is disabled when the XIC instruction is False (input open).

An input can be a connected switch closure or sensor, a contact from a connected output, or a contact from an internal output.

Programming The XIC Instruction

Figure 4-1. Programming the XIC instruction

In Figure 4-1 note that both pushbuttons are represented by the XIC symbol. This is because the normal state of an input (N.O or N.C) does not matter! What does matter is that if contacts need to close to energize the output, then the XIC instruction is used. Since both PB1 and PB2 must close to energize the PL, the XIC instruction is used for both. Figure 4-2 show the PLC connection diagram.

Figure 4-2. PLC connection diagram


4.2 Examine if Open (Examine if OFF) (XIO)

When an input device no longer completes its circuit, the input terminal wired to the device indicates an off state. This off state is reflected in memory for the corresponding bit. When the processor finds an XIO instruction having the same address, the processor determines that the input is off (input open) and sets the instruction logic to true. When the input device completes its circuit, the processor sets the logic for this instruction to false.

If the rung containing this instruction also contains an output instruction, the output instruction Is enabled when the XIO instruction is True (input open); the non retentive output instruction is disabled when the instruction is False (input closed).

An input can be a connected switch closure or sensor, a contact from a connected output, or a contact from an internal output.

Programming The XIO Instruction

Figure 4-3. Programming the XIO instruction

Referring to Figure 4-3 when the pushbutton is open in the hardwired circuit,

relay coil CR is de-energized and contacts CR1 close to switch the PL on. When the pushbutton is closed, relay coil CR is energized and contacts CR1 open to switch the PL off. The pushbutton is represented in the user program by an XIO instruction. This is because the rung must be true when the external pushbutton is open, and false when the pushbutton is closed. Figure 4-4. show the PLC connection diagram.

Figure 4-4. PLC connection diagram


4.3 One-Shot Rising (OSR)

When the rung conditions preceding the OSR instruction go from false-to-true, the OSR instruction will be true for only one scan. After one scan is complete, the OSR instruction becomes false, even if the rung conditions preceding it remain true. The OSR instruction will only become true again if the rung conditions preceding it transition from false-to-true. Figure 4-5 show the OSR-instruction example.

Figure 4-5 OSR-instruction example.

The bit address you use for this instruction must be unique. Do not use it

elsewhere in the program. Do not use an input or output address to program the address parameter of the OSR instruction. 4.4 Out Energize (OTE) or

Use OTE instructions to set a particular bit in memory. If the address of the bit corresponds to the address of an output module terminal, the output device wired to this terminal is energized. The enabled status of this bit is determined by rung logic in your application program.

If a true logic path is established with the input instructions in the rung, the OTE instruction is enabled. If a true logic path cannot be established or rung conditions go false, the OTE instruction is disabled. When rung conditions become false, the associated output device de-energizes.

An OTE instruction is similar to a relay coil. The instruction is controlled by the preceding instructions in its programmed rung. A relay coil is controlled by contacts in its hard-wired rung. A complete logic path of true preconditions is similar to a complete electrical circuit of closed contacts.

Your program can examine a bit controlled by these instructions as often as necessary.

Each set of available outputs (coils) and its respective contacts in the PLC have a unique reference address by which they are identified. For instance (Figure 4-6), coil O:0/1 will have normally open and normally closed contacts with the same address O:0/1 as the coil. Note that a PLC can have as many normally open and normally closed contacts as desired; whereas in an electromechanical relay, only a fixed number of contacts are available.


Figure 4-6. Multiple contacts from a PLC output coil.

Properly formatted outputs 1- An output energize instruction (OTE) referencing a specific output bit should appear only once in a ladder logic program (Figure 4-7).

Figure 4-7. Repeated output.

2- Only one output energize instruction (OTE) should appear in a rung of ladder logic (Figure 4-8).

Figure 4-8. Series outputs.

3- If more than one output is to be controlled by a certain rung of ladder logic, the output energize (OTE) instructions can be placed in parallel (Figure 4-9).

Figure 4-9. Parallel outputs.


4.5 Output Latch(Set) and Output Unlatch (Reset) (OTL), (OTU)

Output latch and output unlatch instructions are retentive output instructions. They are usually used in a pair for any data table bit they control.

When you assign an address to the OTL instruction that corresponds to the address of an output module terminal, the output device wired to this terminal is energized when the bit in memory is set (turned on or enabled). The enabled status of this bit is determined by the rung logic preceding the OTL and OTU instructions.

If a true logic path is established with the input instructions in the rung, the OTL instruction is enabled. If a true logic path is not established and the corresponding bit in memory was not previously set, the OTL instruction is not enabled. However, if a true logic path was previously established, the bit in memory is latched on and remains on, or enabled, even after the rung conditions go false (Figure 4-10).

Figure 4-10. Output latch instruction.

An OTU instruction with the same address as the OTL instruction resets (disables or turns off) the bit in memory. When a true logic path is established, the OTU instruction resets its corresponding bit in memory (Figure 4-11).

Figure 4-11. Output latch and unlatch instruction. 4.6 Internal relay

The internal output operates just as any other output that is controlled by programmed logic; however, the output is used strictly for internal purposes. The internal output does not directly control an output device.


The advantage of using internal outputs is that there are many situations where an output instruction is required in a program, but no physical connection to a field device is needed. Their use in this type of instance can minimize output card requirements. Figure 4-12. show an internal-relay example using Mitsubishi and Siemens manufacturers.

(a) Mitsubishi (b) Siemens

Figure 4-12. internal-relay example 4.7 Data Files

The data file portion of memory stores input and output status, processor status, the status of various bits and numerical data. Data files are organized by the type of data they contain. Figure 4-13 show the file types for data files of SLC 500, 3 through 8 are the default values. Files 9 to 255 can be configured to be bit, timer, counter, control, integer, floating point, or ASCII files. Figure 4-14 show data files table. Figure 4-15. show the data files in the project tree.

Figure 4-13. File types of data files


Figure 4-14. Data files table

Figure 4-15. Data files in the project tree.


4.8 PLC Software and Simulator

A personal computer is most often used to enter the ladder diagram. The computer is adapted to the particular PLC model using the relevant programmable controller software.

The PLC simulator (LogixPro) (Figure 4-16) can be accessed from: Start->Programs->TheLearningPit->LogixPro

Figure 4-16. PLC simulator (LogixPro).

Different screens, toolbars and windows dialog boxes are used to navigate through the Windows environment. Ladder logic elements (instructions) (Figure 4-17). can be dragged and dropped onto the ladder window to create a ladder logic program.

Figure 4-17. Ladder logic elements

The ladder logic program is executed by going online, downloading the PLC program, and switching to run mode. Figure 4-18.

Figure 4-18. PLC online, download and modes of operation

The Logixpro simulator provides a set of built-in simulations, the simulations are shown in Figure 4-19.


Figure 4-19. Logixpro built-in simulations

The programming software needs to know what processor is being used in conjunction with the program. Figure 4-20. show the dialog box of Select Processor Type.

Figure 4-20. Dialog box of Select Processor Type.

Figure 4-21. show the I/O Configuration dialog box. The I/O screen lets you

click or drag-and-drop a module from an all inclusive list to assign it to a slot in your configuration.


Figure 4-21. show the I/O Configuration dialog box

Modes of Operation A processor has basically two modes of operation: the program mode or some

variation of the run mode. Program Mode may be used to enter a new program edit or update an existing program upload files download files document programs change software configurations When the PLC is switched into the program mode, all outputs from the PLC are forced off regardless of their rung logic status, and the ladder I/O scan sequence is halted. Variations of the Run Mode Run Mode is used to execute the user program. Input devices are monitored and output devices are energized accordingly. Test Mode is used to operate, or monitor, the user program without energizing any outputs. Remote Mode allows the PLC to be remotely changed between program and run mode by a personnel computer connected to the PLC processor.


4.9 Examples:

Start-stop-seal circuits

For PLC systems without latch and unlatch instructions, a circuit is needed that will allow a process to start, continue to run after a start button is released, and stop under control of another button. A circuit that implements this functionality is commonly referred to as a start-stop-seal circuit. A feedback path (i.e. a contact) that references the output is normally used to seal around the start contact.

Ex. 1- (a) Write a program that will implement the standard STOP/START motor control circuit shown (start-stop-seal circuit).

Inputs: Stop I:1/0, Start I:1/1 Output: M O:1/0

Solution

(b) Add the necessary programming for a motor run light (O:1/2) and a motor standby or OFF light (O:1/3).

Solution


(c) Add the necessary programming for a second stop pushbutton (I:1/2) and second start pushbutton (I:1/3) (ON/OFF from two position).

In practice several start and/or several stop buttons can be used in a process

Start buttons (with XIC instructions) can be used In series if it is required that ALL be pressed before a process starts In parallel if pressing ANY start button is to start a process (two position) Stop buttons are normally used in series if pressing ANY stop button is to stop a process.

Solution

Ex. 2- Write a program that will implement relay schematic shown. This program demonstrates that the contacts of a single-pole input device can be programmed as a double-pole device.

Inputs: Use only N.C contact of pressure switch (I:1/1).

Outputs: L - O:1/0, H- O:1/1

Solution


Ex. 3- Write a program that will turn ON a light if one or the other of two switches is closed. If both switches are closed simultaneously, an alarm operates that can only be shut OFF by pushing a reset button.

Inputs: Switch (I:1/0), Switch (I:1/1), Reset pushbutton (I:1/2).

Outputs: Light (O:1/0), Alarm (O:1/1).

Solution

Interlock circuits Interlocks can prohibit output(s) from energizing under a certain condition Example: O:2/0 should not energize if O:2/1 is energized (and vise-versa)

Ex. 4- Write a program that will implement the reciprocating motion machine process control schematic shown. The sequence of operation is as follows:

The work-piece starts on the left and moves to the right when the START button is pressed.

When it reaches the rightmost limit, the drive motor reverses and brings the work-piece back to the leftmost position again, and the process repeats.

The reverse pushbutton provides a means of starting the motor in the reverse so that the limit switch LS1 can take over automatic control.

Inputs: Stop (N.C) (I:1/4), Start (N.O) (I:1/2), Reverse pushbutton (N.O) (I:1/3), Limit switch LS1 (N.O) (I:1/6), Limit switch LS2 (N.O) (I:1/7), Overload contact OL (N.C) (I:1/5).

Outputs: F- (O:1/0), R- (O:1/1).


Solution

Ex. 5- Write a program that will cause output pilot light PL to be latched when pushbutton PB1 is closed and unlatched when either pushbutton PB2 or PB3 is closed. Also, do not allow the unlatch to go true when the latch rung is true, nor allow the latch rung to go true when the unlatch rung is true.

Inputs: PB1 (N.O) (I:1/0), PB2 (N.O) (I:1/1), PB3 (N.O) (I:1/2).

Output: Pilot Light PL (O:1/0).

Solution


Chapter 5: PLC Timers and Counters Functions

5.1 Introduction

Timer and counter instructions are output instructions that you can condition by input instructions such as examine if closed and examine if open. Timers time intervals and counters count events, as determined by your application program logic.

Each timer or counter instruction has two values associated with it. These values are:

Preset value (PRE, PR) This is your predetermined set point. You enter this value to govern the timing or counting of the instruction. When the accumulated value is equal to or greater than the preset value, a status bit is changed. You can use this bit to control an output device.

Accumulated value (ACC, AC) This is the current number of ticks that have been measured for a timer instruction; or for a counter instruction, the number of events that has occurred.

Timer and counter instructions require three words of data table, one word each for:

Control word Preset value Accumulated value

5.2 Timer Information

Timer Values. A timer instruction has three important values associated with it: the time base the preset value the accumulated value Timebase: The timebase determines the duration of each timebase interval.

Example: If the timer base is set to 0.01, it would take 200 counts as the preset value (PRE) to equal 2 seconds worth of timing.(see Figure 5-1)

Figure 5-1. Time base illustration

In this example of a timer with a 0.01 time base and a target value of 2 seconds, the preset value would be 200. This value indicates that the timer must wait 200 time bases before timing out. The selection of the time base depends on what is most appropriate for the application.


Each timers has three words associated with it (see Figure 5-2). Each of the three words associated with a timer holds a specific kind of data (see Figure 5-3):

Word 0 holds control data about the status of the timers enable output, whether the timer is actively timing, and the status of the timers done output. Control-word data for timer instructions includes:

EN = Timer Enable bit DN = Timer Done bit TT = Timer Timing Bit

The control word stores this information in bits 15, 14, and 13, respectively. Word 1 stores the timers preset value. This is the target timing value specified in memory. Word 2 holds the accumulated value. This value indicates how much time has actually elapsed since the timer was energized.

Figure 5-2. The timer file showing the three words associated with each timer.

Figure 5-3. The data stored in each word of a timers address.


5.3 On-Delay Timer (TON)

The format of a timer on-delay instruction is:

Figure 5-4. A timer ON-delay instruction.

A timer ON-delay instruction energizes its done output after the timer blocks

input turns on and a specified delay has occurred. (see Figure 5-4).

Following is a description of the operation of the TON instruction:

The TON instruction begins to count time-base intervals when rung conditions become true. As long as rung conditions remain true, the timer increments its accumulated value (AC) each scan until it reaches the preset value (PR). The accumulated value is reset when rung conditions go false, regardless of whether the timer has timed out.

The done bit (DN) is set when the accumulated value is equal to the preset value. It is reset when rung conditions become false.

The timer enable (EN) bit is set when rung conditions are true; it is reset when rung conditions become false.

The timing bit (TT) is set when rung conditions are true and the accumulated value is less than the preset value, it is reset when rung conditions go false or when the done bit is set.

Figure 5-5 illustrates how a timer ON-delay instruction works. When the timer blocks input has logic continuity, the blocks enable output will turn on. As a result, a 1 will be stored in bit 15 of the timers control word. Once the timer is enabled, it will start to time. Thus, a 1 will be stored in bit 14, which is the timer timing bit. As the timer times, the accumulated value increases until it equals the preset value. At that point, the timer timing bit will become a 0, and the done bit will become a 1, meaning that the done output will turn on. This done output is the timers delay action contact. The timers input logic must turn off and then on again before the timer will start


timing again. The timers done output can be referenced throughout the program by XIC and XIO contacts to implement the time delay.

In the ladder program shown in Figure 5-5, the pilot light output will turn on four seconds after the push button (PB) input is pressed. In the ladder diagram, the input logic to the pilot light is a contact that references the done output coil of the timer block. The timers address is T4:18, its preset value is 4, and its time base is 1 second.

Figure 5-5. A timer ON-delay block and its associated timing diagram.

The following (Figure 5-6) shows a ladder diagram program controlling an output device using the TON done bit. By substituting XIC or XIO instructions, you can turn an output on or off depending on your ladder logic. In this figure, when the TON timer enabled by I:101 pushbutton, the output O:301 set on for first 5sec and then it is reset. The output O:300 set on after first 5sec and remain set until reset pushbutton I:100 pressed.

Figure 5-6. A ladder diagram program controlling an output device using the TON done bit.


5.4 Off-Delay Timer (TOF)

The format of a timer on-delay instruction is:

Figure 5-7. A timer OFF-delay instruction. Figure 5-7 illustrates a timer OFF-delay instruction. A timer OFF-delay

instruction de-energizes its done output after the timer blocks input turns off and a specified delay has occurred.

Use the TOF instruction to turn an output on or off after its rung has been off for a preset time interval. The TOF instruction begins to count timebase intervals when the rung makes a true-to-false transition. As long as rung conditions remain false, the timer increments its accumulated value (AC) based on the timebase for each scan until it reaches the preset value (PR). The accumulated value is reset when rung conditions go true regardless of whether the timer has timed out.

The done bit (DN) is set when rung conditions are true. It is reset when rung conditions go false and the accumulated value is greater than or equal to the preset.

The timer enable (EN) bit is set when rung conditions are true; it is reset when rung conditions become false.

The timing bit (TT) is set when rung conditions are false and the accumulated value is less than the preset value, it is reset when rung conditions go true or when the done bit is reset.

The ladder program in Figure 5-8 uses a timer OFF-delay instruction. This circuit works as follows:

The done output will be off when the program is first started and the timers input is off.


When the input logic turns on, both the blocks enable output and done output will turn on. However, the timer will not start timing because it is waiting for an OFF signal instead of an ON signal.

When the blocks input turns off, the enable output will turn off and the timer will start timing. The done output will stay on because it is waiting for the timer to time out before it will turn off.

Once the accumulated value equals the preset value, the timer will stop timing and the done output will turn off, implementing the OFF-delay de-energize function.

Therefore, the done bits action follows the action of the timers input signal, except that the done bit remains on for the specified delay period after the input turns off. All of the timers outputs will now remain off until the input logic turns on again. At this point, the accumulated value is reset to 0.

Figure 5-8. A timer OFF-delay block and its associated timing diagram.

5.5 Retentive Timer (RTO)

The entry format of a retentive timer instruction is the same as a timer on-delay

instruction. A retentive timer, however, can stop timing and then start timing again without its accumulated value resetting to 0.

The RTO instruction begins to count time-base intervals when rung conditions become true. As long as rung conditions remain true, the timer increments its accumulated value (ACC) each scan until it reaches the preset value (PRE). The accumulated value is retained when the rung conditions become false.

When the rung conditions go true, timing continues from the retained accumulated value. By retaining its accumulated value, retentive timers measure the cumulative period during which rung conditions are true. You can use this instruction to turn an output on or off depending on your ladder logic.

The accumulated value must be reset by the RES instruction. When the RES instruction having the same address as the appropriate retentive timer is enabled, the accumulated value and the control bits are reset if the RTO rung


is false. The operation of a reset instruction is explained in the counter section of this chapter.

The done bit (DN) is set when the accumulated value is equal to the preset value. However, it is not reset when rung conditions become false; it is reset only when the appropriate RES instruction is enabled.

The enable bit (EN) is set when rung conditions are true; it is reset when rung conditions become false.

The timing bit (TT) is set when rung conditions are true and the accumulated value is less than the preset value, it is reset when rung conditions go false or when the done bit is set.

Figure 5-9 shows a retentive timer circuit and its timing diagram, which work as follows: When the input logic turns on, the enable output will turn on, and the timer will start

timing. If the input logic turns off, the enable output will turn off, and the timer will stop

timing. The accumulated value, however, will not reset to 0. When the timer starts timing again, it will pick up where it left off. When the accumulated value finally reaches the preset value, the done output will

turn on. Once a retentive timer has timed out, its done output will remain on even if its input

logic and enable output turn off. A reset instruction must be used to turn the done output off and reset the timers accumulated value.

Figure 5-9. A retentive timer circuit and its associated timing diagram.


Examples:

Ex. 1- When a switch is turned on, PL1 and PL2 go on immediately. PL1 turns off after 4 seconds. PL2 remains on until the switch is turned off. Turning the switch off at any time turns both lights off. Write a program that will implements this process. Input: Switch (I:1/0). Outputs: Pilot Light PL1 (O:1/1), Pilot Light PL2 (O:1/2). Solution

Ex. 2- Write a program that will turn on pilot light PL1 10sec after switch S1 is

turned on. Pilot light PL2 will come on 5sec after PL1 comes on. Pilot light PL3 will come on 8sec after PL2 comes on. Pressing PB1 will reset all the timers but only if PL3 is on. Inputs: Switch S1 (I:1/0), Pushbutton PB1 (I:1/4). Outputs: Pilot Light PL1 (O:1/1), Pilot Light PL2 (O:1/2), Pilot Light PL3 (O:1/3).

Solution


Ex. 3- When the lights are turned off in building by S1, an exit door light is to remain on for an additional 10sec. and the parking lot lights are to remain on for an additional 20sec after the door light goes out. Writ a program to implement this process.

Input: Light switch (I:1/0). Outputs: Building light (O:1/0), Exit door light (O:1/1), Parking lot light (O:1/2).

Solution

Ex. 4- Develop a ladder logic program that will control traffic lights in one direction in the following sequence:

RED light on for 12sec. GREEN light on for 8 sec AMBER light on for 4sec Sequence is repeated.

Solution


Homework

Modify the program of Example 4 so as to control the traffic light in both directions.

Red = O:1/00 Green = O:1/02 Amber = O:1/01

Green = O:1/06 Amber = O:1/05 Red = O:1/04

8 Sec. 4 Sec. 8 Sec. 4 Sec.

5.6 Count Up and Count Down Counters (CTU, CTD)

The formats of the CTD and CTU instructions are:

Count up and count down instructions count false-true rung transitions. These rung transitions could be caused by events occurring in the program (from internal logic or by external field devices) such as parts traveling past a detector or actuating a limit switch.

Counter Values. A counter instruction has two values associated with it: the preset value the accumulated value

These values perform the same function as they do in timer instructions. The preset value specifies the target number of counts, while the accumulated value indicates the actual number of counts that have already occurred. In a counter, the preset and accumulated values always increase or decrease in increments of one.

Each count is retained when the rung conditions again become false. The count is retained until an RES instruction having the same address as the counter instruction is enabled.

As with timers, each counter is allotted three words, which are numbered 0, 1, and 2. Each of these three words stores particular data about the counter instruction (see Figure 5-10):


Figure 5-10. The data stored in each word of a counters address.

Word 0 is the control word, which stores data about the counter blocks

operation and status. This word holds information about the status of the count up and count down outputs and data about the counters done, overflow, and underflow status. This information is stored in bits 11 through 15 of the control word.

Word 1 stores the counters preset value, which is the target count value. Word 2 stores the counters accumulated value, which is the actual count

value. A counters preset and accumulated words, words 1 and 2, are addressed with the labels PRE and ACC in the RSLogix software.

5.6.1 Count Up Instruction

A count up instruction is represented by the symbol shown in Figure 5-11. The function of a count up instruction is to increase its accumulated value by one every time the blocks input makes an OFF-to-ON transition. After a certain number of OFF-to-ON transitions have occurred, the count up instruction will energize its output. A count up block has two output coils:

a count up output coil (CU), which indicates that the counter block is energized

a done output coil (DN), which indicates that the count is complete

Figure 5-11. A count up instruction.

The control word for counter instructions includes the following status bits:


In a counter circuit, the counter will continue to count even after the accumulated value has reached the preset value. The done output will remain on as long as the accumulated count is greater than or equal to the preset count. The only way to reset the accumulated value and turn off the done output is to use a reset instruction.

5.6.2 Count Down Instruction

A count down instruction (see Figure 5-12) decreases its accumulated value by one every time the blocks input makes an OFF-to-ON transition. When the accumulated value becomes less than the preset value, the count down instruction de-energizes its output. When the counters accumulated value is greater than or equal to its preset value, the counters output will be on.

Figure 5-12. A count down instruction.

Like a count up instruction, a count down instruction also has two outputs: a count down output, which indicates that the counter is energized a done output, which signals that the target count value has been reached

The control word for counter instructions includes the following status bits:

In practice, a count down instruction is most often used with a count up

instruction to form an up/down counter. In the up/down counter shown in Figure 5-13, both counters share the same address and the same preset and accumulated values. As a result, the up counter increases the accumulated value every time a certain event occurs, while the down counter decreases the same accumulated value if another event occurs.

Figure 5-13. Up/down counter configuration.


5.6.3 Reset (RES)

This output instruction has the format -(RES)-. Use a RES instruction to reset a timer or counter. When the RES instruction is enabled, it resets the Timer On Delay (TON), Retentive Timer (RTO), Count Up (CTU), or Count Down (CTD) instruction having the same address as the RES instruction.

When an RES instruction is enabled, it resets the following:

If the counter rung is enabled, the CU or CD bit will be reset as long as the RES instruction is enabled. If your preset value is negative, the RES instruction sets the accumulated value to zero. This, in turn, causes the done bit to be set by the count down or count up instruction.

A reset instruction can be used with all types of timing and counting instructions except a timer OFF-delay instruction. It cannot be used with a timer OFF-delay instruction because a reset instruction resets the done, timer timing, and enable bits of the timers control word. If the status of these bits is altered while a timer OFF-delay instruction is timing, a machine malfunction, unpredictable machine operation or injury to personnel may occur.

5.6.4 Special Programming Issues

When using counter instructions you must consider some special programming issues:

using a reset instruction to implement a self-resetting counter counting past the maximum count reading fast input signals

Self-Resetting Counter. A self-resetting counter is a counter that resets itself in the same scan after the accumulated value reaches the preset value. Often a reset instruction is used in a counter circuit to implement a self-resetting action. However, this should be avoided in some PLC's unless certain precautions are taken, because the result will be an incorrect count value. Following is an explanation of why.

Figure 5-14 shows a reset instruction used to implement a self-resetting counter. When the counters input turns on, the accumulated count value will increase to 1. At the same time, the counters count up bit, bit 15, will turn on because its action


follows that of the counters input. Since the count up bit reflects the status of the input signal, the PLC uses it to determine if the input signal has made an OFF-to-ON transition. It does this by comparing the current status of the input signal to the value stored in the count up bit address.

Figure 5-14. A reset instruction used to implement a self-resetting counter.

Figure 5-15 shows the self-resetting counter circuit after several subsequent

scans. If the input remains on in the scan following the first OFF-to-ON transition (point A), the PLC will compare this 1 value to the value stored in count up bit 15 in scan 1. Since the count up value is already a 1, the PLC detects that the input has not made an OFF-to-ON transition. The controller will continue to make this same comparison every scan (points B and C). Therefore, when the input signal makes an off-to-on transition (point D), the PLC will know it because the PLC will detect that the current status of the input is 1 and that the previous status of the count up bit was 0. Since the PLC senses an OFF-to-ON transition, it will increase its accumulated count value by one. In this circuit, the done bit will turn on since the accumulated value now equals the preset value.

Figure 5-15. The self-resetting counter circuit after several subsequent scans.

Figure 5-16 shows what will happen after the counters done bit turns on. When

the done output turns on, the reset bit will also turn on since the done bit provides the input logic to the reset coil. The reset instruction will reset the accumulated value, as


well as the count up and done bits, to 0 at the end of the scan. The reset instruction sets the count up bit to 0 (point A), but the input signal has not turned off (point B). This means that in the next scan the PLC will sense an OFF-to-ON transition as it compares the input signal to the count up value (point C), even though no transition has occurred. As a result, the PLC will increase the counters accumulated value, despite the fact that no actual input transition has occurred.

Figure 5-16. An illustration of what will happen after the count up instructions accumulated value is

reset.

Thus, using a reset instruction to implement a self-resetting counter will result in an inaccurate accumulated count value. To avoid this situation, you can use one of the following programming methods to create a self-resetting counter:

Use a clear instruction instead of a reset instruction to set the counters accumulated value to 0.

Use a move instruction to move a value of 0 into the accumulated word at the end of the scan.

Use a reset instruction, but with a one-shot rising instruction programmed at the input to the counter. This one-shot instruction will ensure that the input must turn off and then on again before the PLC will increment its count value.

Following are these methods that can be used to create a self-resetting counter


Counting Past The Maximum Count Value. A counter instructions accumulated value has a range from 32,768 to +32,767. Once a counter reaches a count of +32,767, it cannot go any higher. Therefore, it wraps the accumulated count back around to 32,768 and starts counting up again. To count past the +32,767 count value, you must cascade two counters, making sure that they self-reset in each scan.

When two counters are cascaded, they are programmed so that one counter provides the input to the other counter (see Figure 5-17). This way, the second counter counts how many times the first one has reached its preset value. This figure shows two cascaded counters that implement a count to 100,000.

Figure 5-17. Two cascaded counters that implement a count to 100,000.

Reading Fast Input Signals. If the input events to be counted are happening at a rate faster than the scan, some of the inputs will not be counted (see Figure 5-18). This is because a PLC only detects inputs that are valid at the beginning of each scan. It will not detect inputs that occur during the scan. If an application requires the counting of fast inputs, you must use a high speed counter instruction to count them. This instruction is designed to count fast input signal pulses at a frequency of up to 6.6 kilohertz.

Figure 5-18. If the input events to be counted are happening at a rate faster than the scan, some of the inputs will not be counted.


5.6.5 High-Speed Counter (HSC)

The High-Speed Counter is a variation of the CTU counter. The HSC instruction is enabled when the rung logic is true and disabled when the rung logic is false. The HSC instruction counts transitions that occur at specific input terminal such as I:0/0 (In this case Do not place the XIC instruction with address I:0/0 in series with the HSC instruction because counts will be lost). The HSC instruction does not count rung transitions. You enable or disable the HSC rung to enable or disable the counting of transitions occurring at input terminal I:0/0. We recommend placing the HSC instruction in an unconditional rung.

Examples:

Example (1)

Write a program that will turn a light on when a count reaches 20. The light is then to go off when a count of 30 is reached. The system can be reset manually at any time by the reset button.

Inputs: Count button (N.O) (I:102), Reset button (N.O) (I:103). Output: Light (O:100).

Example (2) (Batch Mixing Simulator) A- Filling the Batch Mixing Tank

Using your knowledge of PLC counters, design a program to meet the following requirements:

o When switch S2 is pressed, pump P1 will be energized and the tank will start to fill. The pulses generated by flowmeter FL1 should be used to increment a counter.

o When the count reaches a value where the tank is approximately 90% full, the pump is to be shut-off and the status panel FULL light is to be energized.

o The filling operation is to halt immediately if the stop switch S1 is pressed.


B- Emptying the Batch Mix Tank

Modify your program so that:

The mixer will run for 8 seconds once the tank is full. When the mixer stops, pump P3 is to be started and the tank is to be drained

till the counter's accumulator reaches zero. Pressing switch S2 will cause the sequence to repeat.


C- Continuous Operation (Homework)

Modify your program so that the filling and emptying sequence will repeat continuously once it has been started by the initial pressing of switch S2.

Ensure that the RUN light is energized when the mixer or either pump is running.

The STANDBY light should light and the process should halt when the Stop button is pressed.

The process should restart where it left off if the Start button is pressed following a Stop.

Use the PSIM or Logixpro batch mixing simulator to simulate the program.


Example (3)

Write a program that will implement the following conveyor motor control process: Operational Sequence:

The start button is pressed to start the conveyor motor Cases move past the proximity switch and increment the counter's

accumulated value. After a count of 13 the conveyor motor stops automatically and the counter's

accumulated value is reset to 0. The conveyor motor can be stopped and started manually at any time without

loss of the accumulated count. The accumulated count of the counter can be reset manually at any time by

means of the count reset button. The process is repeated when the start button is pressed.

Inputs: Stop button (N.C) (I:100), Start button (N.O) (I:101), Count reset button (N.O) (I:102), Proximity switch (N.O) (I:103.

Output: Conveyor Motor (O:100).


Example (4)

Write a P-SIM or Logixpro program that will simulate the parts counting process shown.

Counter C1 count the total number of parts coming off an assembly line for final packaging. Each package must count 10 parts. When 10 parts are detected, counter C2 sets bit B3 to initiate the box closing sequence. Counter C3 counts the total number of packages filled in a day. The maximum number of packages per day is 300. A pushbutton is used to restart the parts and package counters to zero. Use the silo simulator screen and the following addresses to simulate the program.

Inputs: Stop button (N.C) (I:1/00), Start button (N.O) (I:1/01), Reset button (N.O) (I:1/02), Proximity switch (I:1/03).

Output: Conveyor Motor (O:100), Bit B3 (O:1/04) to initiate the box closing sequence.


Chapter 6: PLC Comparison and Math Instructions

6.1 Comparison Instructions

Comparison instructions are used compare the values stored in two memory

locations to condition the logical continuity of a rung. These two values can be the data stored in two different word locations, or one can be the data stored in a word and the other can be a constant value. These instructions are classified as input instructions

The comparisons that may be performed are:

Most of the compare instructions use two parameters, Source A and Source B (MEQ and LIM have an additional parameter and are described later in this chapter). The value specified by source A must be a word location in memory (address). This word location may specify the accumulated value for a timer or counter, the contents of an integer file word, or any other data stored in memory. The value specified by source B may be either a word location (address) or a constant. If source B contains a word location, then it specifies the location of particular data in memory, just as the source A parameter does. If source B is a constant, then this parameter contains a fixed decimal value to which the instruction compares the source A data.

Negative integers are stored in twos complementary form. A brief description for each instruction follows.


Equal (EQU) The format of the Equal instruction is:

Use the EQU instruction to test whether two values are equal. If source A and

source B are equal, the instruction is logically true. If these values are not equal, the instruction is logically false. Not Equal (NEQ) The format of the Not Equal instruction is:

Use the NEQ instruction to test whether two values are not equal. If source A

and source B are not equal, the instruction is logically true. If the two values are equal, the instruction is logically false.

Less Than (LES) The format of the Less Than instruction is:

Use the LES instruction to test whether one value (source A) is less than another

(source B). If source A is less than the value at source B, the instruction is logically true. If the value at source A is greater than or equal to the value at source B, the instruction is logically false. Less Than or Equal (LEQ) The format of the Less Than or Equal instruction is:

Use the LEQ instruction to test whether one value (source A) is less than or

equal to another (source B). If the value at source A is less than or equal to the value at source B, the instruction is logically true. If the value at source A is greater than the value at source B, the instruction is logically false.


Greater Than (GRT) The format of the Greater Than instruction is:

Use the GRT instruction to test whether one value (source A) is greater than

another (source B). If the value at source A is greater than the value at source B, the instruction is logically true. If the value at source A is less than or equal to the value at source B, the instruction is logically false.

Greater Than or Equal (GEQ) The format of the Greater Than or Equal instruction is:

Use the GEQ instruction to test whether one value (source A) is greater than or equal to another (source B). If the value at source A is greater than or equal to the value at source B, the instruction is logically true. If the value at source A is less than the value at source B, the instruction is logically false.

Masked Comparison for Equal (MEQ)

The MEQ instruction is used to compare whether one value (source) is equal to

a second value (compare) through a mask. The source and the compare are logically ANDed with the mask. Then, these results are compared to each other. If the resulting values are equal, the rung state is true. If the resulting values are not equal, the rung state is false. Source is the address of the value you want to compare. Mask is the address of the mask through which the instruction moves data. The mask is displayed as a hexadecimal unsigned value from 0000 to FFFF FFFF. Compare is an integer value or the address of the reference. For example:


The source, mask, and compare values must all be of the same data size (either word or long word). Limit Test (LIM)

Use the LIM instruction to test for values within or outside a specified range,

depending on how you set the limits. Entering Parameters The Low Limit, Test, and High Limit values can be word addresses or constants, restricted to the following combinations: If the Test parameter is a program constant, both the Low Limit and High Limit parameters must be word addresses. If the Test parameter is a word address, the Low Limit and High Limit parameters can be either a program constant or a word address. True/False Status of the Instruction If the Low Limit has a value equal to or less than the High Limit, the instruction is true when the Test value is between the limits or is equal to either limit. If the Test value is outside the limits, the instruction is false, as shown below.

If the Low Limit has a value greater than the High Limit, the instruction is false when the Test value is between the limits. If the Test value is equal to either limit or outside the limits, the instruction is true, as shown below.


Example

Indicate the observed state of the lamps, by circling the appropriate numbers below:

Lamp 0 is On during counts: 01...2....3...4...5...6...7...8...9...10 Lamp 1 is On during counts: 01...2....3...4...5...6...7...8...9...10 Lamp 2 is On during counts: 01...2....3...4...5...6...7...8...9...10 Lamp 3 is On during counts: 01...2....3...4...5...6...7...8...9...10


Example (Traffic Control using Comparison Instructions)

Using your knowledge of word comparison instructions, develop a traffic light control program which operates via a single timer.

Red = O:1/00 Green = O:1/02 Amber = O:1/01

Green = O:1/06 Amber = O:1/05 Red = O:1/04

8 Sec. 4 Sec. 8 Sec. 4 Sec.

Homework

Traffic Light Control With Delayed Green

Modify your program so that there is a 1 second period when both directions will have their RED lights illuminated. Using the timing diagram below, note that two 1 second periods are required in this sequence.


6.2 Math Instructions The majority of the instructions take two input values, perform the specified

arithmetic function, and output the result to an assigned memory location. For example, both the ADD and SUB instructions take a pair of input values,

add or subtract them, and place the result in the specified destination. If the result of the operation exceeds the allowable value, an overflow or underflow bit is set. Entering Parameters

Source is the address(es) of the value(s) on which the mathematical, logical, or move operation is to be performed. This can be word addresses or program constants. An instruction that has two source operands does not accept program constants in both operands.

Destination is the address of the result of the operation. Signed integers are stored in twos complementary form and apply to both source and destination parameters. Add (ADD)

A+B Use the ADD instruction to add one value (source A) to another value (source

B) and place the result in the destination. After an instruction is executed, the arithmetic status bits (Carry (C), Overflow (V), Zero (Z), Sign (S)) in the status file are updated:

Subtract (SUB)

A-B Use the SUB instruction to subtract one value (source B) from another (source

A) and place the result in the destination. After an instruction is executed, the arithmetic status bits (Carry (C), Overflow (V), Zero (Z), Sign (S)) in the status file are updated.


Multiply (MUL)

A*B Use the MUL instruction to multiply one value (source A) by another (source

B) and place the result in the destination. After an instruction is executed, the arithmetic status bits (Carry (C), Overflow (V), Zero (Z), Sign (S)) in the status file are updated. Divide (DIV)

A/B Use the DIV instruction to divide one value (source A) by another (source B).

The rounded quotient is then placed in the destination. If the remainder is 0.5 or greater, round up occurs in the destination. The unrounded quotient is stored in the most significant word of the math register. The remainder is placed in the least significant word of the math register. After an instruction is executed, the arithmetic status bits (Carry (C), Overflow (V), Zero (Z), Sign (S)) in the status file are updated.

Example: The remainder of 11/2 is 0.5, so the quotient is rounded up to 6 and is stored in the destination. The unrounded quotient, which is 5, is stored in S:14 and the remainder, which is 1, is stored at S:13.

Clear (CLR)

Use the CLR instruction to set the destination value of a word to zero.

After an instruction is executed, the arithmetic status bits (Carry (C), Overflow (V), Sign (S)) in the status file are Reset and the status bit Zero (Z) is set. Square Root (SQR)

A When this instruction is evaluated as true, the square root of the absolute value

of the source is calculated and the rounded result is placed in the destination.


The instruction calculates the square root of a negative number without overflow or faults. In applications where the source value may be negative, use a comparison instruction to evaluate the source value to determine if the destination may be invalid. After an instruction is executed, the arithmetic status bits (Carry (C), Overflow (V), Zero (Z), Sign (S)) in the status file are updated. Absolute (ABS)

|A| Use the ABS instruction to calculate the absolute value of the Source and

place the result in the Destination. This instruction supports integer and floating point values. After an instruction is executed, the arithmetic status bits (Carry (C), Overflow (V), Zero (Z), Sign (S)) in the status file are updated.

Sine (SIN)

Sine(A), A: in radians Use the SIN instruction to take the sine of a number (source in radians) and store

the result in the destination. After an instruction is executed, the arithmetic status bits (Carry (C), Overflow (V), Zero (Z), Sign (S)) in the status file are updated. Cosine (COS)

Cos(A), A: in radians Use the COS instruction to take the cosine of a number (source in radians) and

store the result in the destination. After an instruction is executed, the arithmetic status bits (Carry (C), Overflow (V), Zero (Z), Sign (S)) in the status file are updated. Tangent (TAN)

Tan(A), A: in radians Use the TAN instruction to take the tangent of a number (source in radians) and

store the result in the destination. After an instruction is executed, the arithmetic status bits (Carry (C), Overflow (V), Zero (Z), Sign (S)) in the status file are updated. PLC-2012 Eng. Mohammad Al-Arni

Log to the Base 10 (LOG)

Log10(A) Use the LOG instruction to take the log base 10 of the value in the source and

store the result in the destination. After an instruction is executed, the arithmetic status bits (Carry (C), Overflow (V), Zero (Z), Sign (S)) in the status file are updated. X to the Power of Y (XPY)

AB Use the XPY instruction to raise a value (source A) to a power (source B) and

store the result in the destination. After an instruction is executed, the arithmetic status bits (Carry (C), Overflow (V), Zero (Z), Sign (S)) in the status file are updated. Scale with Parameters (SCP)

Use the SCP instruction to produce a scaled output value that has a linear

relationship between the input and scaled values. This instruction supports integer and floating point values. Use the following formula to convert analog input data to engineering units: y = mx + b Where: y = scaled output m = slope = (scaled MAX. - scaled MIN.) / (input MAX. input MIN.) x = input value b = offset (y intercept) = scaled MIN - (input MIN. * m)

The Input Minimum, Input Maximum, Scaled Minimum, and Scaled Maximum are used to determine the slope and offset values. The input value can go outside of the specified input limits and no ordering is required. For example, the scaled output value is not necessarily clamped between the scaled minimum and scaled maximum values.


Entering Parameters Enter the following parameters when programming this instruction: Input value can be a word address or an address of floating point data elements. Input Minimum and Input Maximum values determine the range of data that appears in the Input Value parameter. The value can be a word address, an integer constant, floating point data element, or a floating point constant. Scaled Minimum and Scaled Maximum values determine the range of data that appears in the Scaled Output parameter. The value can be a word address, an integer constant, floating point data element, or a floating point constant. Scaled Output value can be a word address or an address of floating point data elements.

After an instruction is executed, the arithmetic status bits (Carry (C), Overflow (V), Zero (Z), Sign (S)) in the status file are updated. Application Examples Example 1 In the first example, an analog I/O combination module is in slot 1 of the chassis. A pressure transducer is connected to input 0 and we want to read the value in engineering units. The pressure transducer measures pressures from 0 to 1000 psi and provides a 0 to 10V signal to the analog module. For a 0 to 10V signal, the analog module provides a range between 0 to 32,767. The following program rung places a number between 0 and 1000 into N7:20 based on the input signal coming from the pressure transducer into the analog module.

Example 2 In the second example, an analog I/O combination module is in slot 1 of the chassis. We want to control the proportional valve connected to output 0. The valve takes a 4 to 20 mA signal to control how far it opens (0 to 100%). (Assume that additional logic is present in the program that calculates how far to open the valve in percent and places a number between 0 and 100 into N7:21.) The analog module provides a 4 to 20mA output signal for a number between 6242 to 31,208. The following program rung directs an analog output to provide a 4 to 20 mA signal to the proportional valve (N7:21), based on a number between 0 and 100.


Scale Data (SCL)

When this instruction is true, the value at the source address is multiplied by the

rate value. The rounded result is added to the offset value and placed in the destination.

Note that the term rate is sometimes referred to as slope. This instruction can overflow before the offset is added. Entering Parameters The value for the following parameters is between -32,768 to 32,767. Source can be either a constant or a word address. Rate (or slope) is the positive or negative value you enter divided by 10,000. It can be either a constant or a word address. Offset can be either a constant or a word address.

After an instruction is executed, the arithmetic status bits (Carry (C), Overflow (V), Zero (Z), Sign (S)) in the status file are updated.


Application Example 1 - Converting 4 to 20 mA Analog Input Signal to PID Process Variable

Calculating the Linear Relationship Use the following equations to express the linear relationship between the input value and the resulting scaled value:

Application Example 2 - Scaling an Analog Input to Control an Analog Output

Calculating the Linear Relationship Use the following equations to calculate the scaled units:


The above offset and rate values are correct for the SCL instruction. However, if the input exceeds 13,107, the instruction overflows and sets S:5/0 math overflow bit. For example:

To avoid an overflow, we recommend shifting the linear relationship along the input value axis and reduce the values. Notice that an overflow occurred even though the final value was correct. This happens because the

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