Chapter - PLC Operation

A simple computer

K e yb o a rd(Inp ut )

x8 6C P US e ria l

M o use(Inp ut )

1 G B M e m o ry(St o ra ge )

S V G A S c re e n( O utp ut )

3 0 G B D isk( St o ra ge )

A computer in more detail

K e yb o a rd

M o use

Inp ut U a rt

S e ria l Inp ut U a rt

x8 6 C P U

D isk C o ntro lle r

M e m o ry IC s D isk

G ra p hic s

D igita l o utp ut

M o nito r

L E D d isp la y

inp uts inp ut c irc uits c o m p ute r

s to ra ge

o utp ut c irc uits o utp uts

c a rd

The PLC Scan

Selftest

inputscan

logicsolve

outputscan

Selftest

inputscan

logicsolve

outputscan

Selftest

inputscan

logicsolve

0

PLC turns onranges from <1 to 100 ms are possible time

SELF TEST - Checks to see if all cards error free, reset watch-dog timer, etc. (A watchdog timer will cause an error, and shut down the PLC if not reset within a short period of time - this would indicate that the ladder logic is not being scanned normally).

INPUT SCAN - Reads input values from the chips in the input cards, and copies their val-ues to memory. This makes the PLC operation faster, and avoids cases where an input changes from the start to the end of the program (e.g., an emergency stop). There are special PLC functions that read the inputs directly, and avoid the input tables.

LOGIC SOLVE/SCAN - Based on the input table in memory, the program is executed 1 step at a time, and outputs are updated. This is the focus of the later sections.

OUTPUT SCAN - The output table is copied from memory to the output chips. These chips then drive the output devices.

PLC Status

Power On

Program Running

Programming Mode

Fault

Other Issues

Memory Types

Software Based PLCs

1 2 3

4

5 6

7 8

9

10

11

Execution Sequence

A

XB

X

Repeated Outputs

PLC First Scan

firs t sc a nS :F S

light

light

c le a r