instruction manual modbus protocol gauges

Instruction manual Modbus™ protocol Enraf gauges

Instruction manual Modbus™ protocolEnraf gauges

February 2003Part no.: 4416.513_Rev0

Enraf B.V.P.O. Box 8122600 AV DelftNetherlands

Tel. : +31 15 2698600Fax : +31 15 2619574E-mail : [email protected] page : http://www.enraf.com

Copyright 2003 Enraf B.V. All rights reserved.

Reproduction in any form without the prior consent of Enraf B.V. is not allowed. This manual is for informationonly. The contents, descriptions and specifications are subject to change without notice. Enraf B.V. accepts noresponsibility for any errors that may appear in this manual.

The warranty terms and conditions applicable in the country of purchase in respect to Enraf B.V. products areavailable from your supplier. Please retain them with your proof of purchase.

Preface


Preface

This manual describes the Modbus protocol, used in the Enraf servo gauges type 854, the series SmartRadargauges and the field indicator model 877 FDI. The document is intended for a control technician that isexperienced and trained in setting up a Standard Modbus™ interface.

For installation and commissioning of the Enraf gauges, please refer to the Installation guides and Instructionmanuals of the related instrument. Refer also to the list of related documents in Appendix B.

A description preceding the technical procedures gives the technical information necessary to understand itsfunctioning. It is recommended to read this description prior to performing any of the procedures.

Communication

Modbus communication can only be performed via the optional RS-232C / RS-485 communication channel.Then it is not possible to connected an optional analog level output (4 - 20 mA). Also the standard Enraf field buscommunication is not available. There cannot be connected a 977 TSI Tank Side Indicator to the 854 servogauges, 873 SmartRadar and 877 FDI, however a 977 TSI can be connected to the 970, 971 and 973SmartRadar LT series.

Safety and prevention of damage

"Notes" are used throughout this manual to bring special matters to the immediate attention of the reader.A Note points out a statement deserving more emphasis than the general text.

Legal aspects

The information in this manual is the copyright property of Enraf B.V., Netherlands. Enraf B.V. accepts noresponsibility for any errors that may appear in this manual.

Additional information

Please do not hesitate to contact Enraf or its representative if you require additional information.

Table of contents

Table of contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Setup for Modbus RTU mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Modbus protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.1 Communication parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2 Communication procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.3 Function codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.3.1 Function code 03 Read holding registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.3.2 Function code 05 Force single coil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.4 Memory map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.5 On-line switch in Modbus mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 Modbus register contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.1 ProductLevel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.2 ProductLevelStatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.3 ProductLevelAlarms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.4 WaterLevel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.5 WaterLevelStatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.6 ProductTemp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.7 ProductTempStatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.8 VapRoomTemp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.9 VapRoomTempStatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.10 VapRoomPress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.11 VapRoomPressStatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.12 ObsDensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.13 ObsDensityStatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Appendix A ASCII Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Appendix B Related documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Introduction


1 Introduction

The modbus communication takes place via the optional RS-232C / RS-485 communication channel.Reference is made to:

• Instruction manual XPU-2 option RS-232C / RS-485• Instruction manual SmartRadar LT RS-232C / RS-485 output communication

These Instruction manuals describe the commissioning of the optional communication channel.In addition to that information, this manual describes the setup for the optional Modbus protocol and theimplemented Modbus function codes.

The RS-232C communication is used between the instrument and one other user (DCE). This can be a modemor line driver or any host system using RS-232C. The RS-232C communication is limited to 15 m (50 ft) distanceand handshake signals (RTS, CTS, DTR, DSR or DCD) are not supported. The communication is half duplex.

With RS-485 communication a multi-drop bus structure is possible with maximum 32 slaves (instruments) andone master (host). The maximum distance with RS-485 is approximately 1000 m (3000 ft) at 9600 baud.The communication is half duplex.

The Enraf instruments communicate with the Enraf GPU-protocol on the RS-232C / RS-485 communicationchannel. A software switch is used to switch over from Enraf GPU-protocol to Modbus protocol RTU mode.

Setup for Modbus RTU mode

2 Setup for Modbus RTU mode

Before the setup for Modbus RTU mode is made, the gauge should be commissioned and operational. When setfor Modbus RTU mode, access to items is limited to the Portable Enraf Terminal via the infra-red connector.

Item Name Description

W2= Protection level 2 Enter password 2.

BC= Address byte CIU emulation One ASCII character (default: -).For the Modbus RTU mode, it is required that there is no CIUaddress emulation. Therefore, this item must be set to ‘ - ’.

TA Transmission address Two ASCII numbers, ranging from 00 till 99 (default: 00).Read the transmission address and note it for use in the ModbusRTU addressing.

GM= GPU Modbus mode switch One ASCII character (default: G)Selects the communication mode of the RS-232C / RS-485communication channel

G = Enraf GPU protocolM = Modbus RTU protocol

Set to ‘M’, for Modbus communication.

EX Exit Exit protection level.

Note: When the Modbus mode is selected, the command to return to Enraf GPU mode via the RS-232C / RS-485communication channel, is with Modbus Function code 05: Force single coil (refer to section 3.5), or via thePortable Enraf Terminal by setting item GM to ‘G’.

Modbus protocol


3 Modbus protocol

Enraf has implemented in the gauges (as option) the Standard Modbus (RTU mode), as defined by theSchneider Automation Modicon Modbus Protocol Reference Guide (PI-MBUS, Rev. J).Only a limited set of functions is used as the protocol for query / response is straight forward. Gauge commandsand parameter settings (item settings) are not supported within the Modbus implementation.

As with RS-485 communication a bus structure can be built, it is possible to connect a maximum of 32 gauges(as slaves) onto one host (master). Each slave has its unique address. The host (master) initiates thecommunication by addressing one of the slaves in its query. Only the addressed slave (gauge) will respond.With RS-232C there can be only one slave (the gauge) and one master (the host).

3.1 Communication parameters

The following communication settings are fixed and should be taken over by the host:

• Physical layer: RS-232C or RS-485 (no handshake)• Baudrate: 1200, 2400, 4800, 9600 or 19K2 baud as set by item BI• Protocol: Modbus RTU• start bit: 1• data bits: 8• parity: odd• stop bit: 1• message delimiter: ≥3.5 character frame time (2 msec. at baudrate 19K2)• Turn around delay: Selectable between 10 and 900 msec as set by item TH

3.2 Communication procedure

A Modbus message is placed by the transmitting device into a frame that has a known beginning and endingpoint. This allows receiving devices to begin at the start of the message, read the address portion and determinewhich device is addressed, and to know when the message is completed.

RTU mode is a binary mode of data representation. Messages start with a silent interval of at least 3.5 charactertimes. This is most easily implemented as a multiple of character times at the baud rate that is being used on thenetwork (shown as T1T2T3T4 in the figure below). The first field then transmitted is the device address.

The gauges monitor the bus continuously, including during the silent intervals. When the first byte(the address byte) is received, each gauge decodes it to find out if it is the addressed gauge.

Following the last transmitted byte, a similar interval of at least 3.5 character times marks the end of themessage. A new message can begin after this interval.

A typical message frame is shown below:

start address function data CRC check end

T1T2T3T4 8 bits 8 bits n * 8 bits 2 * 8 bits T1T2T3T4

Modbus protocol

start Synchronisation 3.5 character time elapsed

address The address field of a message frame contains eight bits. The address must be the binaryrepresentation of the programmed address in item TA. Item TA can range from 00 to 99 (decimal),which means the largest value in this field can be: 63 (0110 0011 ).H B

A master addresses a slave by placing the slave address in the address field of the message.When the slave sends its response, it places its own address in this address field of the response tolet the master know which slave is responding.

function The function code field of a message frame contains eight bits. With the Modbus protocol in theEnraf gauges, only function code 03 (read holding registers) and function code 05 (force single coil)are valid. When a message is sent from a master to a slave the function code field tells the slavewhat kind of action to perform.

For a normal response, the slave simply echoes the original function code.

data The data field is constructed using sets of two 8 bit bytes (16 bit registers), in the range of0000...FFFF. The data field of messages sent from a master to slave devices contains additionalinformation which the slave must use to take the action defined by the function code.

The data field of a response from a slave to a master contains the requested data.

CRC check The CRC check field contains a 16-bit value implemented as two eight-bit bytes. The error checkvalue is the result of a CRC (Cyclical Redundancy Check) calculation performed on the messagecontents. The CRC field is appended to the message as the last field in the message.

end Synchronisation 3.5 character time elapsed

3.3 Function codes

3.3.1 Function code 03 Read holding registers

Holding registers are located in the memory range (4)0000 ... (4)001D . Register values can range from 0000 toHFFFF . Depending on the use, the registers contain a value or bit coded status in single (16 bit) or doubleH(32 bit) register signed (two’s complement) or not signed.

The query message specifies the starting register and quantity of registers to be read:

Slave address Function 03 Start address No. of registers CRC check

8 bits 8 bits 16 bits 16 bits 16 bits

The register data in the response message is packed as two bytes per register with the binary contents visuallyright-justified within each register:

Slave address Function 03 Byte count (N) Data CRC check

8 bits 8 bits 8 bits (N) x 8 bits 16 bits

The amount of bytes N is double the amount of requested registers, because each register occupies two bytes.

Modbus protocol


3.3.2 Function code 05 Force single coil

Coils are located in the memory range (0)0000 ... (0)0001 .HThe value of a coil can be forced to: FF00 = ON or 0000 = OFF.H H

The query message specifies the coil reference to be forced:

Slave address Function 05 Coil address Force data CRC check


The normal response is an echo of the query, returned after the coil state has been forced:



3.4 Memory map

There is a fixed memory map in the gauges for reading the holding registers.

Modbus unit Modbus address Modbus type No. of(hexa-decimal) registers

ProductLevel 0000 SD40001 2

ProductLevelStatus 0002 40003 1

ProductLevelAlarm 0003 40004 1

WaterLevel 0004 SD40005 2

WaterLevelStatus 0006 40007 1

ProductTemp 0007 SD40008 2

ProductTempStatus 0009 40010 1

VapRoomTemp 000A SD40011 2

VapRoomTempStatus 000C 40013 ... 16 4

VapRoomPress 0010 SD40017 2

VapRoomPressStatus 0012 40019 ... 23 5

ObsDensity 0017 SD40024 2

ObsDensityStatus 0019 40026 ... 30 5

There is no offset applicable for the given data. However, there is a fixed scaling which depends on the selecteddimension of the measured unit.

S = signed data (two’s complement)D = double register (4 bytes; 32 bits)

When a measured unit is in fail, the register value will contain the maximum value, considering the Modbus type.For example: a signed double register then contains: 7FFFFFFF ; a signed single register then contains: 7FFF .H H

Modbus protocol

3.5 On-line switch in Modbus mode

When communicating in Modbus, the command to return to GPU-protocol is by Function code 05: Force singlecoil. The data FF00 must be written at coil address 0000 .H H

When the gauge receives this message, it updates its NOVRAM with item GM=G (for communication inGPU-protocol) and resets itself. After the reset, the RS-232C / RS-485 communication channel communicateswith the GPU protocol.

Example:


8 bits 05 00 00 FF 00 16 bitsH H H

Modbus register contents


*) The characters given here are the defaultsettings in item LU (level status conversion);it may be set to different characters.

4 Modbus register contents

4.1 ProductLevel

Signed number; double register.Depending on the level dimension of the gauge (item LD), this double register contains the product level in:

Level dimension Modbus data Scalingitem LD representation (divide by)

M 0.1 mm 10 for mm, or 10 000 for mF 0.001 ft 1000 for ftI 0.01 inch 100 for inchP / inch 16 for inch1

16

4.2 ProductLevelStatus

This register contains the bit coded product level status. This status information is coded in such a way that anASCII code for a character is generated.

The ASCII characters are listed below (refer to the ASCII table in Appendix A). The product level status dependson the type of gauge.

854 Servo gauge level status: SmartRadar gauge level status:

F - Invalid level data F - Invalid level data• hardware / software error • level fail• 854 in service mode • initial status

C - Motor limit switch reached C*) - 4 character of item LUB - Block or freeze active • out of measuring rangeL - Lock test or calibrate active B*) - 3 character of item LUR - Density scan active (TP or IP) • measurement blockedT - Test gauge (TG), searching for level, M*) - 2 character of item LU

balance test or measure frequency • warningcommand active • general APU/ICU fail

W - Water level found (by displacer) ?*) - 1 character of item LUD - Searching for water (downwards) • reduced accuracy- - Valid level - - Valid level

th

rd

nd

st

877 FDI level status when used as HTG:

F - Invalid level data• general HPU/HCU fail• level fail• P1 and/or P3 out of range• P2 out of range

?*) - Reduced accuracy• manual P3 used• last valid P3 used• manual density used• last valid density used• manual gas density used• level below item LN (minimum HTG level)

- - Valid level


4.3 ProductLevelAlarms

The product level alarm status is represented by the following number in this Modbus register:

Product level Modbus registeralarm status value (decimal)

no alarm 0low level alarm 1high level alarm 2low low level alarm 3high high level alarm 4level alarm failure 255

4.4 WaterLevel

Signed number; double register.Depending on the level dimension of the gauge (item LD), this double register contains the water level in:

Level dimension Modbus data Scalingitem LD representation (divide by)

M 0.1 mm 10 for mm, or 10 000 for mF 0.001 ft 1000 for ftI 0.01 inch 100 for inchP / inch 16 for inch1

16

4.5 WaterLevelStatus

This register contains the bit coded water level status. This status information is coded in such a way that anASCII code for a character is generated. The ASCII characters are listed below (refer to the ASCII table inAppendix A).

F - Invalid water alarm data• general probe failure

A - Water above probe warningZ - Water below probe warning- - Valid water level

4.6 ProductTemp

Signed number; double register.Depending on the temperature dimension of the gauge (item TD), this double register contains the producttemperature in:

Temperature dimension Modbus data Scalingitem LD representation (divide by)

C 0.01 °C 100 for °CF 0.01 °F 100 for °F



4.7 ProductTempStatus

This register contains the bit coded product temperature status. This status information is coded in such a waythat an ASCII code for a character is generated. The ASCII characters are listed below (refer to the ASCII tablein Appendix A).

F - Invalid temperature data• optional board absent• multiple temperature general fail• fail in average product temperature reading

T*) - Reduced accuracy (character from item TU)• out of specified temperature range• exceeding differential temperature range (MTT)• device not calibrated (MTT)• last valid level used• manual level used• level below lowest temperature element• alternative temperature element selected (MRT)

- - Valid temperature

*) The character ‘T’ is the default value of item TU (temperature status conversion); it may be set to adifferent character.

4.8 VapRoomTemp

Signed number; double register.Depending on the temperature dimension of the gauge (item TD), this double register contains the vapour room(gas) temperature in:

Temperature dimension Modbus data Scalingitem LD representation (divide by)

C 0.01 °C 100 for °CF 0.01 °F 100 for °F

4.9 VapRoomTempStatus

These four registers contains the bit coded vapour room temperature status. With exception of the first register,the status information is coded in such a way that an ASCII code for a character is generated. The 8 bits ASCIIcoded character is placed in the Low byte of the register; the High byte remains empty (zero).The ASCII characters are listed below (refer to the ASCII table in Appendix A).

Modbus address 000C 000D 000E 000FH H H H

Register contents item MQ, byte 0 item MQ, byte 1 item MQ, byte 2 item MQ, byte 3

Status byte 0:Contains one of the characters ‘0’ to ‘F’ (as a hexa-decimal number), indicating the highest immersed(spot)element of the temperature element.


Status byte 1: Status byte 2:bit 0 : General temperature fail bit 0 : Last valid level used

1 : Fail in average product temperature 1 : Manual level used2 : Fail in average vapour temperature 2 : Level time out3 : Level exceeds lowest (spot)element 3 : Device not calibrated (MTT)4 : Level exceeds highest (spot)element 4 : Exceeding differential temp. range (MTT)5 : (Spot)element fail 5 : Out of specified temperature range6 : 1 6 : 17 : 0 7 : 0

Status byte 3:bit 0 : No previous store command Note:

1 : Alternative element selected (MRT) Only the bits which are set to ‘1’ have an active 2-5 : 0 status.

6 : 17 : 0

4.10 VapRoomPress

Signed number; double register.Depending on the pressure dimension of the gauge (item PI), this double register contains the vapour pressurein:

Pressure dimension Modbus data Scalingitem PI representation (divide by)

P 0.001 bar 1000 for barI, S 0.001 psi 1000 for psiK 1 Pa no scaling

4.11 VapRoomPressStatus

These five registers contain the bit coded vapour room pressure and observed density status. The statusinformation is coded in such a way that an ASCII code for a character is generated. The 8 bits ASCII codedcharacter is placed in the Low byte of the register; the High byte remains empty (zero).The ASCII characters are listed below (refer to the ASCII table in Appendix A).

Modbus address 0012 0013 0014 0015 0016H H H H H

Register contentsitem QF, item QF, item QF, item QF, item QF, byte 0 byte 1 byte 2 byte 3 byte 4

Status byte 0: Status byte 1:bit 0 : General option board fail bit 0 : P1 exceeds min. / max. trip pressure

1 : Low level alarm *) 1 : P2 exceeds min. / max. trip pressure2 : Low low level alarm *) 2 : P3 exceeds min. / max. trip pressure3 : High level alarm *) 3 : Exceeding range P14 : High high level alarm *) 4 : Exceeding range P25 : Level time out *) 5 : Exceeding range P36 : 1 6 : 17 : 0 7 : 0



Status byte 2: Status byte 3:bit 0 : Fail P1 bit 0 : Last valid density used

1 : Fail P2 1 : Manual density used2 : Fail P3 2 : High density alarm3 : Manual P3 used 3 : Low density used4 : Last valid P3 used 4 : HTG level fail *)5 : Manual level used *) 5 : No previous store command6 : 0 6 : 17 : 1 7 : 0

Status byte 4:bit 0 : Manual gas density used Note:

1 : Level below LN Only the bits which are set to ‘1’ have an active2 : Last valid level used status.3 : Invalid level reading4 : °API underflow/overflow or negative density5 : 06 : 1 *) These bits represent typical HIMS / HTG status7 : 0 messages. Ignore for vapour pressure and

observed density measurement.

4.12 ObsDensity

Signed number; double register.Depending on the density dimension of the gauge (item DI), this double register contains the observed densityin:

Density dimension Modbus data Scalingitem DI representation (divide by)

K 0.01 kg/m 100 for kg/mA 0.001 °API 1 000 for °APIL 0.0001 lbs/ft 10 000 for lbs/ft

3

3

3

3

4.13 ObsDensityStatus

These five registers contain the bit coded observed density and vapour room pressure status. Refer to section4.11 VapRoomPressStatus.

Appendix

Appendix A ASCII Table

Appendix B Related documents

Instruction manual 854 ATG level gaugeInstruction manual 854 series XTG level gaugeInstruction manual 877 FDI Field Display & InterfaceInstruction manual 873 SmartRadarInstruction manual 973 SmartRadar LTInstruction manual 971 SmartRadar LTiInstruction manual 970 SmartRadar ATi

Instruction manual XPU-2 option RS-232C / RS-485Instruction manual 973 SmartRadar LT RS-232C / RS-485 output communication

instruction manual modbus protocol gauges

Documents

ft inch inch

232c rs

force single coil

portable enraf terminal

modbus rtu mode

query message specifies

manual gas density

modbus register contents