Omega Products D1000M Installation Manual

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User’s Guide

D1000M SERIES

Modbus Digital Transmitters

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D1000M MODBUS PROTOCOL

USERS MANUAL

September 14, 1998

Omega Engineering
One Omega Drive
PO Box 4047
Stamford, CT 00

TABLE OF CONTENTS

MODBUS PROTOCOL OVERVIEW .............................. 1

GETTING STARTED ..................................................... 2

MODBUS FUNCTION CODES ...................................... 4

MODBUS ENABLE/DISABLE ........................................ 9

MODBUS EXCEPTION RESPONSES ........................ 10

D1000M SETUP COMMAND ....................................... 11

CALIBRATION ............................................................. 17

D1000M USERS MANUAL 1

MODBUS PROTOCOL OVERVIEW

This document describes the Modbus RTU protocol option included in
the D1000 series of data acquisition modules. This implementat ion of
the Modbus protocol is a subset of the protocol as described in the
Modicon Modbus Protocol Reference Guide PI-MBUS-300 Rev F. Only
the RTU version of the protocol has been implemented.

Modbus RTU mode communicates in standard NRZ asynchronous
format with one start bit, eight data bits, one parity bit, and one stop bit.
Even and odd parity is supported. If no parity is specified, the number of
stop bits can be user configured for either one or two stop bits.

Baud rates supported at this time are: 300, 600, 1200, 2400, 4800,
9600, 19,200, 38,400, 57,600 and 115,200 baud.

Modbus uses the RS-485 electrical specification for multidrop
communications. The RS-232 electrical specification is not supported.

Modbus is a registered trademark of AEG Modicon Inc.

The Modbus RTU protocol transmits data in 8-bit binary bytes (not
ASCII). To illustrate the data in this document, the 8-bit byte is described
as two hexadecimal nibbles. For example, the binary byte value “0101
1101” will be written as 5D.

A typical Modbus RTU command may look like this:

01 04 00 00 00 01 31 CA

Remember, this command string and others throughout this
document are actually transmitted to a module as eight 8-bit binary
characters.

The actual format of the data is dependent on the type of command
desired. The example above is the Modbus ‘Read Input Registers’
function.

The ‘01’ is the address of the slave device (D1000 module) being
commanded. Each slave device must have its own unique address.

The ‘04’ specifies the Modbus ‘Read Input Registers’ function. This is
equivalent to the ‘Read Data’ command to obtain analog input data.

The next two characters ‘00 00’ specify the starting address of the
registers to be read. The first Modicon input register 30001 is addressed
as ‘00 00’. Register 30005 is addressed as ‘00 04’, etc.

2 D1000M USERS MANUAL

The next two characters of this command specify the number of
registers to be read, including the starting register. In this case the two
binary characters ‘00 01’ indicates only one register is to be read.

The final two characters of the command string make up the Cyclical
Redundancy Check (CRC), used to check for errors in the message.

There are no prompt or terminating characters in the messages. All
messages must be transmitted as continuous strings. Messages are
terminated by a ‘silent’ interval of at least 3.5 character times. A ‘silent’
interval of more than 1.5 character times marks the beginning of the
next message. Therefore it is mandatory that the RS-485 bus must be
biased in the MARK condition during the ‘silent’ interval. This is usually
accomplished by pull-up and pull-down resistors on the
communications line .

A typical response to this example command could be:

01 04 02 80 00 D8 F0

The ‘01’ and ‘04’ characters echo the slave address and the command
function.

For this particular command function, the ‘02’ character indicates the
number of data characters to follow, in this case, 2 characters.

The two character string ‘80 00’ is the value read from Modicon input
register 30001. Register data is read back as 16 bits.

The remaining two characters, ‘D8 F0’ is the CRC for the response.

The A1000 series of RS-232 to RS-485 protocol converters and
repeaters will not operate with the 9-bit data characters used by the
Modbus protocol.

Getting Started

The D1000M series modules are initialized at the factory to
communicate using the D1000 ASCII protocol. This allows for all setup
and configurations to be easily performed using the D1000 setup
software or a dumb terminal. After the se tup process has been
completed the D1000M can be placed in Modbus RTU protocol mode
using the "MBR" command. Disable the Modbus RTU mode using the
Modbus Disable ("MBD") command.

Quick start steps:

D1000M USERS MANUAL 3

1 Connect a power supply to the D1000M between +Vs terminal and
GND terminal. The supply voltage must be between +10 and +30Vdc.

2. Properly connect the D1000M series to a computer using the "quick
hook-up" diagrams in chapter #1 of this manual using either an RS-232
or RS-485 serial port.

3. Locate the S1000 Utility software diskette and copy files from the
S1000 sub-directory on the computer hard drive and run the 1000.bat
file.

4. Configure the main menu "Host" RS-232 Port settings and correct
COMx: port baud rate. Note: If the "Default*" pin on D1000M is
connected to GND then select 300 baud as host computer baud rate
and select no parity.

5. Select main menu "Setup" and enter the D1000M device address and
four digit model number. For example, enter 1112 for a D1112M analog
input module.

6. At the next configuration screen make alterations to Baud Rate, Parity
type and any other required parameters. Press the <F10> function key
to transmit the new setup values. Once the values have been
transmitted press the <ESC> key back to the program main menu.

7. Select "Misc" followed by "Enable Modbus Mode" to specify the
Modbus Slave address. Using the <+><-> keys, or left mouse button,
increment the screen address value to desired Modbus Slave address
and press <F10> to transmit the value.

8. Remove the connect ion between "Default*" and GND, which
performs internal reset, to enable Modbus RTU mode. If there was no
connection between "Default*" and GND then cycle the power on
device to force a reset and enable Modbus Mode.

The device is now configured for Modbus RTU mode and can be
connected to a RS-485 based Modbus master system.

D1000 Modbus Protocol SetUp Notes

D1000M do not have local alarm output capability or Events counters.
Therefore, the D1000 ASCII protocol commands CA, CE, DA, EA, EC,
HI, LO, RE, RH a nd RL are not supported. A COMMAND ERROR will
be generated in response to any of these commands.

4 D1000M USERS MANUAL

Each D1000M module must contain a unique Modbus device address
and the communications settings must be configured properly before
installation into a Modbus s ystem..

MODBUS Function Codes

Modbus protocol compatible devices communicate using a master­slave technique similar to that used in ASCII protocol. In a master-slave
communications system only one device (the master) can initiate a
communications sequence. All others devices (the slaves) respond
when requested by the master. Typical master devices can be personal
computers or PLCs. Typical slave devices are D1000M modules.

The master can address any slave device. Slave devices return a
message to any command that was addressed specifically to them. The
returned messages are considered response messages.

The Modbus protocol format used by a master consists of a device
address, a command function code which defines the operation to be
performed, data required with the command, and an error checking
value. The slave response message contains any required data and an
error checking value. If an error occurs, an exception code will be
generated. The supported master function codes are discussed below.

01 - Read Coil Status (Digital Inputs)
04 - Read Input Register (Analog Inputs)
05 - Force Single Coil (Digital Output)
06 - Preset Single Register (Return to D1000 ASCII protocol)
15 - Force Multiple Coils (Digital Outputs)

Function (01) Read Coil Status (Digital Inputs)

Modbus function (01) Read Coil Status will read the status of both the
digital inputs and digital outputs. Digital outputs are read as the state of
the data on the microprocessor output port before being buffered by the
open-collector transistor. If the coil status of a digital output returns as
'1', this means that this particular bit (coil) is turned "on" or sinking
current on the corresponding module digital output pin. Depending on
the module type, some of the digital outputs may not be implemented.

Modbus relay input coils are considered digital inputs on the D1000M
series modules. Modbus relay output coils are considered digital
outputs on the D1000M series modules. This function can be used to
read status of the digital inputs or the combined status of both the digital
inputs and digital outputs.

D1000M digital output bits DO0 to DO7 correspond to Modbus coils 00
00 to 00 07.