Carbon Monoxide Transmitter Modbus
Installation Manual
May 5, 2017
IN-CMD5B4MOD-01-01 Page 12
Network Communication
The device parameters must be set before connection to the network and will ensure each device will have a unique ModBus
address for startup. Details of the device setup can be found in the User Menu section. Once set, all parameters are saved in
non-volatile memory. The local menu and LCD are used to set the ModBus device address (1-255) and the baud rate. The
factory defaults are address 01 and 9600 baud.
The menu is also used to select RTU/ASCII mode, the parity, number of stop bits, the CRC value and ModBus delay values
so the device can be completely configured for the communication parameters before being connecting to the network.
Modbus Trouble-shooting
The CO Transmitter operates as a slave. It will not communicate unless a master is connected to the network and sends a
request for information, then the slave will answer. If the device does not communicate properly, first check that the
communication wires are not reversed. Then check the communication parameters in the menu in the following sequence:
Slave address, baud rate, transmission mode, parity bit, stop bit, RTU mode CRC polynomial and slave response delay.
The factory default Modbus address is 01 and each device must have its unique address to communicate properly on the bus.
Use the menu to change the Slave address to a unique number for each unit.
The default Modbus baud rate is 9600. Use the menu to change the baud rate to the correct setting.
The default transmission mode is RTU. If this is incorrect, use the menu to change the transmission mode to ASCII.
The default Modbus parity is None. If this is not correct, use the menu to change the parity from None to Odd or Even.
The default stop bits is 1. Use the menu to change the stop bit setting to 2. For some configurations the value is fixed.
The default Modbus CRC value is A001. The menu can be used to change this setting. This only applies to RTU mode and
has no effect in ASCII mode. It is the CRC polynomial setting and can be changed between A001, 1021, 8005 or 8408.
The default Modbus delay is minimum (0). This can be changed in the menu. It is the slave response delay and can be set
from minimum to 350ms. For example, the minimum delay means 3.5 character time delays or 4ms for 9600 baud rate.
ModBus Protocol
This section describes the implementation of the Modbus protocol used in the CO Transmitter. It is intended to assist control
system programmers who may need to add support to their systems to communicate with this device. The CO Transmitter
communicates on standard Modbus networks using either RTU or ASCII mode transmission. It operates as a slave device
(address from 01 to 255) and expects a Modbus master device to transmit queries, which it will answer.
RTU Mode Message Format
Modbus Framing
8 bit binary
Data Bits
start bits --- 1
data bits --- 8
parity bits --- none, odd or even
stop bits --- 1or 2
Baud Rate
300, 600, 1200, 2400, 4800, 9600 or 19200
Duplex Half
duplex
Error Checking
Cyclical Redundancy Check (CRC)
CRC-16 --- polynomial x16+x15+x2+x0 0x8005 or reversed version 0xA001
or CRC-CITT --- polynomial x16+x12+x5+x0 0x1021 or reversed version 0x8408
Latency
More than 3.5 characters --- minimum, 50, 100, 150, 200, 250, 300 or 350 mS
