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6 Remote Communications
6.4 Modbus Communication
6.4 Modbus Communication
Modbus is a common industrial communication protocol. The ERX350 supports the Modbus protocol (as
a slave device) on the TB4 RS-232/485 port as well as the front panel IrDA port. Through the RS-232/485
port it can also be used with modems including Messenger, CNI2, and Cloud Link. Function codes 01, 02,
03, 04, 05, 06, 15 and 16 are supported, in both RTU and ASCII modes.
The Modbus protocol standard defines the format of the data and the techniques used to control the flow
of data. It supports one master device and up to 247 slave devices on one bus (although a maximum of
32 slave devices are permitted on an RS-485 network, due to physical layer limitations). The master
always initiates a communication exchange. Each slave on a Modbus network has its own unique
address (1...247) so that it can be addressed independently from other slaves. This address is sent by the
master as part of every message. All slaves on the network see the message, but only the slave with the
matching address will respond to the message. A message sent to a slave from the master is called a
request, the answer sent back to the master is called a response. Request and response messages are
also called packets or frames. The master can also broadcast a message to all slave devices, using a
slave address of 0. In that case there is no response.
Modbus configuration items are found in the 'Modbus Protocol' and 'Modbus Map Registers' function
groups of MasterLink:
If the external device communicates via Modbus protocol, item 985 must be set to 1. Item 994 will need
to be set to the proper Modbus variation, either RTU or ASCII. The correct setting for all of these items
will be determined by the external device used.
Modbus registers can be 'mapped' to items in the ERX350 via the items in the following group. For
example, pressure can be mapped to register 7003 by enabling Float mapping in item 935 and setting
item 943 to '8'.
Honeywell
166
2023
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