Modbus Communications - Emerson Liebert OpenComms-485 Reference Manual

7/6/2007
REV. 5
Liebert Monitoring Group –Development

Modbus Communications

Connectivity to OpenComms-485 using Modbus
This design specification describes the Modbus communications protocol as
supported by the OC-485 interface card. It includes information on how to pass
information to and from the OC-485 card via Modbus. It is also intended to help
facilitate answering questions regarding supported types, frame format, function
code support etc.
Implementation Basics
Protocol controls the language structure or message format between devices in
other words, the rules for communication. The rules for communication include
how master and slave devices initiate communications, as well as unit
identification, message handling and error checking. Modbus protocol simply
refers to the control of the query and response cycles between master and slave
devices.
The OC-485 card is configured to act as a slave device on a common network.
This common network can be a multi-drop configuration over EIA-485, where
multiple slaves reside on a common wire or loop.
Transmission Format
The OC-485 interface card supports Modbus RTU (Remote Terminal Unit)
transmission modes. . See chart below.
Physical Transmission Baud Rate Data Bits Parity Bits Stop Bits Default
Port Mode
EIA- RTU 9600, 19200 Configurable Configurable Configurable No
485/422 or 38400
2 wire
1