Modbus Communications

REV. 5 7/6/2007

Liebert Monitoring Group –Development

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