Chatsworth Products Remote Infrastructure Management System Protocol Converter User Manual
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RIM-Protocol Converter User Guide - Version 1.0
Chatsworth Products, Inc. (CPI) • 800-834-4969
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C H A P T E R
4
MODBUS COMMUNICATIONS
This chapter describes the Modbus communication protocol as supported by the Protocol Converter. The content includes
details and information on how to configure the Protocol Converter for communications via Modbus network.
4.1.
Implementation Basics
The Protocol Converter is capable of communicating via the half-duplex EIA-485 serial communication standard. The Protocol
Converter is configured to act as a slave device on a common network. The EIA-485 medium allows for multiple devices on a
multi-drop network. The Protocol Converter is a slave only device and will never initiate a communications sequence.
4.1.1 Modes of Transmission
The Modbus protocol uses ASCII and RTU modes of transmission. The Protocol Converter supports only the RTU mode of
transmission, with 8 data bits, no parity and one stop bit. Every Modbus packet consists of four fields:
• Slave Address Field
• Function Field
• Data Field
• Error Check Field (Checksum)
4.1.1.1 Slave Address Field
The slave address field is one byte in length and identifies the slave device involved in the transaction. A valid address range
is between 1 and 254. The slave address is set on the Modbus Device section of the Configuration page.