Modbus/RTU Protocol

Modbus/RTU is one of the serial RS-232 and RS-485 protocols supported by the RMC75S. This topic describes the Modbus/RTU protocol as it applies to the RMC75. For details on configuring the RMC75 for serial communications, see the Serial Configuration topic.

General

Modbus/RTU is a standard protocol managed by Schneider Automation (Modicon). The specification is available through Schneider Automation (https://www.modicon.com/openmbus) and Modbus.org (https://www.modbus.org). The Modicon Modbus Protocol Reference Guide (PI-MBUS-300) from Modicon is a good reference for this protocol. It is available through the Modicon web site. At the time of this writing it was necessary to select the MODBUS/TCP Protocol documents link to find an Acrobat Reader (.pdf) format version of this document.

RMC75 Support for Modbus/RTU

Modbus, in its various forms such as Modbus/ASCII, Modbus/RTU, Modbus Plus, and Modbus/TCP, is a request/response protocol. That is, a Modbus master makes a request from a Modbus slave, and the slave responds. A number of functions are defined under Modbus. The following functions are supported by the RMC75:

Read Multiple Registers (FC 3)
Read Input Registers (FC 4)
Write Single Register (FC 6)
Get Diagnostics (FC 8)
Write Multiple Registers (FC 16)
Read/Write Registers (FC 23)

Each of the above functions acts on 4X or Holding registers. These 4X registers are mapped in the RMC75 as described in the following topic:

RMC Register Map (Modbus/RTU and /TCP)