Modbus/RTU is a standard protocol managed by Schneider Automation (Modicon). The specification is available through Schneider Automation (http://www.modicon.com/openmbus) and Modbus.org (http://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.
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 RMC SERIAL module:
03: Read Holding Registers
06: Preset Single Register
16 (10 Hex): Preset Multiple Registers
23 (17 Hex): Read/Write 4X Registers
Each of the above functions acts on 4X or Holding registers. The RMC has these 4X registers mapped as described in the following topic:
Protocol-Specific Settings
The RMC Modbus/RTU implementation has one protocol-specific setting: the node address. Enter this address in the Serial Module Options dialog box, as described in Configuring the RMC SERIAL.
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