Omega CN1166 Modbus Communications, Introduction, Modbus Functions Supported, Message Formats

4MODBUS COMMUNICATIONS

4.1INTRODUCTION

This Section specifies the MODBUS communications protocol as implemented on the 161 -DIN Ramp/Soak Profile Controller.

Certain restrictions have been imposed upon this implementation:

(i)Baud rates may be set to 1200, 2400, 4800 and 9600 only

(ii)Support for multi-parameter Writes is limited to support of the Multi-word Write Function (Number 16) but will permit writing of one parameter only per message

(iii)The multi-parameter Read function supports a maximum of 10 parameters in one message.

4.2MODBUS FUNCTIONS SUPPORTED

In the following list, the original Gould MODBUS function names have been used, followed by the JBUS names in italics, where such an equivalence exists. The MODBUS Function number follows the names.

A	Read Coil Status (Read n Bits)	01/02
B	Read Holding Registers (Read n Words)	03/04
C	Force Single Coil (Write 1 Bit)	05
D	Preset Single Register (Write 1 Word)	06
E	Loopback Diagnostic Test	08
F	Preset Multiple Registers (Write n Words)	16

The instrument will identify itself in reply to a Read Holding Registers message which enquires the values of parameter numbers 121 & 122, as specified in the CNOMO documentation, and MODBUS Function 17 (Report Slave ID) will not be supported.

4.3MESSAGE FORMATS

The first character of every message is an instrument address. The valid range of such an address is 0 to 255. The second character is always the Function

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