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4 MODBUS COMMUNICATIONS
4.1 INTRODUCTION
This Section specifies the MODBUS communications protocol as implemented on
the
1
16
-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.
4.2 MODBUS 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.
4.3 MESSAGE 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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