Standard Screw Compressor - RWB Series
C-8
M&M
R
EFRIGERATION
I
NC
.
MODBUS Protocol Description
All transactions will be performed using the Modbus RTU transmission mode. All mes-
sages will be terminated with a two-byte CRC-16. The Modbus ASCII transmission mode
is not supported.
The default software configuration for Modbus is shown below. Where a parameter can
be changed, a range of options is also shown. It is the responsibility of the user to ensure
that the software configuration is the same for both the master and slave controllers.
• Baud rate: 9600 bps (2400/4800/9600/19200)
• Data bits: 8 (7/8)
• Parity: None (None/Odd/Even)
• Stop bits: 2 (1/2)
Currently there are only three Modbus message types supported by the M&M protocol:
• Function Code 03 (0x03): Read Holding Registers
• Function code 06 (0x06): Preset Single Register
• Function Code 16 (0x10): Preset Multiple Registers
Other Modbus message types may become available later or if the need arises. A brief
description of the format for each function code will be provided below. However, for a
more thorough explanation, please refer to the Modicon Modbus Protocol Reference
Guide (PI-MBUS-300)
Read Holding Registers
The Read Holding Registers function code (0x03) is used to read the contents of one
or more registers within the M&M controller. Registers within the Status Message,
Control Message and Setpoint Messages may be requested singularly or in groups as
long as the number of registers requested does not extend beyond the valid range.
The following table is an example of the use of the Read Holding Registers function
code to request all 40 words in the Status Message.
Unit Address 0X??
Function Code 0x03
Starting Address Hi 0x00
Starting Address Lo 0x00
No. of Points Hi 0x00
No. of Points Lo 0x28
CRC-16 0x??
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