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MODBUS Protocol Description
All transactions will be performed using the Modbus RTU transmission mode. All messages 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 (2400/4800/9600/19200/38400/76800)
• 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 Vilter 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 Viltech 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.
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