2 MODBUS communication
The serial communication port of the thyristor unit is two-wire RS485 type.
This port use an half-duplex system.
When a Unit must transmit active the transmission line, and when there are not units in
transmission the outputs are fixed to high impedance.
The serial communication port allows to communicate between the thyristor units and a MASTER
device (ex. an computer or a terminal). The cable must be rated for use to data transfer
2.1 MODBUS RTU Protocol
The communication is based on the standard industrial MODBUS RTU with the following
• The Baud rate can be 4800-9600- 9200 Baud (Standard 9200).
• The Preset Multiple Registers (Funct. 6) is limited to the writing of a single
parameter for message.
The following MODBUS functions are supported:
Function Descri tion
03 Read Holding Registers
6 Preset Multiple Registers
The unit support the Broadcast messages:
It' possible send a Broadcast messages using the address 0, all the units respond at
the message without sending back any reply.
2.2 Message Format
The transmission format is a bit start, 8 date bit, and bit stop with no parity verification.
Each message terminate after a said time of "time out", equal at 3.5 time of a character
transmission, where there are not transitions on the transmission line.
The first Byte of each message is always the address of the unit that is a value from to 255 or 0
for the broadcast messages, the second is always the function number, and the rest of the
message depends of the function demand.
When a Slave receive an message, the unit send an answer with the same structure
but with the information demanded.
Each message is followed by CRC (Cyclic Redundancy Check) with two byte. The CRC identify the
incongruity situations of the message, in this case the receiver ignore the message.
The CRC is calculated in accordance with a formula that imply a recursive division of the data by a
polynomial.
The polynomial divisor is:
2
6
+ 2
5
+ 2
2
+ (Hex 8005)
but is modified in two ways:
• Since the bits order are reversed, then the binary pattern is also reversed, and the most
significant bit (MSB) is the right-most bit.
• Since interest only the remainder, the right-most bit could be discarded.
Therefore, the polynomial divisor has value: Hex A00
Normal bit order:
Most significant bit
