IFIX Modbus RTU

I have a PLC SAIA PCD1 and would like to connect to the iFix 5.5 via MBE Modbus Ethernet v7.xx I/O driver, how do I do it?

[Shaleen]

Most Modbus/TCP devices use the slave’s IP address, not the slave node’s ID address like Modbus RTU requires, but some early vintage PLC firmware does “Modbus RTU over Ethernet”, which requires the slave node address to be configured and working.

The software ordinarily calculates the CRC bytes.

The actual Modbus message frame looks like this:



The Modbus message starts counting at zero. Your decimal registers are likely one-offset from zero.
(3)0000 = register 0001
(3)0001 = register 0002
(3)0098 = register 0099
(3)0099 = register 0100
(3)0100 = register 0101
(3)0101 = register 0102
(3)0200 = register 0201

(3)0101 = register 0102 = 0066 hex
(3)0200 = register 0201 = 00C9 hex

Quantity of 100 input registers = 0064 hex

For Slave address 02, the Modbus RTU message would be

You can use a generic Windows Modbus master with the correct hardware connections, to test the connection as well as the functionality of the Modbus slave, presumably the PLC. This is a very good way to test that the slave is setup the way one thinks it is.