Modbus communication between Idec Microsmart and Mitsubishi F700

[todell]

Hello all,

Hopefully someone can help me with the setup of modbus-rtu between a Idec Microsmart PLC and a Mitsubishi F700 VFD. My project is to write a program to control two drives for duplex pump applications. I have written programs that used hard wiring between the devices but never used modbus. I am using a Idec FC5A-D16RS1 plc with a FC4A-HPC3 RS-485 module. It has terminals A, B, and SG.

Questions

1- Do I have to program the PLC to make the RS-485 module work? I set port 2 to a modbus-rtu master. Does that tell the module to operate?

2- Which terminal is TX and which is RX?

 

[danw]

For RS-485, the lines are positive or negative polarity, not Tx or Rx (which is RS-232).

The bad news is that there is no standard or even convention for whether A or B is positive or negative.

The good news is that it won't hurt the line drivers if they are connected backwards.

As an example, Phoenix Contact has one division that assigns A as positive, another division that assigns A as negative. It's just the way the world is.

Standard advice to those who can't get comm running is to swap the A and B lines on one end and try again.

SG is signal ground. 2 wire RS-485 is a misnomer, because it should be 3 wire: A, B and signal ground. Some manufacturers do not provide a signal ground.

I can't help on model specifics. Your product manual should define which pin/terminals on the connector are designated D+ or D-, or A or B or whatever.