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ModBus Port Connections

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I need to connect a V570 to a servo drive with Modbus. Both have six pins. The port pin designations are shown below. Can someone please tell me which pins connect to which.


1. A signal (+)

2. (RS232 signal)

3. (RS232 signal)

4. (RS232 signal)

5. (RS232 signal)

6. B signal (-)

Servo Drive

1: GND (0V)

2: RS-232 TX

3: RS-422 RX+

4: RS-232 RX, RS-422 RX-

5: RS-422 TX+

6: RS-422 TX-



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Thanks Joe.

That configuration worked fine for a single drive. I should have mentioned that I am connecting two drives, and have initialized the comm port to RS485, 115200 baud rate, 8 data bits, odd parity, 1 stop bit. I came across a similar application using some other plc and copied their connections as shown below.

PLC -----Servo

1-------3 & 5

3 & 5

6-------4 & 6

4 & 6

This seems to work with both drives, but only as fast as 57,600. When I try 115,200 it is hit and miss...mostly miss. This is only a bench test, with short cables(1/2m). I have only tried reading from the drives. I will try writing to them and see how that goes. I wonder if I should have the servo pin 1 connected to the PLC pin 2?


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  • MVP 2023

RS 485 can be tricky, especially at high speeds. It sounds like you've got the wiring and the DIP switches set right or it wouldn't work at all. You most probably are getting reflections with your short cables. Unitronics recommend that you set the terminator at both ends, but I've found that it sometimes attenuates the signal too much. If you have access to a scope hook it up across the A & B lines and see what the waveform looks like and it's amplitude. You want to set your terminators so the peak-to-peak voltage is somewhere between 6 and 9 volts.

Joe T.

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