We got AB micrologix 1400ML PLC and use modbus serial to communicate with four frequency inverters telemeq. ATV31 and ATV61. We are experiencing troubles with the frequency inverters. The communication port on the inverters gets damaged after some time in use. I have spoken to Schneider support and they says that we need Pull up and Pull down resistors on master in our system. Is there somebody experiencing the same problems?

 

You do not specifically mention it but is this an RS485 connection? I suspect that it must be because you state that you are communicating with 4 inverters. If this is correct, then also make sure that you have a ground conductor connected between all the devices to insure that the Common Mode Voltage on the RS485 signal wires stay within the specifications.

The pull up and pull down resistors might do the trick because they will provide a relatively high impedance connection to ground and that MIGHT help but is not really the right choice.

You need to insure that the ground voltages at each location are kept to a reasonable level. As I recall the Common Mode Voltage must be within the range of -7 to +12 volts.

There is lots of further information available on the net. Just Google "RS485 Common Mode Voltage" and start reading!

The very first reference that comes up on my search:

www.circuitcellar.com/library/ccofeature/perrin0799/c79bp4.htm

is pretty good and I did not spend any more time reading the others. They may be equally good or better. Another good story is here:

http://www.circuitcellar.com/library/ccofeature/perrin0799/c79bppdf.pdf

about what other things you should consider when you experience a high failure rate.

RS485 is a good connection method for communication but it is nowhere near as good as an isolated type of connection!

My advice is to "spend the money" and move to electrically isolated connections. There are many manufacturers of this type of equipment.

Jerry Miille