BMXNOM0200 Link failing

We are using a Schneider BMXNOM0200 with an RS-485 loop with four devices daisy chained. If one device is powered down, we lose communication to all four devices. This is contrary to how RS-485/ Modbus RTU should work. Anyone run into this before or know how to resolve this?

Sounds like it's time to dig out the scope.

Does this happen when just a certain device is powered down? Or any of them?
Do you lose the terminating resistor when this happens? (which would be crazy, but I have to ask).
Does the powered down device pull the data lines down? Physically disconnecting the powered down device might get things going again in this case.

Or does your polling program get hung up trying to communicate with the offline device?

As an alternative to the scope, connect a PC up to your Modbus network with a USB to RS485 converter and listen in on what is going on. You can also issue commands from your PC that would mimic what the Master should be doing.

Is there 120 Ohm resistor mounted on last device?
Also 1nF capacitor in series with capacitor goes well also.

Pretty sure that you have timing error and new modbus querys are sended even if answer isn't arrived yet from faulty device back to master.
(Modbus data collision on cable, which makes data messed also to/from non faulty slaves)


You should send new query only after message before is succesful or errored on sender side. (You need query all messages on modbus RTU.)

"Does this happen when just a certain device is powered down? Or any of them?"

This happens when any of the devices(starters) is powered off.

I am note sure what you mean by "lose the terminating resister". There is a terminating resister installed.

The program is written to poll one of the starters at a time, if there is no response by a certain amount of time then it polls the next device.

What is configurated timeout (Hardware timeout) and retries on NOM0200 channel?
What is timeout for read and wrie var blocks (software timeout)?

Software timeout should be more than (retries*timeout + timeout).