Unistream - Problem with Modbus RTU Connection

[Joa]

Hi, I'm having trouble with a Modbus RTU connection and unfortunately I don't have enough experience to understand the problem. In my project I would like to control 2 or more control valves that can be operated digitally and analogue. Communication with the first valve takes place via a USC-X10-RA28 expansion module (2-wire serial Modbus RTU interface), all other valves are connected with a daisy chain cable. When sending setpoints/operating modes I sometimes have to click the button twice and after a while the connection may hang (status 1). I noticed that the Com Rx buffer is running full to the value 4096 and then the Rx drop counter starts counting. I assume that the PLC can no longer buffer the received bytes and so the data is lost. Am I right and how could this problem be solved?

Thank's & BR

Joachim

 

[Ausman]

Depending on the speeds you are using and length, do you have all termination resistors in the chain set correctly?  If none onboard the valves etc, you need to fit one to the last one in the chain.  This would be the first thing to try.  Some people say that termination can supposedly be ignored on lower speeds and lesser distances.  I always fit it regardless.  Also, are you using proper shielded cable earthed correctly?

I'll repeat an often quoted thing to do;  trial all your connections on your PC first.  Once you've learnt all that and got it working ok, THEN shift the info you've found that works ok to the PLC.  It is so much easier to discover eveything using the PC.

cheers, Aus

[Joa]

Hi Ausman, thank's a lot for taking time. I checked again, The dip switch of the last valve for the terminating resistor is set. What I do next is asking the supplier why signal cable is not drilled and shielded. It was included in the package..I would also like to measure the signal quality...

Cheers

Joachim