Error 56 (Timeout) on Modbus RTU read

[Oksavik]

It's not exactly random, but I don't think there's anything there.

Thanks for the concern though. 🙂

[elnaiem10]

Hi Oksavik,

How are you getting on with this issue? If you are still getting the error, this article has a number of steps you can go through to troubleshoot this error:

Timeout Error 56 When Communicating Between Network Devices Using LabVIEW - National Instruments

Kind Regards

Elmunzir

[Oksavik]

Thank you. I must admit I had forgotten to follow up on this post.

I verified that it was not the host computer, by trying a secondary computer.

The communication between host and cRIO is working fine.

To verify that the serial (RS485) port on the cRIO is not malfunctioning I did a loopback test.

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z0000019LyOSAU

The port seemed to work fine, as I was able to write and read with no problem.

This makes me believe that the issue must be the daisy-chain of instruments.

I will troubleshoot this by

• Disconnecting all of them
• Connect only the first one
• Test
  • If it works, connect the next
  • Continue until it doesn't work
• Try to understand what is not working

The article you linked was informative, but mainly with regards to TCP/IP communication. Thank you!

[ikaiser]

Hey Oksavik, did you find the cause for your problem?

[Oksavik · Accepted solution]

Hi,

We were able to fix the problem, though what caused it remains a bit of a mystery.

When we started looking at how the instruments were connected, we quickly saw that they were in fact not daisy-chained, but connected in a star configuration.

This was much easier to do at the time, and since we have relatively short leads it was not seen as a problem.

When we started disconnecting instruments from the star we found that a single pump was the cause of the issue.

When it was connected, nothing worked; when we disconnected it, everything worked fine.

Turned out that the pump was in a "bad state" of some sort, and after cycling its power it was fine.

How come this pump in a "bad state" messed up the communication for all the other instruments, I don't know. Maybe someone here is able to explain.

Lesson learned:

Try turning it off, and on again.

In this case, not just the computer stuff, but the main circuit breaker as well.

[Oksavik]

I'm not sure if accepting my own post as "solution" is the right thing to do, but it sort of closed the case for my part.

I would mention that in regards to the error message received when trying to open the Visa Test panel, I read somewhere that this was the expected behavior for a cRIO.

I haven't been able to verify this, but if you're getting this error message when trying to open the test panel on a cRIO, it might not be a symptom of any problems with the HW.

[ikaiser]

I'm not sure if accepting my own post as "solution" is the right thing to do, but it sort of closed the case for my part.

---

Yes, that's absolutely the right thing as you described the solution in it!

I am a bit baffled that I did not ask about your instruments' connections earlier: Generally, Modbus RTU RS485 is not meant to support star topology wiring. If it works - good. Unfortunately in your case, it produced a tricky problem. Good to hear you solved it!