Problem with Labview RS485 comms

[Jvi]

I am trying to read the display values from four identical panel meters using an RS485 port and USB-485 with labview.

When using Measurement explorer and "Communicate with Instrument" everything is fine (I have to change to 2-wire auto mode to test communication).

I select the required meter by writing a two digit address as part of the command. 

When I try to communicate from Labview in exactly the same way one of the four meters gives a timeout.

What causes an error in Labview when the USB-485 is obviously working?

This applies for all meter control codes I have tried.

A simple write/read vi for one meter is attached.

[Ben]

I guess it is a different device that times out each time, otherwise the answer is trivial.

What happens if you change the "Wait until next..." in frame #1 with a "Wait (ms)" function?

I ask because the former is (in my book) really a random delay and not a fixed delay as is the case with the latter.

Ben

[Jvi]

It is the same device that times out. What is confusing me is that it is an identical device to the others and it does not timeout when I test it in Measurement explorer. The other three identical devices don't timeout either so something is different in Labview and/or the device. I am investigating the device as well but I need to understand the difference between Labview Read VISA and Measurement Explorer communication test.

John

[Ben]

M&A does not look for a specific byte count.

Could it be that the bad device is sending less bytes?

Ben

[Jvi]

I have checked the byte count that M&A receives and it is the same for all devices (8).

I have tried varying the Byte count that Labview reads with no effect.

John

[Ben]

The failure matrix is confusing me!

1) All units work with M & A (T/F) ?

2) Three of four units ALWAYS work with LV (T/F) ?

3) One and ONLY one unit ALWAYS fails with LV (T/F) ?

If the answer to all of the above is true, then there is something wrong with the ONE failing unit. Carefully examine all of its settings but concider tossing it.

Old Joke:

Q: How does a field service engineer fix a flat a tire?

A: He keep s swapping them until he finds the one that was flat!

That unit sounds sorta flat to me.

Ben

[Jvi]

Your assessment is correct ie all answers are true but why does it work in M&A not LV?

I have had some feedback from the meter supplier that a previous customer had RS232 problems and found that Labview was sending a "break" signal which was killing the comms. A new serial driver for Labview solved the problem.

Does this sound possible?

John

[Ben]

"

Does this sound possible?

"

Yes, if all of the device failed.

Have you attempted to re-address the failing unit to use one of the addresses of the uints that are working? This could indicate a termination character related issue.

The matrix still says there is something strange about that unit.

Ben

[Ben]

"but why does it work in M&A not LV?"

All of the differences between M&A and LV can be controlled using the configure serial port.

There is something weird about that one unit.

Ben