Hello! We have a set of VI we ship with our devices. These didn't have this problem before. The problem itself occurred quite suddenly and makes us wonder why it occurs now. When trying to reconstruct I found that TCP Read randomly reads less bytes than expected. Following setup: LabView 2015, Windows 10, VISA 18.5. The test VI (attached) shall read ModBus register 151 (ModBus RTU over Ethernet) from IP 192.168.0.2 and port 5025. See attached video 2019-09-20 at 09-41-46_Trim.mp3. The incoming message is randomly truncated. When doing the same in a different tool (Docklight Scripting), this never happens. See video 2019-09-20 at 10-04-20.mp4 We are pretty sure, this didn't happen in the past years with older LabView and VISA versions. Any idea?

You are correct, Modbus does not have a termination character. However the protocol is well defined and you can determine the number of bytes you need to read. My suggestion to you would be to replace your code and use the NI Modbus library to handle the communication with the device. It actually implements the complete protocol and knows how to read the messages. You can get the library from the tools network. If you have to use your code, change your read method. When I need to read messages of unknown length I will read a single byte using a longer timeout. This covers the case when no data is actually sent. Once I read a single character, I continue reading chunks of data with a very short timeout. I continue to read until I actually get a timeout. This generally means that I have read the complete message. There is no guarantee but it it far better than using red immediate. You need to determine what makes sense for the short timeout. You don't want it too long so you cause delays in your application yet it shouldn't be so short that you stop reading too soon. I find that something between 50 and 100 ms usually works.

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TCP Read - messages randomly truncated

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Re: TCP Read - messages randomly truncated

rolfk

Knight of NI

‎09-30-2019 02:44 AM

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Why would you consider your own answer a solution if all it does is propose a solution that is not feasable and hence not implemented by LabVIEW? If any post should be marked as solution I think the one from Mark deserves a lot more merits. He knows what he talks about as he has worked on his own Modbus library.

Rolf Kalbermatter
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