Hi everyone, I am trying to read holding registers values off an Omega USB DAQ device using a virtual serial port that uses Modbus RTU protocol. First, I have, what I think is a timing problem. I have attached the portion of VI in question. The error I keep getting, which comes from the "Read Holding Registries VI," is: Error 56, RTU Data Unit.lvclass:Read ADU Packet.vi:5030001 Can anyone help me resolve this? I know that the serial port configuration is correct because I can read the registers with the software provided by Omega. One additional question: The DAQ manual states that the Modbus query delay be 0ms and the response delay be 3ms. It is not obvious to me if this can be set using the Modbus VI library. Is this a standard setting, or can I change the values? Thanks for any help.

@nschweit302 wrote: Thanks for the quick reply RavensFan, quick update: I changed the register address, but I still get the same error. When I deleted "VISA Close," the port is left open. I can't access it from the Omega software I mentioned earlier. I have to keep "VISA Close" in there in order for the other software to be able to access the port. Maybe this is a bug in the Shutdown VI? Strange, I would have thought it would close it down. Perhaps there is a bug, or they never programmed it to. It is hard to know because those VI's are password protected. Take a look and try an older Modbus library given here. MODBUS Library for LabVIEW I used that from early on before the newer ones came out and has always worked for me. It is completely unlocked so you can drill into the subVI's and see what is happening. In terms of the port settings, I checked them again and the defaults are the same as the Omega software except for flow control. In the other software, flow control is set to "RTS only", and it doesn't read correctly if I change it to RTS/CTS. In LabVIEW, I noticed that "RTS only" is not an option. Is this the same as "none"? I don't know much about this particular serial setting... Who Dey! One other thing about the older library I use is that the VISA wire continues throughout and doesn't get hidden within the object/class wire that the purple headered library uses. One other thing is that I don't use the MB Init on that older library and always set the serial settings myself using the Serial Config like you have at the beginning of this VI. I have never heard of "RTS Only" flow control, and NI-VISA doesn't have that as an option. Only RTS/CTS. But honestly, it is very unusual to see any devices nowadays that require hardware flow control.

LabVIEW

Modbus serial communication

Solved!

What's the actual error that occurs? What is the point of the outer while loop and the second stop button? You only need one loop.

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Also, if you want more help with this, you might want to post it as a new topic, since this one was marked solved by the original poster, meaning fewer people are likely to read it.