Labview Modbus works with PCI-485/4 or USB-485, but not with PCI-485/8?

[Kevin_R]

I have run into a very strange problem. I am interfacing a Eurotherm T800 Visual Supervisor via ModBus to my LabVIEW application. This is an expansion based on a prototype system which is running successfully. No matter what I did, however, the newer system kept returning Error code -1073807298 == "VISA:  (Hex 0xBFFF003E) Could not perform operation because of I/O error."

After much effort and frustration, I finally thought to run a long cable from the new T800 to the PC running the prototype system. When connected to that system, everything worked as it was supposed to!

The main difference between the prototype and the full implementation is one of scale, so the PC for the larger system has a PCI-485/8 8-port serial card in it, while the prototype uses a (non-isolated) PCI-485/4 4-port system.
I happened to have a USB-485 interface, so I plugged that into one of the USB ports on the problematic system... and my ModBus-based software worked immediately!
On the chance that my 485/8 was defective, I swapped it out for a new card (with a new fanout cable). The software will not work with the new card either.

So it appears that for some reason the ModBus vis are working with the PCI485/4 and with the USB-485, but *not* with the PCI485/8.

I can limp along using the USB interface for now, but I would really prefer to get this working with the 8-port card.

Any suggestions?

[K C]

Hi Kevin,

I had the same error with my USB devices. Using the serial buffer function solved it. Maybe you can try this.

Otherwise NI should be able to help you since the 4 port works for you.