I am trying to connect a new device using Modbus to a system that's been operational for 8 years and has been updated several times. I've used this device on other systems without issue. I have a UDT that I've used before that's connected and it's polling several data points. What's interesting i…

How big is the wireshark file? Try zipping and uploading here. If too big though, you'll have to host it elsewhere and send a link

AMMPS working.pcapng (86.8 KB) The file is not big, so I uploaded it. ip.addr == 172.21.43.210

Correction, it's Ignition 8.1.54, so I believe the modbus module is the correct version.

In the wireshark file, the first 15 seconds are where it's not connecting properly. After that, I changed the tag group and it started working.

Does anybody have any ideas what to look at next?

Look at configuration settings for your device (not in Ignition). It seems that it closes the TCP connection if there's no traffic/requests from Ignition within 2 seconds. There's also some spontaneous responses earlier in the capture with no matching request. Not sure what that's about.

Thanks for the reply. The device is a black box that converts proprietary CAN bus data into Modbus TCP. We can't change any of its settings and I believe early on with this device we were told that it would disconnect if data wasn't requested every second or two. The Tag Group used for the UDT ta…

Probably just lucky with timing. The Modbus driver, and other drivers of that vintage (old AB drivers, old Siemens driver) based on the legacy driver framework, have a ~2 second delay between the connection being established and the polling starting. There's nothing that can change this.

Got it. Thanks for the information. Knowing that, I can write a script to kick start it when it breaks. A bit clunky, but it should work. Interesting that at our other site this has worked for years. As you said, must be lucky timing. Something on that server triggers at 1.99 seconds instead o…

I'll contact the vendor to see if they can extend the timeout at all.

Modbus device going from disconnected to connected and back. No data comes through

Ignition

Matrix_Engineering July 20, 2026, 3:14pm 16