No TCP ACK for Modbus TCP message with exception?

Hello All,

I am attaching a redacted screen grab due to the nature of site security, but I am hoping is enough enough maybe someone can point me in right direction or offer some knowledge.

The text portion in an Intouch .txt log file capture for a OI.MBTCP field device. It tells me that transaction 1695 timed out.

Wireshark capture tells me that transaction 1695 was "textbook" to what I would expect.

However, transaction 1694 in WS shows a modbus exception with the bridge device. I was curious because I never did see a TCP ACK for the frame 96480. Would this be expected? K.

I don't understand why the Intouch OI.MBTCP says 1695 is the message that timed out.

I am open to any suggestions as I am sort of stuck with as to what direction. I wish I could just attach the capture file, but this is part of my struggle here and why I decided to try my luck here. I am trying to clean up modbus commss and this seems to be a re-occuring thing with these particle devices. So if I can resolve these issues, I can focus on my next round of cleanup.




I assume that the bridge device is some sort of Modbus/TCP to Modbus RTU router or gateway, is this correct?

I can't speak to the timeout error you're seeing in Intouch, but the exception response you're seeing is exception 11, which is Gateway Target Device Failed to Respond, defined by the spec as:
"Specialized use in conjunction with gateways, indicates that no response was obtained from the target device. Usually means that the device is not present on the network."

It sounds like there is an issue on your Modbus RTU network (assuming that's the protocol used on the other side of the bridge device). A device is not responding to a request or there is a communication issue that causes the response to not be received correctly by the bridge device. I recommend that you check your wiring, ground references, and shielding terminations and make sure the RS-485 wiring is not run near any sources of EMI like motors or power wiring.