This is why Visual BACnet has three diagnostic checks for MS/TP traffic.
I’m going to focus on the top two diagnostic checks: using “Average Token Round-Trip Time” and the “Standard Deviation of Token Round-Trip Time” to assess your MS/TP network. (Need help capturing an MS/TP file in Wireshark? Steve Karg can help you here!)
Average Token Round-Trip Time
The industry best-practice for Average Token Round-Trip Time in an MS/TP network is maximum 80ms. If this is the case in your network, the diagnostic check will pass. You will see a warning for the check if your average time is up to 2000ms, and a fail if it is any longer than that.
Long round-trip times usually indicate a high amount of traffic, and can also point to potential buffer overflow.
Clicking on the diagnostic check allows you to see how long each token pass took. Sorting by the maximum and mean times will give you some insights into which sources and destinations may be problematic.
Standard Deviation of Token Round-trip Time
The Standard Deviation of Token Round-trip Time shows how much the length of a token round-trip varies. While it is normal for a round-trip to be slightly longer approximately every 50 trips (because the system is polling for new devices), it is unusual for round-trip time to vary dramatically. If this is the case, there is quite likely something interrupting the token and potentially a master being skipped in the token passing.
Drilling into the diagnostic check, you can sort by standard deviation to see which devices are the worst offenders and start your troubleshooting here.
It is important to look at these two diagnostic checks together. A long average round-trip time with large standard deviation can point to big problems and disruptions in your network.
Conversely, a long round-trip time with little variation may mean that although the system is not optimal, it is working consistently.
Drilling down into these diagnostic checks also shows you each token pass that occurred during the capture. If you notice a source device passing the token to more than one destination regularly, that means the destination device (next master of the source) did not receive the token at some point and was skipped. The problem may be in the destination device itself or the wiring between the source and destination devices.
While each system is different, our diagnostic checks can help you understand what is happening in your system and what is causing your problems.