I had the opportunity to troubleshoot an MS/TP BACnet bus using my Rigol DS1052E oscilloscope. Oscilloscopes are much more useful for bus diagnostics than a multimeter. Although they don’t tell you where the problem physically is, they can immediately confirm that you are in fact looking at a bus fault instead of a software problem and also identify when it is corrected.
The bus in question appears to have a short. Communication does still work, but it takes a long time to poll points or override them. I presume there’s a lot of error-correction and trying in the background making up for the bad bus on the platform I’m using here (Tridium AX).
You can’t really tell there’s a problem from the software side besides the slowness and some strange errors about frame errors printed in a log. With the oscilloscope I can see immediately that there is a problem and I know what conductor the problem is on. That doesn’t exactly solve it for me, but it’s a clear indication at least which has a clear, tedious solution.
I’ll update as I investigate!
Yup, that conductor was shorted to the shield which is grounded. This is a floating floor system, and the bus is cable-tied to struts under the floor. It looks like someone pulled a little too zealously. Very easy to repair.