Common BACnet MS/TP issues

Common BACnet MS/TP issues
Untangling typical MS/TP problems

It is a truth universally acknowledged, that an MS/TP network rife with problems probably has bad wiring. 

MS/TP has been a foundation for building automation for a long time. While many newer buildings are shifting towards IP and Ethernet, lots of old structures are slower to move away from their traditional, hardwired roots. Once the building is up, it’s hard to alter the network’s foundation without a serious overhaul. There’s also an “if it ain’t broke, don’t fix it” attitude in the industry, and a lot of professionals don’t see MS/TP as “broke.” 

Learn more about the differences between MS/TP, IP, and Ethernet

The downside with MS/TP, though, is that wiring is extremely finicky, and it can slowly get worse and worse over time. There are any number of problems that could arise: loose wiring, flipped polarity — you name it. And to confirm what the problem even is, you usually have to go and physically check the wiring. 

Common MS/TP problems

Problems do occasionally arise out of misconfigurations — perhaps duplicate device IDs or duplicate MAC addresses — but MS/TP issues are usually the result of bad wiring. 

There can’t be any loose strands, any untwisted or unterminated wires, any insulation that isn’t stripped back far enough. These are very small details, but even one error can cause big problems.

Read more about problematic MS/TP networks in our blog post, or watch our 10-minute webinar. 

How to solve those issues 

1. Identify the problem

Visual BACnet highlights MS/TP issues, like Longest Response Time, Average Token Roundtrip Time, and Standard Deviation of Token Roundtrip Time. Use Visual BACnet to see if you’re dealing with wiring (e.g. long Response Times or long Average Token Roundtrips), or a configuration issue (e.g. Duplicate Device IDs).  

MSTP scored checks in Visual BACnet

2. Pinpoint the device(s) 

Dig into the checks and pcap data to see which device is causing the issue. You can click on the failed check(s) to see which devices are causing the problem, and with what frequency. Hover over the entries in that failed check to view the Device ID and BACnet Address. You can also see the vendor behind the device, so you know who to call for any technical support. 

MSTP token passing issues

3. Resolve the problem(s)

In all likelihood, this digging will guide you to check your wiring because, again, that’s the most common cause of BACnet issues. While you will still have to check the physical device, at least you’ll know which one to go to, and won’t have to rip out dozens of ceiling tiles. Check the wiring and terminations, and confirm that all is connected as it should be.  

Keep an eye on your MS/TP networks now and into the future, with Visual BACnet Capture Tools for MS/TP. Install these devices on your MS/TP network and they’ll passively monitor, capture data, and send the information up to Visual BACnet for analysis. When issues arise, you’ll get an alert so you can dig in, identify problems, and quickly solve them. Learn more.  

Learn about solving common BACnet problems.

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