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The 3 Most Common MS/TP Mistakes (And How to Fix Them)

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We’ve asked some of our fellow industry experts to weigh in on what they’ve consistently seen to be the most common issues and mistakes in MSTP networks. Here's what they had to say.

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BACnet MS/TP, and BACnet MS/TP troubleshooting, has been around for a long time. And while smart building systems continue to transition towards IP-based solutions, there are still a lot of MS/TP networks everywhere. They remain a robust, cost-effective way to create and maintain building automation systems. 

So what does that mean for those who may not have decades of experience under their belt with a 30-year-old system? It’s good news and bad. The bad news: you’re going to need working knowledge of BACnet MS/TP systems for some time to come. The good news: there’s still a wealth of professionals and experts who have “been there, done that” when it comes to maintaining a healthy MS/TP system. 

We’ve asked some of our fellow industry experts to weigh in on what they’ve consistently seen to be the most common issues and mistakes in MS/TP networks – and how to get them resolved without (too much) of a headache. We’ve come up with three main areas to look at – wiring, overloading, and device configurations.

1. Wiring

Most, if not all, of our experts agreed: When we’re talking about BACnet MS/TP troubleshooting, it’s almost always a wiring issue. MS/TP is essentially one long electrical wire, daisy-chained between devices. Joshua Durston, Controls Engineer at Modern Niagara reminds us, “MS/TP networks often go through many devices, making it easy to affect the whole network when a single device goes down.”

It’s those many physical connections between devices that are likely the source of your issues. The chief culprit? Unless your network is brand new (and maybe even then!) age and vibrations can work a wire loose from its termination point. Once you’ve isolated your problem areas, usually by splitting your network, start by checking the terminal block for loose connections or touching (shorting) wires.

You can get pretty deep into the weeds at this level dealing with electrical engineering, but there are a few other common issues that should be at the top of your wire troubleshooting checklist: 

  • Flipped polarity is a pretty common mistake you’ll see, and can be a little tricky to diagnose. That’s because your MSTP network will still power up, but you’ll get no communication across the system. Quite simply, you’ve got your positive and negative wires in the wrong termination points. Some controllers will have a fault light or code to indicate reversed polarity, but you can also test for inverted polarity with a multimeter or oscilloscope
  • Grounding issues. Not using a ground reference (or 3rd conductor) risks blowing ports and unstable operation across your entire network. 
  • Bad cabling. This can refer to a few issues. As mentioned above, always use a quality cable with a 3rd conductor. If you are running a new wire, you want to look for RS485 cables with low capacitance. And finally, think about length. The further your length, the slower your network will run. Keep it short to run at a higher baud rate, or consider making an entirely new subnet, or employing a signal repeater/booster if the length of the run can’t be shortened.
  • Lack of end-of-line (EOL) resistance. Adding EOL resistors to the ends of the daisy chain (and nowhere else!) suppresses electrical reflections at each end improving the strength of communications for the entire chain. 
A photograph of a Johnson Controls WMA1630 MSTP controller
Controllers, like this one from Johnson Controls, have a fault indicator light. (Image: eBay user jacyus9)

If you’re BACnet MS/TP troubleshooting using OptigoVN, you’ll see a number of diagnostic results that will clue you into possible physical issues, including Checksum Errors, Lost Tokens, and Unacknowledged Requests.


2. Overload

It’s hard to overstate just how easy it is to overload an MS/TP network. MS/TP networks are old technology. Their data processing capacities are objectively tiny – 78 Kilobauds  at the top end. These days, they are also frequently integrated into larger IP networks so servers, and monitoring software can centralize IoT management. 

For the messages MS/TP networks need to send, that tiny capacity is perfectly fine! The typical packet rate on isolated MS/TP networks is measured in bytes per second. 


"It’s like trying to use an original iPhone with applications for an iPhone 15."

IP networks, on the other hand, are orders of magnitude larger and faster than an MS/TP network – 1Gpbs and beyond. That makes it easy for bleed-through from an IP network – say a typical discover (who-is) message on a complete system will completely flood a BACnet MS/TP router with more traffic than it can handle. “It’s like trying to use an original iPhone with applications for an iPhone 15,” says Optigo CTO Pook-Ping Yao. “It’s probably already struggling to display something resource-heavy, like a YouTube video, and then layer on background processes and other application alerts? It’s going to be completely overwhelmed.” 

BACnet MSTP troubleshooting
As seen in OptigoVN, MSTP devices often exhibit long response times due to a lack of computing resources.

An IP network isn’t the only source that can lead to overload. Consider your MS/TP network itself: 

  • Chaining together too many MS/TP devices on a single network can create long round-trips for data, leading to communication gaps and timeouts. 
  • Poor “tuning” of your devices can also result in far too many requests for data and COV alerts than are needed, pushing even more data traffic that will need to wait to be processed. 
  • Since MS/TP networks don’t keep a record of how many devices are present, it’s also easy to mistakenly set your Max Master settings. Too high and your network will continue to poll for devices that aren’t there. Too low and your system will simply skip over any devices beyond what you’ve told it to look for. Joshua Durston adds, “Don’t forget to set the Max Master on higher MAC address devices, too. I often set it one higher than the actual max address to avoid the scenario where a device is added but is not visible because the max master is set too low on the preceding device.”

In each case, segregation of devices into manageable MS/TP networks (less than 35 per subnet), and managing the amount of broadcast traffic on the overall network will be your go-to steps to take in managing overload. 

If you’re BACnet MS/TP troubleshooting using OptigoVN, you’ll see a number of diagnostic results that will clue you into possible overload issues, including Excessive Number of Devices on Single MS/TP Network, Excessive Token Hold Time & Lost Tokens, Excessive MS/TP Round Trip Token Time, Busy Router Backpressure, Device Global Discovery, Slow Response Time, and several Excessive Change of Value messages.


3. Device Configurations

Configuring devices on a BACnet MS/TP network is still largely a manual affair. While some newer devices allow for software commissioning, there’s still a fair bit of dip switches and cabling to deal with. Alongside this, MS/TP networks do not benefit from full duplex communication that BACnet/IP networks have. That combination inevitably leads to devices misbehaving due to their configuration. Here’s a couple to keep in mind.

One of the most common device configuration errors our experts come across is duplicate addressing. Because addresses for MS/TP network devices need to be manually assigned, it’s pretty easy to accidentally assign the same one twice. This error can occur in MAC addressing, BACnet Device ID assignment, and also when configuring network numbers in BACnet routers. 

A few others to look for include: 

  • Addressing gaps. While this isn’t a critical fault, keep in mind that your system will continue to poll for these “missing devices” in your network every 50 token cycles, adding delay on your network. Take the time to do some housekeeping and keep your addresses gap-free. 
  • Conflicting baud rates can also be a common source of conflict as well. If some or part of your network is set to a different transmission speed, you run the risk of a communication breakdown. Here’s a tip from Robert Lastinger at Distech Controls : Skip the “auto baud” setting. Configure all your devices at the same baud rate manually to avoid any software-based issues. 

If you’re BACnet MS/TP troubleshooting using OptigoVN, you’ll see a number of diagnostic results that will clue you into possible config issues, including Gap in MS/TP Device Addressing, Excessive Max Master, Device Global Discovery, Duplicate Device, ID, and Network errors, Error Messages of Interoperability Type, Error Messages of Programming Type, Partially and Fully Unreachable Devices, Router Rejecting Network Messages, and Unacknowledged Requests.

An image of a laptop with OptigoVM Diagnostic results displayed, showing BACnet MSTP troubleshooting issues.
OptigoVN Diagnostic results showing several issues related to device configurations.

4. Bonus: Network Design Can Save You MSTP Troubleshooting Time

Finally, network design – the actual layout of your connected devices – is worth mentioning. Network design can play an important factor in avoiding small issues becoming system-wide downtime. Some of these issues are best addressed when you’re first building out your BACnet MS/TP networks, but if you have the opportunity to make layout changes, here’s some of the common suggestions our experts made:

  • It was mentioned above, and it bears repeating: it’s easy to overload an MS/TP network. Too many devices is a surefire way to create traffic rates too large for MS/TP devices to process. While the default max number of devices may be 127 per network, it’s a well-known best practice to limit the number of MS/TP devices on a network to 35 or less. 
  • Another great tip from Joshua Durston, keep critical devices on their own dedicated network. When it comes to servicing or troubleshooting smart building systems, your aim should be to limit the number of devices you take offline. “Don’t succumb to the temptation to pick up a couple of hallway VAVs or FCUs that are in adjacent areas,” he recommends. 
  • Make sure that your backups and redundant devices can properly function in the event they’re needed, so ensure they’re not on the same network as the critical devices they’re meant to serve. 
  • Finally, document your network! A lack of documentation will only add to your headache and slow down efforts to resolve network issues. Always take the time to note the as-built drawings and the present state of your network, and update any changes immediately. That way, anyone coming in to service the network can avoid hours establishing the lay of the land. As Ping describes, “It’s a huge timesaver if you know the order and the path of the network wires behind the ceiling tiles.”

This list is a great place to start your BACnet MS/TP troubleshooting processes. And by adding in the power diagnostic tools of OptigoVN and Site Scopes, you’ll get faster and deeper insights into your BACnet networks than you’ve ever had before. Ready to learn more? Sign up for free today.

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