Troubleshooting BACnet MS/TP issues can be frustrating, especially if you’re flying blind. But with the right tools, and a bit of structure, most problems aren’t just solvable — they’re preventable.
In our experience, almost every MS/TP problem falls into one of three categories:
- Internal Issues — Physical problems like faulty wiring, misconfigured settings, or outdated device patches and firmware.
- External Errors — Bottlenecks or traffic floods from the larger BACnet/IP network that cascade into the MS/TP side.
- Just Needs a Tweak — Common tuning opportunities that improve capacity, reduce latency, or bring an overloaded device back under control.
OptigoVN’s diagnostics are built to surface the most common (and costly) issues fast — so you don’t have to dig through hundreds of packets to guess where the problem lives. There are seven key diagnostics that specifically target MS/TP-related problems:
- Checksum Error
- Excessive Max Master
- Excessive MS/TP Round Trip Token Time
- Excessive number of devices on an MS/TP network
- Excessive Token Hold Time
- Gap in MS/TP Device Addressing
- Lost Tokens
Let’s break down what each of these means, how to fix them, and when to look outside the MS/TP chain for a root cause.
Internal Issues
Sometimes the problem is isolated to the MS/TP subnet itself. Less than 10 years ago, this would be a sign that you’d be putting in some overtime splitting, testing, decoding packets, and repeating until you found the issue. But with OptigoVN, you can quickly determine, with the help of our diagnostic results, where your efforts need to be focused.
If you see any of these diagnostic errors returned after your packet analysis, you can be sure your issue is originating from an MS/TP subnet:
1. Checksum errors
These almost always mean a physical issue. A frayed or disconnected serial cable or a hardware failure are your likely culprits. Combine this alert with the Lost Tokens diagnostic and a Site Scope+ add-on to pinpoint the exact device address or network ID for the device in question. Hours saved.
2. Excessive Max_Master errors
A commonly ignored configuration issue that gets even the best of us! This diagnostic lets you know the max_master configuration setting for your MS/TP subnet exceeds the actual number of devices by more than two. While it’s not mission-critical, an incorrect max master setting will cause unnecessary poll-for-master calls, slowing down communications on the network.
3. Excessive MS/TP Round-Trip Token Time errors
This error is also a likely symptom of wiring errors introducing conflicts, leading to lost messages. Like the Lost Token diagnostic, a Site Scope+ identifies devices with excessive round-trip times. You’ll have to physically inspect and fix any faulty wiring terminations at these devices, then repeat your capture to confirm the problem is resolved.
Tip: failing hardware and poor connections can also send out garbled data, so you may also see “Error Messages of Unknown Type” alerts as an indication of a physical issue.
4. Excessive number of devices on an MS/TP network errors
The official protocol may list the capacity of an MS/TP subnet at 127 devices, but that’s a far cry from what an on-the-ground pro would recommend. This alert is tuned to help you adjust the number of devices on your MS/TP network to a number it can realistically handle before data processing and capacity start to be a drag on throughput. For us, that’s 35. If you receive this message, it’s time to think about splitting your daisy chain into two or more subnets.
5. Excessive Token Hold Time errors
A device that needs to hold on to the token for longer than 500ms is likely overloaded. It’s likely trying to keep up with too many messages and running out of time before its hold on the token expires. OptigoVN will tell you which devices are having trouble “moving on”, allowing you to increase the max_info_frame device attribute (default is 1) to flush out the buffer and/or reduce the amount of messages being sent.
6. Gap in MS/TP Device Addressing
It’s a common mistake: removing an MS/TP device from the network and leaving a space in the address list you forget to update. While not an error itself, gaps can lead to communication delays as devices take time to repeatedly look for (poll-for-master) those missing MAC addresses every 50 token cycles. Lots of gaps, lots of wasted time. Site Scope+ will identify exactly which addresses are missing in the sequence, so you can quickly re-configure device addresses to fill any gaps.
7. Lost Tokens
This is exactly what it sounds like: the token used to designate the device communicating never appeared at the next device in line. While this can sometimes result from adding a new device while capturing packet data, it’s most likely a physical wiring issue somewhere on the network. Site Scope+ will quickly identify the source and destination devices with lost tokens. You’ll have to physically inspect and fix any faulty wiring issues, then repeat your capture to confirm the problem is resolved.
External Issues
Just because a problem shows up on MS/TP doesn’t mean that’s where it started. In many cases, issues on the BACnet/IP side — especially around routers, traffic floods, or poor segmentation — can spill over into the MS/TP subnet. These problems tend to create bottlenecks at the MS/TP router, leading to familiar symptoms like lost tokens, excessive token hold times, or devices dropping off the network.
If your MS/TP diagnostics don’t point to an obvious issue on the subnet itself, it’s time to consider what’s happening upstream. Here are common external causes worth investigating:
1. Router bottlenecks
A BACnet router is the only path in or out of a MS/TP subnet. If the IP network is sending too much traffic (broadcasts, large COVs, etc.), the router can’t keep up — and MS/TP devices start missing messages. Use Site Scope+ to see which IP devices are hammering the router.
2. Traffic floods from BACnet/IP devices
Poorly tuned IP controllers can easily fire off Who-Is, Read-Property, or COV messages faster than an MS/TP router can process. This often causes Excessive Token Hold Time or Lost Tokens as the router queues or drops traffic. Identify the sources in OptigoVN and rate-limit or tune your IP devices accordingly.
3. Upstream blockages
Discovery or polling attempts that never reach the MS/TP router won’t make it to the MS/TP subnet. This shows up as timeouts or “offline” MS/TP devices, even though the subnet is healthy. Check for VLAN issues, firewall blocks, or routing delays between your head-end and the router. Retest with OptigoVN to confirm if the traffic made it to the router or not.
For Everything Else, There’s OptigoVN
The examples we’ve covered here — whether they’re wiring issues, misconfigurations, or IP traffic bottlenecks — are just a sample of the many ways problems can show up on your MS/TP network. In reality, BACnet issues are rarely isolated. A slowdown on IP can choke MS/TP. A single device can bring an entire subnet to a crawl.
That’s why OptigoVN’s full suite of diagnostics is designed to help you spot all the hidden troublemakers — not just the obvious ones. From high-level traffic analysis to deep-dive protocol diagnostics, OptigoVN gives you a complete picture of your BACnet network health.
Whether it’s device-level tuning, physical troubleshooting, or finding the root cause of a nagging latency issue, OptigoVN helps you fix it faster — and with confidence. request a demo to see how OptigoVN helps teams keep MS/TP networks running, or create your own free account and start exploring today.
