BACnet master slave token passing, or MS/TP, is a protocol frequently used in building automation systems (BAS). Developed by the HVAC industry association, ASHRAE, the BACnet MS/TP communications bus allows equipment from multiple vendors to communicate with a BAS.
An open communication protocol like BACnet MS/TP is a fantastic advancement for building owners and the building automation industry. Of course, no technology is perfect, and that includes BACnet MS/TP. Here are a few common issues we at Conexus have seen.
Problems with BACnet MS/TP
1. Wiring errors or problems
One troublesome aspect of BACnet MS/TP communication is its daisy chain configuration—if communication is shorted, all controllers past that point will go offline. Using star wiring or T-Taps would eliminate this problem; however, BACnet MS/TP wiring is unable to accommodate these configurations.
Not meeting the exacting wire specifications for BACnet MS/TP (such as length, conductor size, impedance, capacitance, or shielding requirements), will also cause issues.
2. Improper grounding
Manufacturer-specific grounding requirements must be followed to avoid problems with BACnet MS/TP.
Some building control manufacturers require a two-conductor bus and may utilize ground as a reference; therefore, proper controller grounding is critical. On a system using BACnet, the cable shield is often only grounded at one end of the daisy chain, failure to follow this leads to ground loops.
3. Incorrect termination
Proper termination prevents reflections and noise on the communications bus. Problems arise if a BACnet MS/TP network is not terminated according to the control manufacturer’s documentation.
4. Failed controller
A bad controller can cause a short on either the communication bus or from the communication bus to ground. A failed controller could even cause the entire bus to go offline.
5. Improper software configuration
Each device communicating over BACnet MS/TP has a unique ID number, which means that an improperly configured system with duplicate IDs will cause issues.
When devices use different baud rate speeds, that is, different rates for signal change per second, communication is not possible. Devices on the same bus must use the same baud rate.
Like BACnet MS/TP, BACnet IP is an open, non-proprietary protocol. However, unlike MS/TP that uses a serial connection, BACnet IP uses a modern, Ethernet connection for communicating between devices.
BACnet IP is faster than MS/TP, it is more configurable than MS/TP, it allows for a greater number of connected devices than MS/TP, and it isn’t prone to wiring issues like the MS/TP. In short, choosing BACnet IP controllers for your BAS is hugely beneficial, allowing you to keep your existing equipment while increasing reliability and flexibility.