BACnet MS/TP over RS-485 is one of the most common and cost-effective solutions for a network of Building Management System (BMS) controllers. I know that the current trend is for BACnet over IP based networks, but the tried and true BACnet over RS-485 networks have their merits. There are tremendous potential savings in reusing an existing functioning BACnet over the RS-485 network. A BACnet over the RS-485 network adds a layer of separation from the IP network and reduces IP traffic. This communication reduction being true, especially if one is judicious with the points that are passed up to the front end graphics, alarm database, and history database on the BMS server and the BMS workstations.
One of the complaints of BACnet over RS-485 is network stability. I have been dealing with this issue for years, and in most cases, the most common causes have been as outlined in this article. Here are some of what I have discovered over the years working with great talented teams in implementing and troubleshooting BACnet over RS-485 networks.
Some common causes of BACnet communications issues are listed below
- A short or open of the wiring can affect communications. An intermittent short is incredibly difficult to pinpoint.
- A faulty transceiver on one of the controllers can affect communications. I determine this by removing the controllers one at a time from the network to see if the problem goes away.
- Some brands just do not play well with others. This cause of unreliable communications can be confirmed by disconnecting the suspected culprit brand and seeing if reliable communications occur when the suspect controllers are off the network. The solution when this happens is to put the brand that does not play well with others on a separate segment of the bus, or one can optically isolate them.
- Termination resistors or a termination resistor in a location other than each end of a proper daisy chain can affect communications.
Tee tapped segments or other non-daisy chained topology of the RS-485 bus. A tee tap is a segment of wiring that is a single pair to a single controller extended out of the daisy chain topology. See the following image
Duplicate MAC address or device IDs or incorrect baud rates are also common issues that will affect a network adversely.
An Oscilloscope is a powerful tool when troubleshooting an unstable network. A good BACnet signal looks like the image below:
One of the most common problems I have noticed is that there are conflicting opinions and documentation. Below is the BACnet wiring standard that a team of experienced engineers and I came up with these details. See the following RS-485 BACnet Wiring Detail image.
The critical items for stable BACnet over RS-485 MSTP communications are the use of a quality low capacitance wire, true daisy-chain without any tee taps, optically isolating troublesome devices, and proper termination at each end without termination anywhere else. An item that I have found helpful when a network is communicating with borderline stability is to use the Loytec model LT-B4 (or equal) Network terminator, 1 x RS‑485 (bus topology, ANSI TIA/EIA-485) with biasing circuit (failsafe biasing). The optical isolator that I like to use is an Advantech model BB-485OP (or equal) optically isolated RS-422/485 signal repeater. The repeater is required when you have a full-wave rectified power supply on one controller and a half-wave rectified power supply on another controller. I have seen this situation create ground loop shorts, usually destroying a controller in the process as the magic smoke leaves the controller. It always seems to blow up the $10,000.00 controller and not the $300.00 one!
Equally important is to document the BACnet RS-485 network with clear and accurate preliminary, construction, and as-built documents. See the following image for an example of what I mean.
If one does not document the BACnet over the RS-485 network, it makes installing and servicing the network much more difficult. I have noticed that the as built drawings by many companies are more like wannabe built drawings and do not reflect reality. This attention to detail is essential and often overlooked. A manager must allow for a proper amount of time that is required for a project to update the drawings and documentation at the end of the job. Even after following all the steps outlined previously in this article, sometimes the only thing that would get two different branded controllers to communicate on the same RS-485 segment was to isolate the two brands with an optical isolator or put them on their own segment. This issue has happened with certain older Carel and Liebert based legacy controllers, but these two brands are not the only ones by any means that did not play well with others on the same segment. The default baud rate and settings should be checked against the network settings.
If one follows the recommendations outlined in this article, they should have reliable BACnet over RS-485 communications. I have been called in as an expert to diagnose unreliably communicating BACnet over RS-485 networks many times over the years. Most times, experienced engineers and technicians had already spent quite a few hours attempting to pinpoint the problem without being able to conclude what the issue was that was affecting communications. I had spent many hours as part of a talented team zeroing in on the source of the unreliable communications issue, with the intermittent problems being the most difficult to diagnose. I would estimate that over 98% of the time, the culprit was one of the items outlined in this article. I hope that you found this article useful if I have overlooked anything, please feel free to contact me with your suggestions or subjects that you would like discussed by my team or me in the future. I am available for consultation at https://hvacbydavid.com/
David Smith (2020)