RS-485 bus grounding and termination seems like a magical rite at times. Some people get lazy with the details of termination because buses often work even when done incorrectly. I had some untwisted, shielded AWG22 lying around, so I used it to show a quick comparison between bus wiring in a controlled environment. I used a single MS/TP BACnet controller and 120 Ω termination resistors. The cable is of inconsequential length and I don’t have any weird equipment near my desk, so the electromagnetic noise should be minimal.
Shielded, Grounded Reference, No End-of-line Termination
Shielded, No Reference, Terminated Shielded
Grounded Reference, Terminated
Note that the waveform is inverted – that’s just because I had the probes the other way. Sorry about that!
Please note that this is a simple look on a desk in a residential area. All of the important things to do with RS-485 buses really show up in larger systems with real electromagnetic noise. The lack of noise in these captures is not similar to real-world applications. I wanted to have this as a baseline for reference. If you want to see what happens when you don’t ground your bus when you need to, check out this post!