Engineering notes
RS-485 on a BMS: termination, node count, noise
Modbus RTU and BACnet MS/TP ride RS-485. Missing termination, too many nodes, or a shared power tray — the bus goes quiet.
The gateway is on the truck, the meters have a 485 port, the DDC has a driver — and slave 7 never answers. More common than it sounds. RS-485 likes long cable. It does not like “just add one more” and no resistors at the ends.
Terminate both ends at the maker’s value, often 120 Ω — not in the middle of the cabinet. Node count has a ceiling; past that you split the trunk or add a repeater. Twisted pair, screen, earth one end as the sheet says. Run it beside a drive output for tens of metres and you get CRC errors, not “bad software”.
We name the cable, the estimated length, and whether discrete terminators are needed. Do not let the site use untwisted bell wire. BACnet and Modbus · Signal cable