BACnet can't discover a device — MS/TP & BACnet/IP

BACnet is the building & HVAC communications standard (chiller, AHU, VAV, BMS). When the BMS or a tool can't discover a device, or a controller “disappears”, the cause is usually a duplicate Device Instance, MAC/baud (MS/TP), or BACnet/IP (BBMD) config — not a faulty controller.

Common causes

1. Duplicate Device Instance — two devices on the same number → BMS sees them flicker / drop.
2. MS/TP: duplicate MAC / baud mismatch — wrong DIP, different baud on the same RS-485 segment.
3. RS-485 terminator — missing/wrong on the MS/TP segment.
4. BACnet/IP: BBMD / subnet — across routers without BBMD, the broadcast (Who-Is) doesn't arrive.
5. Port/UDP 47808 — firewall blocks it, or wrong network number.

Step-by-step diagnosis

1. Check Device Instance Ensure each device has a unique number system-wide; fix duplicates first.
2. MS/TP: MAC + baud Verify unique MAC, same baud across the segment; check RS-485 terminators at both ends.
3. Run a Who-Is scan Use a tool (e.g. YABE) to send Who-Is; see if an I-Am returns to know the device is “alive”.
4. BACnet/IP: subnet & port Check IP/subnet, UDP 47808, network number; open the firewall if blocked.
5. BBMD across routers Configure BBMD (or Foreign Device) so broadcasts cross subnets.
6. Isolate Connect the tool directly to the device to rule out the upstream network.

When to call an expert

Mixing BACnet MS/TP and IP, BBMD across many subnets gets confusing fast. DeepDebug localizes quickly (instance/MAC/network) and supports remote configuration, including multi-vendor BMS integration.