ASHRAE 135 (BACnet) — Annex J
What it means on the job
The shield on a BACnet MS/TP trunk must be grounded at one end only to prevent ground loops. Grounding at both ends creates circulating currents that introduce noise and cause intermittent communication failures.
Tested in 1 CraftIQ puzzle
- whats-wrongBACnet Network Wiring
Related ASHRAE sections
ASHRAE 135 (BACnet) — Annex J, MS/TP Physical Layer
Every BACnet MS/TP trunk requires a 120-ohm termination resistor at both physical ends of the bus. Missing the end-of-line resistor causes signal reflections that corrupt data frames, leading to intermittent communication failures and token-passing errors across the entire trunk.
ASHRAE 135 (BACnet) — Annex J, Wiring Topology
BACnet MS/TP requires a strict daisy-chain (bus) topology with no stubs or tees. A stub creates an impedance mismatch that causes signal reflections, corrupting communications for all devices on the trunk. Branch devices must be wired in-line on the bus, not teed off.
ASHRAE 135 (BACnet) — Clause 12
Simply using the BACnet physical layer (MS/TP) does not make a device BACnet compliant. True BACnet compliance requires supporting standard BACnet object types (Analog Input, Analog Output, Binary Input, Binary Output, etc.) and standard services. A device using only proprietary objects cannot interoperate with other BACnet devices. Look for BTL (BACnet Testing Lab) certification.
ASHRAE 135 (BACnet) — Clause 12.2
BACnet device instance numbers must be globally unique across the entire BACnet internetwork, not just per trunk. Duplicate device instances cause routing conflicts and make one or both devices unreachable from the operator workstation.
ASHRAE 135 (BACnet) — Clause 6.1
Network number 0 is reserved and means 'local network only' in BACnet. A routed segment must have a unique non-zero network number (1-65534) assigned. Using 0 prevents the router from forwarding messages to and from this segment.
ASHRAE 135 (BACnet) — Clause 9, MS/TP MAC addressing
MS/TP reserves MAC 255 for broadcast and 128-254 for slave devices; 127 is the very top of the master range. Parking a field controller at 127 forces max_master to its ceiling, so every token rotation polls the entire 0-127 address space looking for masters that aren't there — network performance craters. Keep field controllers in a contiguous low range and set max_master just above the highest used address.
CraftIQ explanations are original educational commentary written in trade language and verified against the cited standards. Always confirm requirements against the code edition adopted by your jurisdiction — this is training material, not a substitute for the code book or your AHJ.