ASHRAE 135 (BACnet) — Clause 9, MS/TP MAC addressing
What it means on the job
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.
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- whats-wrongUnion Hall Training: BACnet Network Troubleshooting
Related ASHRAE sections
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) — MS/TP Physical Layer
BACnet MS/TP requires a daisy-chain (bus) topology, not star topology. Star wiring creates signal reflections and communication failures. Each device must connect in series on the bus.
ASHRAE 135 (BACnet) — Object Types
Connecting a 0-10VDC analog output signal to a binary (digital) input terminal will not control the device properly. Binary inputs only read on/off states. Analog signals must connect to analog input terminals for proportional control.
ASHRAE 135-2016
A BACnet gateway does not satisfy a native BACnet requirement. The specification explicitly requires BACnet per ASHRAE 135-2016 and prohibits proprietary protocols. A gateway creates a single point of failure, adds latency, limits functionality (not all BACnet services translate through gateways), and locks the owner into one vendor for the field-level devices. Native BACnet means every controller communicates BACnet natively — either BACnet/IP or BACnet MS/TP — without translation. This is a critical specification requirement designed to protect the building owner from vendor lock-in.
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.