ASHRAE 135 (BACnet) — MS/TP Physical Layer

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.

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.

ASHRAE 135-2016 / Crestline BAS Specification

Crestline specifications require all building automation systems to comply with ASHRAE Standard 135-2016 (BACnet). Proprietary, closed-protocol systems lock the owner into a single vendor and prevent competitive service and integration. Open-protocol BACnet is mandatory for all new installations.