Building a large BACnet MS/TP network across multiple floors and buildings. What's the best way to segment the network? Can you daisy chain 200 devices together?
Solution
MS/TP Segment Limits
Maximum devices per daisy chain: 64 (protocol limit)
Maximum cable run: 1200 meters (≈4000 feet) at 9600 baud
Practical limit: 32–48 devices per segment (performance degrades with too many)
Design Multi-Segment Networks
For 200+ devices, divide into multiple daisy chains (each ≤64 devices)
Use a BACnet/IP gateway to interconnect chains
Example: 50 devices on Floor 1 chain, 50 on Floor 2 chain, 50 on Floor 3 chain, all connected via IP gateway
Wiring Standards
Use 2-core shielded RS-485 cable (not generic twisted pair)
All cables in a segment must be same gauge (18 or 20 AWG, not mixed)
Recommended: 20 AWG for better signal integrity
Shield must be grounded at only one point (usually the gateway/master device)
Termination
Place 120Ω terminating resistor at both ends of each daisy chain
No terminator in the middle or on stubs
Only devices at the absolute ends of the chain terminate
Baud Rate Coordination
All devices on same segment must use same baud rate (9600, 19200, or 38400 bps)
Slower baud (9600) is more robust for long runs or noisy environments
Faster baud (38400) is better for high-frequency polling, but requires good wiring