BACnet/IP vs MS/TP: Wiring and Network Practice
Two physical layers, one protocol. Here's how to decide between Ethernet-based BACnet/IP and RS-485 MS/TP on your next job — and how to wire each one correctly.
How we plan the physical layer
Map devices per floor
Count controllers and field devices per floor or wing, and note the distance to the nearest IT closet — these two numbers drive most of the IP vs MS/TP decision.
Clear it with IT first
BACnet/IP rides on UDP port 47808 across the existing network. Confirm early whether the client's IT team will allow new broadcast traffic and where VLAN segmentation needs to sit.
Terminate the MS/TP trunk properly
Run the RS-485 twisted pair as a daisy-chain, fit 120Ω termination at both physical ends of the bus, and set one consistent baud rate (9600-76800 bit/s) for every device on the segment.
Bridge subnets with a BBMD
IP routers don't forward broadcasts, so Who-Is/I-Am discovery breaks across subnets unless you register a BBMD or set up Foreign Device registration on each segment.
What actually differs between IP and MS/TP
One port, two very different buses
BACnet/IP uses UDP port 47808 (0xBAC0) over your existing Ethernet. MS/TP uses RS-485 twisted pair and a token-passing bus that has nothing to do with your IT network at all.
BACnet/IP: fast to add, IT-dependent
New controllers join over existing Ethernet cabling — no separate bus to pull. The tradeoff is that broadcasts don't cross IP subnets without a BBMD or Foreign Device registration.
MS/TP: independent, but a physical bus to design
RS-485 daisy-chain with token-passing access keeps field devices off the IT network entirely. It's cheaper per device but needs correct topology and termination to work reliably.
Termination is not optional
Every MS/TP segment needs 120Ω resistors at both physical ends of the chain — the same rule as any RS-485 bus. Skip it and you get intermittent token errors that are painful to trace.
Baud rate is a segment-wide decision
MS/TP runs at 9600, 19200, 38400 or 76800 bit/s. Every device on the same trunk has to agree, so it's set once for the segment, not per controller.
Most real buildings run both
An IP backbone connects BACnet/IP routers between risers and plant rooms, with MS/TP field trunks hanging off local controllers for zone-level devices. It's the standard commercial pattern, not a compromise.
What's in a wiring & network review
Before we touch a controller, we work out which physical layer belongs where.
- Device count and distance mapping per floor/wing
- BACnet/IP network plan: VLAN segmentation, BBMD/Foreign Device placement, firewall rules for UDP 47808
- MS/TP trunk design: daisy-chain routing, 120Ω termination points, baud rate selection
- Device Instance and Object_Name naming convention, so the BMS graphic doesn't fill up with AI-1/BO-14
- PICS review against your BMS front end before integration starts, not after
Frequently asked questions
Can BACnet/IP and MS/TP run in the same building?
Yes — it's the normal setup. An IP backbone links BACnet/IP routers between plant rooms and risers, while MS/TP trunks handle field-level devices off local controllers.
Why doesn't Who-Is/I-Am discovery work across subnets?
IP routers don't forward broadcast traffic. To let discovery cross a subnet boundary you need a BBMD on each subnet, or a Foreign Device registration back to one.
What happens if I skip termination on an MS/TP trunk?
You get reflections on the RS-485 line and intermittent token-passing errors — the classic symptom is devices dropping off randomly. Fit 120Ω resistors at both physical ends, not just one.
Will our IT department need to get involved?
For BACnet/IP, yes — they need to allow UDP 47808 traffic and ideally give you a segmented VLAN. That's usually the deciding factor between choosing IP or MS/TP for a given area.
Which baud rate should we use on MS/TP?
Anything from 9600 to 76800 bit/s is standard; the whole segment has to run at the same rate. Match it to what your devices support and confirm before wiring, not after commissioning.
Planning a BACnet install?
Send us the floor plan and device count and we'll work out the IP/MS/TP split, BBMD placement and trunk design before you order cable.