If you work with building automation systems long enough, you will eventually face this question on a real project: should the devices communicate over BACnet/IP or BACnet MS/TP?
Both are official data link options defined in the BACnet standard (ASHRAE 135), and both carry the same BACnet objects, properties, and services.
The difference is not what they say; it’s how the message travels. One rides on Ethernet and standard IP networking. The other runs on twisted-pair RS-485 cable daisy-chained from device to device.
BACnet/IP vs. BACnet MS/TP: Which Should You Use?
Use BACnet/IP for your network backbone, supervisory controllers, and anywhere Ethernet already exists; use BACnet MS/TP for field-level devices like VAV controllers, thermostats, and unitary equipment where low cost per point matters more than speed. Most real buildings use both, connected through BACnet routers.
The longer answer, including speed, cost, wiring, troubleshooting, and the failure modes of each, is what this guide covers.
Where IP and MS/TP Fit in BACnet
BACnet is a layered protocol. The application layer (the objects and services: Analog Inputs, Binary Outputs, ReadProperty, and COV subscriptions) is identical no matter which network you use.
Underneath that, the standard defines several data link/physical layer options, and these two dominate the market.
BACnet/IP
BACnet/IP encapsulates BACnet messages inside UDP/IP packets (default port 47808, or 0xBAC0 in hex) and sends them over standard Ethernet infrastructure, the same switches, routers, and cabling as your IT network.
BACnet MS/TP
BACnet MS/TP (Master-Slave/Token-Passing) runs over an RS-485 serial network: a shielded twisted-pair cable daisy-chained through up to 32 devices per segment (more with repeaters), passing a software token that controls which device may transmit.
Because the application layer is shared, a chiller on BACnet/IP and a VAV box on MS/TP can read and write each other’s points seamlessly as long as a BACnet router connects the two networks.
BACnet/IP Explained
BACnet/IP treats the building automation system as just another application on the IP network.
Each controller gets an IP address, plugs into an Ethernet switch, and communicates at 100 Mbps or 1 Gbps.
Key characteristics
Speed
Ethernet is thousands of times faster than any MS/TP segment. Trend data uploads, graphics refreshes, schedule downloads, and firmware updates that crawl on serial networks happen almost instantly over IP.
Topology freedom
In a star topology with switches, a damaged cable takes down one device, not the whole chain. You can also use fiber for long runs and VLANs to segment traffic.
Broadcast management
BACnet relies on broadcasts for device discovery (Who-Is/I-Am). IP routers block broadcasts between subnets, so BACnet/IP uses BBMDs (BACnet Broadcast Management Devices) to forward broadcasts across subnets, plus Foreign Device Registration for devices like remote workstations.
Getting BBMD configuration right exactly one per subnet, is the single most common stumbling block on multi-subnet BACnet/IP jobs.
IT involvement
IP addressing, switch ports, VLANs, and firewall rules usually mean coordinating with the IT department. On some projects that’s a five-minute conversation; on others it’s the longest lead-time item on the schedule.
BACnet MS/TP Explained
MS/TP was designed for exactly one purpose: cheap, reliable communication for the hundreds of small controllers scattered through a building.
Key characteristics:
Token passing
Devices on the segment take turns transmitting by passing a token in address order. It’s deterministic and collision-free, but every device added to the trunk slows the token rotation, and one misconfigured device can disrupt the entire segment.
Baud rates
Standard speeds are 9,600, 19,200, 38,400, and 76,800 bps. Even at the top rate of 76.8 kbps, an MS/TP trunk is more than a thousand times slower than 100 Mbps Ethernet. Every device on a segment must run the same baud rate.
Wiring rules
RS-485 demands discipline: a single daisy chain (no stars, no tees), shielded twisted-pair cable, end-of-line termination resistors at both physical ends of the trunk, proper biasing, and consistent polarity (+/−) at every device.
In my experience, the majority of “MS/TP communication problems” traced back in the field are physical-layer issues: a swapped pair, a missing terminator, or a star splice hidden above a ceiling tile.
Addressing
Each master device needs a unique MAC address from 0 to 127 on its segment, and the segment’s Max Master setting should match the highest address in use; leaving it at the default 127 when your highest device is 24 wastes token time polling addresses that don’t exist.
Cost
This is MS/TP’s entire reason to exist. A twisted-pair trunk looping through forty VAV boxes costs a fraction of pulling Ethernet home runs and providing a switch port for every controller. The controllers themselves are also cheaper without an Ethernet interface.
BACnet/IP vs. BACnet MS/TP: Side-by-Side Comparison
| Factor | BACnet/IP | BACnet MS/TP |
|---|---|---|
| Physical medium | Ethernet (Cat5e/6, fiber) | RS-485 shielded twisted pair |
| Typical speed | 100 Mbps – 1 Gbps | 9.6 – 76.8 kbps |
| Topology | Star (via switches) | Daisy chain only |
| Devices per segment | Limited mainly by subnet design | 32 per segment (more with repeaters), max address 127 |
| Cost per device | Higher (switch port, Ethernet interface) | Lower (shared trunk cable) |
| Failure behavior | One cable fault equals one device down | One trunk fault can take down the whole segment |
| Common failure causes | BBMD misconfiguration, duplicate IPs, IT firewall rules | Termination, polarity, baud mismatch, duplicate MACs |
| Best suited for | Backbones, supervisory controllers, plant equipment, large AHUs | VAV boxes, fan coils, thermostats, unitary field devices |
| IT coordination | Usually required | Rarely required |
Performance: How Big Is the Speed Gap Really?
On paper, 100 Mbps versus 76.8 kbps looks absurd, and for bulk operations, it is. But for steady-state control traffic, the honest engineering answer is that a well-designed MS/TP trunk performs perfectly well.
A VAV controller reporting zone temperature and receiving a setpoint doesn’t need megabits.
The gap becomes painful in three situations. First, long trunks with many devices: token rotation time grows with device count, so a 60-device trunk at 38.4 kbps can see multi-second update latency.
Second, trend log uploads and graphics-heavy front ends that hammer the trunk with read requests.
Third, firmware updates and database downloads, which can take the better part of an hour per device over serial versus seconds over IP.
Practical rule
Keep MS/TP segments to a sensible device count (many integrators target 20–30 devices per trunk rather than the theoretical limits), run 76.8 kbps if all devices support it, and put anything data-hungry on IP.
Cost and Installation: Where MS/TP Still Wins
Consider a floor with 30 VAV controllers. On MS/TP, you pull one twisted-pair trunk that daisy-chains through all thirty.
On BACnet/IP, you either pull 30 Ethernet home runs to an IDF closet and provide 30 switch ports, or you distribute small switches through the ceiling, adding cost, power requirements, and points of failure either way.
Multiply that across a mid-size commercial building, and the cabling and hardware difference is substantial.
This is why, even as IP-based controllers get cheaper, MS/TP still dominates the terminal-unit layer of most new construction.
That said, the gap is narrowing. If a building is already getting dense Ethernet for other systems (lighting, cameras, access control), the marginal cost of adding BAS controllers to that infrastructure drops, and some owners now mandate IP-to-the-edge for exactly that reason.
Troubleshooting and Reliability
This comparison matters more over the 15–20 year life of the system than on installation day.
MS/TP failure modes are physical and shared
Because every device sits on the same electrical trunk, a single failed transceiver, water-damaged splice, or contractor who tees into the cable can degrade or kill communication for the entire segment.
Diagnosing it often means splitting the trunk in half repeatedly until the fault is isolated tedious work above ceiling tiles. Tools: a multimeter, a protocol analyzer with an RS-485 tap, and patience.
BACnet/IP failure modes are logical and isolated
A bad cable takes out one device. The harder problems are configuration-level: duplicate IP addresses, duplicate device instance numbers, more than one BBMD on a subnet, or an IT firewall silently dropping UDP 47808.
These are invisible to a multimeter but quick to find with Wireshark and a BACnet discovery tool.
Neither is inherently more reliable, but IP faults tend to be isolated and diagnosable from a desk, while MS/TP faults tend to be shared and diagnosable from a ladder.
When to Use BACnet MS/TP
Choose MS/TP when the devices are numerous, small, and geographically clustered and when per-point cost drives the budget.
Typical cases: VAV and fan coil controllers on a floor, zone thermostats and sensors with native MS/TP, small packaged rooftop units, retrofit projects where pulling new Ethernet is impractical, and any job where the IT department cannot or will not support BAS devices on the network.
When to Use BACnet/IP
Choose BACnet/IP for the building backbone connecting supervisory controllers and the operator workstation.
For large equipment with many points (chillers, boilers, large AHUs, and plant controllers); for anything that generates heavy trend or graphics traffic.
For campus-wide integration across multiple buildings and for any device you’ll want to update or commission remotely.
If structured Ethernet cabling already reaches the device location, IP is usually the better long-term choice even at slightly higher first cost.
The Real-World Answer: Use Both, Connected by Routers
Walk into almost any commercial building automation system installed in the last decade, and you’ll find the same architecture: a BACnet/IP backbone at the top, with supervisory or plant controllers acting as BACnet routers down to multiple MS/TP trunks serving the field devices.
This hybrid design gives you Ethernet speed where traffic is heavy and RS-485 economy where device counts are high.
The BACnet standard was explicitly built for these network numbers, and routing is native to the protocol, so a workstation on IP reads a point on trunk 3 as transparently as a point on its own subnet.
Design tips for hybrid networks: assign unique BACnet network numbers to every MS/TP trunk and to the IP network itself; keep Device Instance numbers globally unique across the entire internetwork, not just per trunk; document baud rate, Max Master, and MAC addressing per trunk; and resist the temptation to overload any single router with too many segments.
FAQ: BACnet/IP vs. BACnet MS/TP
Is BACnet/IP faster than MS/TP?
Yes, dramatically, 100 Mbps or more versus a maximum of 76.8 kbps. For routine control traffic, the difference is often unnoticeable, but for trend uploads, graphics, and firmware updates, IP is far faster.
Can BACnet/IP and MS/TP devices talk to each other?
Yes. A BACnet router (often built into a supervisory controller) joins the two networks, and devices on each side exchange data transparently because the application layer is identical.
How many devices can be on one MS/TP trunk?
Up to 32 devices per electrical segment without repeaters, with master addresses ranging from 0 to 127. In practice, many integrators limit trunks to 20–30 devices to keep token rotation fast.
Does MS/TP need termination resistors?
Yes, one at each physical end of the trunk, and only at the ends. Missing or extra terminators are among the most common causes of intermittent MS/TP communication failures.
Is MS/TP obsolete?
No. Despite the growth of IP-to-the-edge controllers and newer options like BACnet/SC, MS/TP remains the most cost-effective choice for dense terminal-unit networks and continues to be installed in new buildings today.
What port does BACnet/IP use?
UDP port 47808 (0xBAC0 in hexadecimal) by default. Firewalls between BACnet subnets must allow this traffic.
Final Verdict
Don’t frame it as either/or. BACnet/IP is the right backbone; BACnet MS/TP is the right last hundred feet.
Put your supervisory controllers, plant equipment, and anything bandwidth-hungry on IP. Put your dense field-level devices on well-designed MS/TP trunks that are short, properly terminated, correctly addressed, and at the highest common baud rate.
Connect them with BACnet routers, keep your network numbers and device instances clean, and both data links will serve the building reliably for decades.
