BACnet comes in multiple "data link layers" — the underlying network technologies that carry BACnet messages. The two most common are BACnet/IP (which runs on standard Ethernet networks) and BACnet MS/TP (which runs on simple twisted-pair wiring).
Understanding when to use each helps you design better building automation systems — and avoid costly mistakes.
- ✓ 100 Mbps – 1 Gbps speed
- ✓ Standard Ethernet cabling
- ✓ Unlimited network size
- ✓ Remote access capable
- ⚠ Higher device cost
- ⚠ Requires IT infrastructure
- ✓ Lower per-device cost
- ✓ Simple twisted-pair wiring
- ✓ No switches needed
- ✓ Easy daisy-chain topology
- ⚠ 9.6–76.8 kbps speed
- ⚠ 128 devices, 4000 ft max
BACnet/IP: The Basics
BACnet/IP transmits BACnet messages over standard UDP/IP networks — the same infrastructure you use for computers, phones, and other IT equipment.
Best Applications for BACnet/IP
Backbone Connections
Connect major building systems — supervisory controllers, servers, and building-level controllers.
Large Equipment
Chillers, AHUs, and other equipment with significant data benefit from IP's higher bandwidth.
Remote Access
Systems requiring remote monitoring, diagnostics, or control from off-site locations.
BACnet MS/TP: The Basics
BACnet MS/TP (Master-Slave/Token-Passing) uses EIA-485 (RS-485) serial communication over simple twisted-pair wiring. It's the workhorse protocol for field-level devices.
- Cost: A VAV controller on MS/TP costs significantly less than one with an Ethernet port
- Simplicity: No switches, no IP addresses, no IT coordination required
- Scale: When you have 200 VAV boxes, those cost savings add up fast
Best Applications for MS/TP
- VAV box controllers — The classic MS/TP use case
- Fan coil controllers — Simple devices with few points
- Zone temperature controllers — Low data volume, many devices
- Smart sensors and actuators — BACnet-native field devices
The Typical Hybrid Architecture
Most real-world BACnet systems use both protocols in a hierarchical design. Here's what that looks like:
BACnet/IP Backbone
The main building automation network connects:
- Supervisory controllers and servers
- Building-level controllers (NAEs, NCEs, JACEs, etc.)
- Major equipment with native BACnet/IP
- Integration gateways
BACnet MS/TP Field Networks
Building controllers host MS/TP trunks connecting to field devices. A single controller might have 2-4 MS/TP ports, each supporting its own trunk of devices.
"A VAV box on MS/TP can communicate with a chiller on BACnet/IP through the router function — the protocols are different, but the BACnet data model is the same."
Common Mistakes to Avoid
All-IP systems: Putting every device on BACnet/IP increases costs unnecessarily. A VAV box doesn't need gigabit Ethernet.
Overloaded MS/TP trunks: MS/TP is slow. Keep trunk sizes to 30-60 devices maximum for responsive operation.
Poor bus design: MS/TP requires proper daisy-chain topology, termination, and cable quality. Poor wiring causes intermittent problems.
Decision Framework
When deciding between IP and MS/TP for a specific application, consider:
- Device has 50+ data points
- Remote access is required
- Ethernet already exists at location
- Integration with IT systems needed
- High-speed response is critical
- Connecting many similar devices
- Device has few data points
- Cost per device matters
- Only control wiring exists
- Simple installation preferred
BACnet/IP Network Considerations
IP Addressing
BACnet/IP devices need IP addresses. Plan your addressing scheme:
- Static IPs are typical for BACnet devices (DHCP adds complexity)
- Separate subnet for BACnet traffic
- Logical organization by floor, system type, etc.
- Room for growth in your address ranges
Broadcast Management
BACnet uses broadcasts for device discovery. On large networks, BACnet Broadcast Management Devices (BBMDs) control broadcast traffic. Understanding BBMD configuration is essential for multi-segment BACnet/IP networks.
MS/TP Network Considerations
Bus topology: Daisy-chain between devices (no stars or T-taps)
Termination: Resistors at both ends of the trunk
Grounding: Shield grounded at one point only
Baud rate: All devices must match (typically 38.4 or 76.8 kbps)
Specifying for New Projects
When specifying BACnet systems, document these decisions:
- Network architecture — What's on IP vs. MS/TP?
- Trunk sizing — How many devices per MS/TP trunk?
- Infrastructure — Switch specs, VLAN plan, cable requirements
- Documentation requirements — Network diagrams, address schedules
Need Help with BACnet Design?
At Controls NYC, we design BACnet systems that balance performance, cost, and maintainability. Whether you're planning new construction, upgrading an existing system, or troubleshooting network problems, we can help.
Contact us to discuss your BACnet network design.
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