BACnet/IP to MS/TP Router Configuration

Problem Description

BACnet/IP and BACnet MS/TP operate on fundamentally different physical and data link layers. BACnet/IP runs over Ethernet and UDP (port 47808), while MS/TP runs over RS-485 using a token-passing protocol. A supervisory controller or head-end workstation sitting on an IP network cannot directly communicate with field controllers on an MS/TP trunk—the frames are incompatible, the media are different, and there is no shared addressing scheme between the two layers.

This is the most common topology in commercial building automation: an IP-based supervisor (Tridium JACE, Johnson Controls NAE, Schneider Electric AS-P, or a BACnet-aware front-end) needs to read points, issue commands, and manage schedules on dozens of MS/TP field controllers—VAV boxes, AHU controllers, fan coils, and zone sensors. Without a router, those MS/TP devices are invisible to everything on the IP side.

The solution is a BACnet/IP to MS/TP router. This device sits at the boundary between the two networks, holds a valid address on each side, and performs frame-level translation per ASHRAE 135. From the BACnet internetwork's perspective, it is a standard BACnet router connecting two distinct network numbers. The most widely deployed standalone device for this role is the Contemporary Controls BASRT-B (BASrouter), though many supervisory controllers include built-in routing capability.

Step-by-Step Configuration

Step 1: Plan Your Network Numbers

Every BACnet network segment—whether IP or MS/TP—must have a unique network number within the BACnet internetwork. Network numbers range from 1 to 65534. The router connects two segments, so it needs two distinct numbers: one for the BACnet/IP side and one for the MS/TP side.

Establish a numbering scheme before you configure anything. A common convention is to assign IP networks in a low range and MS/TP trunks in a higher range:

# Example network number plan
# ──────────────────────────────────────────────
# Network 1      BACnet/IP backbone (all IP devices)
# Network 2001   MS/TP Trunk 1 - Building A, Floor 1
# Network 2002   MS/TP Trunk 2 - Building A, Floor 2
# Network 2003   MS/TP Trunk 3 - Building A, Floor 3
# Network 3001   MS/TP Trunk 1 - Building B, Floor 1
# ──────────────────────────────────────────────
# Rule: Every network number must be globally unique
#       across the entire BACnet internetwork.

Two network numbers must never be the same. If two MS/TP trunks share a network number, the BACnet routing tables become ambiguous and devices on one or both trunks will be unreachable. Similarly, the MS/TP network number must differ from the IP network number. This is the single most common configuration error with BACnet routers.

Step 2: Assign the Router's IP Address

The router's BACnet/IP port needs a static IP address on the same subnet as your BACnet/IP supervisor. Never use DHCP for a BACnet router—if the lease expires or the address changes, every MS/TP device behind it drops off the network until the supervisor rediscovers the new address.

For the BASRT-B, IP configuration is done through its built-in web interface or by temporarily connecting to the unit's default IP address:

# BASRT-B Default Network Settings (factory reset)
# ──────────────────────────────────────────────
# IP Address:       192.168.92.68
# Subnet Mask:      255.255.255.0
# Default Gateway:  192.168.92.1
# BACnet/IP UDP Port: 47808 (0xBAC0)
#
# To access the web interface:
#   1. Set your laptop to 192.168.92.x (e.g., 192.168.92.100)
#   2. Connect directly to the BASRT-B Ethernet port
#   3. Browse to http://192.168.92.68
#   4. No username/password required on factory-default units

Once connected to the web interface, navigate to the BACnet IP configuration page and set:

• IP Address — A static address on your BACnet/IP subnet (e.g., 10.1.10.50)
• Subnet Mask — Must match the subnet (e.g., 255.255.255.0)
• Default Gateway — Required only if the supervisor is on a different IP subnet and you are using BBMD. For same-subnet deployments, set it to the subnet's gateway address.
• UDP Port — Leave at 47808 unless your site uses a non-standard port (rare)
• BACnet/IP Network Number — The network number for the IP side (e.g., 1)

Step 3: Configure the MS/TP Port

The MS/TP side of the router requires four parameters: MAC address, baud rate, Max Master, and network number. All four must be correct for the trunk to function.

MAC Address

The router participates in the MS/TP token ring as a master device, so it needs a MAC address in the range 0–127 that is unique on the trunk. Assign the router a MAC address that does not conflict with any existing field controller. A common practice is to give the router MAC address 0 or 1, reserving addresses 2–127 for field controllers.

# BASRT-B MS/TP Configuration
# ──────────────────────────────────────────────
# MS/TP MAC Address:     0       (unique on this trunk)
# MS/TP Baud Rate:       38400   (must match all devices)
# MS/TP Max Master:      127     (adjust to highest MAC)
# MS/TP Network Number:  2001    (unique in BACnet internetwork)
# Max Info Frames:       1       (default; rarely changed)

Baud Rate

Every device on an MS/TP trunk must use the same baud rate. There is no auto-negotiation on RS-485. The BASRT-B supports 9600, 19200, 38400, 76800, and 115200 bps. If you are adding a router to an existing trunk, match the baud rate already in use. For new installations, 38400 bps is the most common choice and provides a good balance of speed and reliability over typical RS-485 cable runs (up to 4000 feet / 1200 meters at 38400).

If you set the router to 38400 and the field controllers are running 76800, the router will generate constant CRC errors on the trunk. From the supervisor's perspective, devices will appear to be offline. This symptom is identical to wiring faults and is easy to misdiagnose.

Max Master

Set Max Master to the highest MAC address on the trunk, or one value above it. The BASRT-B defaults to 127, which works but is inefficient—the router will poll every address from 0 to 127 during Poll For Master cycles, wasting time on addresses that have no device. If your highest MAC on the trunk is 24, set Max Master to 24 or 25.

MS/TP Network Number

Enter the unique network number you planned in Step 1 for this MS/TP trunk. On the BASRT-B, this is the MS/TP Network Number field on the MS/TP configuration page.

Step 4: Set the Router's Device Instance

The router itself is a BACnet device and must have a globally unique Device Instance number (0–4194302). This is separate from the MAC address and the network numbers. Pick a value that fits your site's device instance numbering scheme.

# Device instance numbering example
# ──────────────────────────────────────────────
# Supervisor (JACE):       100001
# BACnet Router (BASRT-B): 100050
# VAV Controller 1:        201001
# VAV Controller 2:        201002
# AHU Controller:          201010
# ──────────────────────────────────────────────
# Rule: Device Instance must be unique across the
#       entire BACnet internetwork (all networks).

Step 5: Wire the MS/TP Trunk

Connect the BASRT-B's RS-485 terminals to the MS/TP trunk. The BASRT-B has three screw terminals for MS/TP:

• + (A / Data+) — Connect to the + terminal on the daisy-chain
• − (B / Data−) — Connect to the − terminal on the daisy-chain
• SC (Shield/Common) — Signal reference ground; connect to the cable shield

MS/TP wiring must follow a strict daisy-chain topology. Do not use star or T-tap configurations—they cause signal reflections and unreliable communication. Install 120-ohm termination resistors at the two physical endpoints of the chain only. If the router is at one end of the trunk, enable its built-in termination (the BASRT-B has a termination jumper or DIP switch) or install an external 120-ohm resistor between + and −.

Step 6: Apply Settings and Reboot

On the BASRT-B, click Submit on each configuration page after making changes. The device requires a reboot to apply new IP or MS/TP settings. Use the Restart Device button in the web interface, or cycle power. The unit takes approximately 15–30 seconds to come back online.

Step 7: Verify Device Discovery

From your BACnet/IP supervisor or a BACnet exploration tool (YABE, CAS BACnet Explorer, or the supervisor's built-in discovery), send a global WHO-IS. You should see:

• The router itself responding with its Device Instance
• All MS/TP field controllers behind the router, with their Device Instances and the MS/TP network number in the source address

If the router appears but the MS/TP devices do not, the issue is on the MS/TP side—check baud rate, wiring, and MAC address conflicts. If the router itself does not appear, the IP configuration is wrong or there is a network connectivity issue between the supervisor and the router.

# Expected WHO-IS / I-AM results from supervisor
# ──────────────────────────────────────────────
# Device Instance  Network  MAC       Description
# 100050           1        10.1.10.50  BASRT-B Router
# 201001           2001     03          VAV-1 Controller
# 201002           2001     04          VAV-2 Controller
# 201010           2001     10          AHU-1 Controller
# ──────────────────────────────────────────────
# Note: MS/TP devices show their MS/TP MAC (1 byte)
#       IP devices show their IP address as MAC (6 bytes)

Configuration Snippets for Other Platforms

While the BASRT-B is the most common standalone router, many supervisory controllers have built-in BACnet/IP to MS/TP routing. The configuration principles are identical—unique network numbers, matching baud rate, unique MAC, correct Max Master.

Tridium Niagara JACE (Built-in Router)

# Niagara Workbench > Station > Config > Drivers > BacnetNetwork
#
# BACnet/IP Port:
#   Enabled:            true
#   IP Address:         (inherited from station NIC)
#   UDP Port:           47808
#   Network Number:     1
#   Register As Foreign Device: false (unless crossing subnets)
#
# MS/TP Port:
#   Enabled:            true
#   COM Port:           COM1 (or platform-specific serial port)
#   MAC Address:        0
#   Baud Rate:          38400
#   Max Master:         31
#   Network Number:     2001
#
# The JACE automatically routes between IP and MS/TP
# when both ports are enabled with different network numbers.

Schneider Electric AS-P (SpaceLogic / EcoStruxure)

# EcoStruxure Building Operation > System Tree > Controller > Communication
#
# BACnet/IP:
#   Network Number:     1
#   UDP Port:           47808
#
# MS/TP Port:
#   Network Number:     2001
#   MAC Address:        0
#   Baud Rate:          38400
#   Max Master:         127  (adjust to match highest trunk MAC)
#
# Note: The AS-P supports up to 2 MS/TP trunks.
#       Each trunk must have a unique network number.

Chipkin BACnet Router (Software-Based)

# Chipkin CAS BACnet MS/TP to IP Router configuration
# settings.ini
# ──────────────────────────────────────────────
[BACnetIP]
IPAddress=10.1.10.60
SubnetMask=255.255.255.0
Gateway=10.1.10.1
UDPPort=47808
NetworkNumber=1

[MSTP]
COMPort=COM3
BaudRate=38400
MACAddress=0
MaxMaster=31
NetworkNumber=2001

[Device]
DeviceInstance=100060
DeviceName=Chipkin-Router-01

Common Mistakes

• Duplicate network numbers. This is the most frequent cause of failed BACnet router deployments. If the IP network and the MS/TP trunk share the same network number, or if two routers assign the same number to different MS/TP trunks, the BACnet routing tables break. Devices become unreachable, WHO-IS responses are inconsistent, and the supervisor may show devices appearing and vanishing. Every network segment in the entire BACnet internetwork must have a unique number.
• Baud rate mismatch on the MS/TP trunk. The router's baud rate must exactly match every field controller on the trunk. There is no auto-negotiation on RS-485. A router running 38400 on a trunk of devices at 76800 generates constant CRC errors, and the supervisor sees no MS/TP devices at all. Verify the baud rate on every device, including any that were added by a different contractor.
• MAC address conflict between the router and a field controller. The router occupies a MAC address on the MS/TP trunk just like any other master device. If a field controller already holds the same MAC (commonly MAC 0 or 1, which are popular defaults), both devices compete for the token, causing intermittent failures on the entire trunk. Always verify that the router's MS/TP MAC is unused before connecting it.
• Using DHCP for the router's IP address. When the DHCP lease expires and the router gets a new IP, the supervisor loses contact with the router and every device behind it. This often manifests as a floor or zone going offline after a network event, DHCP server reboot, or lease timeout. Always assign a static IP and document it in the as-built records.
• Leaving Max Master at 127 with low-numbered MACs. A BASRT-B with Max Master set to 127 polls all 128 possible master addresses during each Poll For Master cycle. On a trunk with five devices at MACs 0–4, this wastes significant time sending PFM frames to 123 empty addresses. The result is sluggish point reads and slow discovery. Set Max Master to the highest actual MAC on the trunk plus one.

Platform Compatibility

BACnet/IP to MS/TP routing is defined in ASHRAE Standard 135 and is interoperable across vendors. The router does not need to be the same manufacturer as the field controllers or the supervisor. Any combination of BTL-listed routers and devices will work, provided the configuration parameters (network numbers, baud rate, MAC addresses) are set correctly.

When selecting a standalone router, confirm it carries BTL (BACnet Testing Laboratories) certification for the B-RTR device profile. This ensures the routing implementation has been tested for interoperability. The BASRT-B is BTL-listed as a B-RTR device.

For large sites with many MS/TP trunks, it is more cost-effective to use supervisory controllers with built-in routing (JACE, NAE, AS-P) rather than deploying a standalone router per trunk. Reserve standalone routers like the BASRT-B for situations where you need routing without a full supervisory controller—for example, bridging a third-party trunk into an existing IP backbone.

Source Attribution

The technical guidance in this entry is informed by the following sources:

• Contemporary Controls — BASrouter (BASRT-B) Product Documentation and Installation Guide
• Contemporary Controls — BASRT-B Quick Start Guide
• Chipkin Automation Systems — How to Set Up a BACnet IP to MS/TP Router
• Schneider Electric Community — BACnet MS/TP Bus Communication Troubleshooting Checklist
• Contemporary Controls — BACnet Networking Best Practices for Routers and BBMDs
• ASHRAE Standard 135 — BACnet—A Data Communication Protocol for Building Automation and Control Networks (Clause 6: MS/TP Data Link Layer; Annex J: BACnet/IP)