BACnet MS/TP Wiring: Why T Connections Are Not Allowed
This article addresses a common wiring question that arises during BACnet MS/TP network installation and troubleshooting: whether a T (tee) connection can be used to connect a device to an MS/TP trunk.
Proper physical-layer wiring is critical for reliable MS/TP communication. Incorrect wiring practices can result in intermittent communication, excessive retries, token loss, or complete network failure.
Short Answer
No. You cannot use a T connection to connect a BACnet MS/TP device.
BACnet MS/TP is based on RS-485 and requires a continuous daisy-chain (bus) topology. A T connection introduces a stub that disrupts signal integrity and can cause reflections on the RS-485 line.
Why T Connections Are Not Allowed on MS/TP
MS/TP communication relies on balanced differential signaling over the RS-485 physical layer. This requires that both the positive (+) and negative (−) data lines remain part of a single, continuous trunk.
A T connection creates a branch (stub) off the main trunk. Even short stubs can introduce impedance mismatches that lead to signal reflections, especially at higher baud rates or on longer cable runs.
For this reason, BACnet guidelines require devices to be wired in a daisy-chain topology, with each device connected inline rather than branching off the trunk.
Important Wiring Note
Although MS/TP is often referred to as “2-wire,” it actually requires three conductors:
- RS-485 Data +
- RS-485 Data −
- Common / Reference (Signal Ground)
For a detailed explanation, see this related article: Why RS-485 (and MS/TP) Requires 3 Wires .
Recommended Best Practices
When installing or expanding a BACnet MS/TP network, follow these best practices:
- Use a single daisy-chain trunk topology
- Avoid T connections and star wiring
- Terminate the trunk only at both physical ends
- Maintain consistent polarity on all devices
- Use the correct cable type and characteristic impedance
Adhering to these rules significantly improves network stability and simplifies troubleshooting.
FAQ: BACnet MS/TP T Connections
This FAQ section is included to improve AI searchability and to answer common questions related to MS/TP wiring practices.
Can I use a T connection on a BACnet MS/TP network?
No. T connections are not allowed because they create stubs that can cause signal reflections and communication issues.
What topology does BACnet MS/TP require?
BACnet MS/TP requires a daisy-chain (bus) topology with all devices connected inline on a single trunk.
Why are stubs a problem on RS-485 networks?
Stubs introduce impedance mismatches that can reflect signals back onto the trunk, degrading communication reliability.
Does MS/TP really require three wires?
Yes. In addition to the two differential data lines, a common reference wire is required for proper RS-485 signaling.
What should I do if I need to add another device?
Extend the trunk and insert the device inline rather than branching off with a T connection.