BACnet MS/TP Wiring Specifications

BACnet MS/TP Wiring Specifications

This guide covers the physical layer requirements for BACnet MS/TP (Master-Slave/Token-Passing) networks per ASHRAE Standard 135.

MS/TP Overview

BACnet MS/TP is an RS-485 based communication protocol designed for building automation:

Parameter	Specification
Physical Layer	EIA-485 (RS-485)
Topology	Daisy-chain (bus)
Max Devices	127 (practical: 64)
Max Segment Length	1200m (4000 ft) at 9600 baud
Baud Rates	9600 to 76800 (common: 38400, 76800)

Cable Specifications

Recommended Cable Types

Cable Type	Description	Use Case
22 AWG STP	Shielded twisted pair	Standard installation
18 AWG STP	Shielded twisted pair	Long runs, high noise
Cat5e/Cat6	Use one twisted pair + shield	Acceptable alternative
Belden 9841	RS-485 specific	Recommended
Belden 9842	RS-485 with drain	Outdoor/industrial

Cable Requirements

• Twisted pair: Minimum 1 twist per inch
• Shielding: Foil or braid shield required
• Impedance: 100-120 ohms characteristic impedance
• Capacitance: <30 pF/ft recommended

Reference: ASHRAE 135-2020, Clause 9 - MS/TP Data Link Layer

Wiring Topology

Daisy-Chain Configuration

[T] = 120Ω Termination Resistor

Critical Rules

1. Single continuous bus - No star or branch topology
2. Termination at both ends - 120Ω resistors at first and last device only
3. No stubs - Maximum stub length: 0.3m (1 ft) from trunk
4. Single ground point - Ground shield at one end only

Maximum Distances

Distance limits depend on baud rate:

Baud Rate	Max Total Length	Max Stub
9600	1200m (4000 ft)	0.3m (1 ft)
19200	1200m (4000 ft)	0.3m (1 ft)
38400	1200m (4000 ft)	0.3m (1 ft)
76800	1200m (4000 ft)*	0.3m (1 ft)

*At higher baud rates, practical limits may be shorter in noisy environments.

Termination

Why Termination Matters

• Prevents signal reflections
• Ensures clean signal edges
• Required for reliable communication

Termination Requirements

Location	Termination
First device	120Ω resistor
Middle devices	No termination
Last device	120Ω resistor

Termination Methods

1. Built-in jumper - Many controllers have onboard termination
2. External resistor - 120Ω ±5%, 1/4W minimum across data lines
3. Biased termination - Some systems use bias resistors for idle state

Shield Grounding

Proper Grounding

• Ground at ONE end only - Typically at the supervisor/router
• Drain wire - Connect shield to earth ground
• Avoid ground loops - Never ground at multiple points

Grounding Diagram

Device Limits

Per Segment

Type	Maximum
Total Devices	127 (per BACnet spec)
Practical Limit	64 devices
Masters	127
Slaves	Unlimited (but counted in total)

When to Add Routers

• More than 64 devices needed
• Segment length exceeds limits
• Isolation between areas required
• Different baud rates needed

Common Installation Errors

Error	Problem	Solution
Star wiring	Signal reflections	Rewire as daisy-chain
Missing termination	Communication errors	Add 120Ω at both ends
Over-termination	Signal attenuation	Remove extra resistors
Multiple ground points	Ground loops, noise	Ground at one end only
Long stubs	Reflections	Keep stubs under 1 ft
Wrong cable	Signal degradation	Use shielded twisted pair

Testing and Verification

Visual Inspection

• Verify daisy-chain topology
• Check termination placement
• Confirm shield grounding
• Inspect connections for quality

Electrical Testing

Test	Expected
Continuity (Data+ to Data+)	<10Ω end-to-end
Resistance (Data+ to Data-)	~60Ω (two 120Ω in parallel)
Shield continuity	<10Ω end-to-end
Shield to ground	Connected at one end

Communication Testing

• Check for CRC errors
• Monitor token passing
• Verify all devices responding
• Test at expected traffic load

References

• ASHRAE Standard 135-2020 - BACnet Protocol, Clause 9 (MS/TP Data Link)
• TIA/EIA-485-A - Electrical Characteristics of Generators and Receivers
• Manufacturer documentation - Device-specific requirements