RS-485 Cable Specifications for BACnet MS/TP Networks
BACnet MS/TP networks use RS-485 (EIA-485) as the physical layer. To ensure reliable communication, the BACnet specification defines clear requirements for cable type, impedance, capacitance, shielding, and maximum segment length.
Following these cable specifications is critical for stable MS/TP operation, especially in larger networks with many devices or long trunk runs.
BACnet MS/TP Cable Requirements
The BACnet specification (2004) defines the MS/TP physical media as follows:
“An MS/TP EIA-485 network shall use shielded, twisted-pair cable with characteristic impedance between 100 and 130 ohms. Distributed capacitance between conductors shall be less than 100 pF per meter (30 pF per foot). Distributed capacitance between conductors and shield shall be less than 200 pF per meter (60 pF per foot). Foil or braided shields are acceptable. The maximum recommended length of an MS/TP segment is 1200 meters (4000 feet) with AWG 18 (0.82 mm² conductor area) cable. The use of greater distances and/or different wire gauges shall comply with the electrical specifications of EIA-485.”
Practical Interpretation
In practical terms, this means BACnet MS/TP networks should use shielded, twisted-pair RS-485 cable designed for industrial communications. Cable impedance and capacitance directly affect signal quality, especially at higher baud rates or longer distances.
Staying within the recommended cable length and electrical limits helps reduce reflections, noise susceptibility, and communication errors on the MS/TP trunk.
FAQ: RS-485 Cable Specifications for BACnet MS/TP
What type of cable is required for BACnet MS/TP?
Shielded, twisted-pair cable with a characteristic impedance between 100 and 130 ohms is required.
Is shielding mandatory for MS/TP networks?
Yes. The BACnet specification requires shielded cable. Both foil and braided shields are acceptable.
What is the maximum recommended MS/TP segment length?
The maximum recommended length is 1200 meters (4000 feet) when using AWG 18 cable.
Can I exceed 1200 meters or use a different wire gauge?
Longer distances or different wire gauges may be used only if they comply with the electrical specifications of EIA-485.
Why are capacitance limits important?
Excessive capacitance can degrade signal edges, increase noise sensitivity, and cause communication errors on RS-485 MS/TP networks.