Issue
What specification should BACnet MS/TP cable meet?
Product Line
Andover Continuum, EcoStruxure Building Operation
Environment
MS/TP BACnet devices
Cause
Often an incorrect cable type is used when wiring MS/TP devices causing unreliable communications.
Resolution
The BACnet Controller Technical Reference defines the following cable specification for a MS/TP cable:
RS-485 Cable Specifications:
- Cables used to form the RS-485 network should conform to the following specifications:
- Wire Size: 22-24 AWG
- Cable Type: Twisted-pair, copper wire, tinned
- Shield: Braid
- Nominal Impedance: 100-120 Ohms
- Velocity of propagation: 78%
- Capacitance: <12.5 pF/ft (~ 41pF/M) between conductors and < 22 pF/ft (~72pF/M) between the conductor connected to ground and the next conductor.
An example of a cable that meets this specification is Belden 3105A.
Also check out the SmartStruxure-RS485-Network Installation Quick-Help video on the Exchange.
