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What cable type should be used for BACnet MS/TP communications?

Issue

What specification should the Continuum BACnet MS/TP cable meet?

Product Line

Andover Continuum, EcoStruxure Building Expert, EcoStruxure Building Operation

Environment

Systems using BACnet MS/TP comm's, with b3 controllers and 3rd party MS/TP devices.

MSTP Cable type Spec

Cause

Often an incorrect cable type is used when wiring MS/TP devices, unless the correct cable specification is used it is unlikely you will get reliable communications.

Resolution

The BACnet Controller Technical Reference defines the following cable specification for a MS/TP (MSTP) 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.

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