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RS485 Networks – BACnet MSTP

Cable Specifications for RS485 Networks – BACnet MSTP


The BACnet spec defined the media for MSTP as follows (2004) ;

“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 that 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.”


BACnet MSTP (RS485), Cable selection does make a difference.


All cables offer impedance (resistance). Some cables are designed so that the impedance is relatively independent of distance. You want one of these cables. A clue to knowing if you selected one is to look at the cable’s Nominal Impedance. If they quote a number such a 100Ohms you have a good cable. If they quote an impedance per meter/foot you have chosen the wrong kind. Wrong in the sense – to determine the value of terminating resistors now requires measurements and calculations. Choose low capacitance cables.

Can you use Cat5 cable ? Yes. Use one pair for Tx,Rx and a conductor from another pair for the ground reference signal.

We recommend these two cables.

Belden 3106A

Belden 3106A - RS485 Cable 

Multi-Conductor – EIA Industrial RS-485 PLTC/CM
22 AWG stranded (7×30) tinned copper conductors, Datalene® insulation, twisted pairs, overall Beldfoil® shield (100% coverage) plus a tinned copper braid (90% coverage), drain wire, UV resistant PVC jacket. 
 

Belden 3107A

Belden 3107A - RS485 Cable 

Multi-Conductor – EIA Industrial RS-485 PLTC/CM
22 AWG stranded (7×30) tinned copper conductors, Datalene® insulation, twisted pairs, overall Beldfoil® shield (100% coverage) plus a tinned copper braid (90% coverage), drain wire, UV resistant PVC jacket.



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