RS485 - BACnet MSTP (RS485), Does 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 what sense? Determining 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 3107A

 

Multi-Conductor - EIA Industrial RS-485 PLTC/CM

22 AWG stranded (7x30) 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.