Modbus on RS485

Modbus on RS485 Advice 1 - 3

Advice #4


 

Take care where you run your cables. It seems obvious not to wind your cable around other cables or sources of electricity / magnetism. People are often surprised to find that the worst source of induced noise are switching DC loads. Another big culprit are Variable Frequency drives.

Advice #5



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

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. 

 
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.