# Finding cable impedance, to determine RS485 line termination

We have some Belden 6341PC cable which will be used with some RS-485 communications equipment, which of I'm learning will need line terminators. I'm told the resistor values needed should match the characteristic impedance of the cable. The problem is I'm not seeing anything on the datasheet which explicitly mentions the impedance.

I also see that, according to the spec sheet, this cable is rated as being for 'Pro Audio and Intercom Systems', not RS-485 systems.

1) Based on the info on the spec sheet, is there a way to determine which resister termination value should be used?

2) Should I recommend another cable be used instead? The problem is the cable has already been pulled, so we're hoping it'll work without issues.

• IMO, step 1 is to try 120 Ohms. If it works, problem solved! Commented Jan 8, 2018 at 19:02

$Z_o=\sqrt {\frac{L}{C}}$ for L= 0.16 uH/ft for each line and C= 44 pF/ft, line to line thus;
$Z_0= \sqrt{\frac{0.16uH}{44pF}}=60 Ω$ for each line and thus differential Zo = 120 Ω