Which able to use for RS-485 connection?


Thread Starter



Thanks you all in advance.

I search on the web about which cable should I use to connect 5 gas meter to a Mgate device through RS-485 connection. Distance should be less than 300 ft (100 m) between the Mgate device to the last meter.

Some folks recommend STP Cat6, others Belden 3106A.

I currently have the following cables:

1- Belden 9941
2- Belden 9322
3- Belden 9463
4- Honeywell 12065509
5- STP Cat6

Which of those cables should be my best choice?

I am also read about to add a resistor of 120 ohm in the Mgate RS-485 connector and in the RS-485 connector of the last meter. My questions about this:

No matter what cable I chose do I always have to add the resistor at the end of both ends?

Thank everyone for your help.
If I had STP CAT6, I'd use it. I have in the past and it works great.

Use a 'pair' as a single conductor to provide a lower conductor resistance. With four pair in the cable, you've even got a pair available for a signal ground connection. The extra conductors would not be used anyhow. The shielding always helps in an industrial location.

The only thing is that CAT 5/6 cable has a 100 ohm characteristic impedance, so use 100 ohm termination resistors, instead of 120 ohm resistors.

I find that CAT 5/6 is readily available off-the-shelf compared to trying to find other cable like Belden 3106A (the official Belden RS-485 cable).
Thank you David!

Then STP Cat6 will be the best option over the others Belden cables I currently have.

To double check I understand correctly your recommendation. I will use each pair for each connection. Ex. orange and white orange for A, blue and white blue for B, and green and white green for COM? And then going meter after meter with each pair for each connection.

I only have 2 120 Ohm resistor since a read about this resistor on the web. I didn't realize that should be the same impedance of the cable. What is the result if I use 120 Ohm on both end of the cable with STP Cat6 cable?

Thank you for your help.
I didn't do an engineering evaluation of the other cables you suggested. I just want to let you know that I've used STP CAT6 for RS-485 at 38K baud at 100m distances several times and the CAT6 worked and has continued to work fine.

As to the effect of using 120 ohm termination resistors, I suspect it'll work OK at moderate baud rates. The effect can be calculated knowing all the 485 driver's specs, number of drops and bias values, but why not try and see? By the way, resistors are cheap (pennies each) so I terminate with a 100 ohm resistor at each end. If you only have 120 ohm resistors, try them.

I would avoid using RJ45's for connectors because the world thinks they're always Ethernet. But most RS-232 connectors are screw terminal anyhow.
Hello J.F.

What baud rate do you plan to use?

The reason I ask is because the lower the baud rate, the more forgiving the RS-485 communications will be when using less-than-ideal cable and/or less-than-ideal termination resistors.

Hope this helps!