Is there a known problem using a 2 pair x 0.75mm2 (24/0.2mm) armoured screened cable (27.2 ohm/km, 88 pF/m) for RS485/Modbus comm, approx distance of 100 metres - will it be advisable just to use a twisted pair, e.g. from Belden to avoid any problems (noise, etc)? The signal is from/to a variable speed drive - DCS. Should the capacitance of the cable has to be lower than 88 pF/m to have a good signal for Modbus RTU?

 

Using the cable you discribed will work in many cases (I have used similar myself).

The big BUT is if the line drivers at either end are flaky - such as most large manufactures 'economy' product range.

If in doubt, Beldon 9842 is the optimum solution.

 

24G should be fine. Running 2-pair is nice - aids trouble shooting - once you've only run 1 pair you are stuck with the limitations. At 100m it must be twisted pair - although I have seen "analog loop" cable used at 800m at 4800 baud (they wanted 19.2, but the error rate was too high).

Since RS-485 is not "reversing" the current flow like RS-232, capacitance has much less impact on RS-485 and isn't normally part of the central cable spec.

At 100m and with a drive, common-mode ground problems will be your most likely issue - your Master RS-485 device won't always agree with the
drive circuit as to what is ground. Hopefully at least 1 end has an isolated/floating ground - in which case you can use one of wires in the extra pair as a shared ground reference.

- LynnL, www.digi.com