Helo everyone,
I have an instrumentation cable (Twisted pair) that has aproximatelly 135nf/km capacitance and 0.6mH/km of inductance, I would like to know if I can use it as RS485 cable with 9600 baudrate speed. Calculating the impedance sqrt(L/C) it's aprox. 65ohm. I Know that higher capacitance has worst performance, but I would like to know What else that makes it not suitable in a physical way.
The characteristic impedance is 67 Ω based on your numbers and, as such you need to terminate the cable at both ends with the appropriate value resistor (a resistor of 68 Ω would do).
However, the formula you used is for high frequencies (usually above 100 kHz) and you are operating at much lower frequencies. But, at 5 kHz, the wavelength is 60 km so, I don't think you'll get any extra problems due to reflections. Even if you take into account harmonics of the square wave shape of the data, I can't see any extra problems over and above getting a decent enough signal amplitude.
Another answer that gives more details about characteristic impedance.
The low speed setting indicates that you're using a very long cable; please state approximate cable length.
Speed: In general the only reason for lowering the speed is to use lengthy cabling.
The speed that provides a stable connection depends on the actual electrical environment (not only cable parameters), the cable layout, positioning relative to disturbance/interference and length.
At least for short cables e.g. 10 meters or so
the actual physical layout of the cable might not matter very much.
RS485's "differential signaling" generally reduces interference to nothing.
