# RS485 Cable Lengths and Reflections

In our lab we have setup a RS485 network with short cables and the network is working fine. I know that terminating resistors are required, especially with longer cables lengths due to reflections on the line. Why do shorter cables work correctly without terminating resistors, and why do we not see a lot more reflections on the shorter line?

Oh, the reflections are there, don't worry :-). It's just that for short distances the delay of the reflected signal is so short that it won't distort the original signal much. Think of a 1 µs pulse, in a typical cable that will stretch over 200 m (at 2/3 $c$). Then at a 2 m cable the reflection will come after 1 % of the pulse width, and the distortion will be limited because the phase difference between signal and reflection will be negligible.