Proper termination of long cables
Something I've always struggled with, and not found a lot of resources on is how to send reliable digital data down long cables. I understand transmission line theory, impedance matching, etc etc, but I'm missing something between that and practical applications.
Say I have a long cable and I need to send a 5V digital signal down it. I can measure the short circuit inductance and open circuit capacitance of the cable and calculate the characteristic impedance, but if I then terminate both ends of the cable with that impedance for "maximum power transfer", I end up halving the voltage, which means I only have 2.5V signal at the far end across the termination resistor - not good for a digital signal. So for a digital system, it seems like maximum power transfer is not the ideal goal for cable termination, as it is in RF.
I also saw this post earlier https://www.reddit.com/r/rfelectronics/comments/1t5l2km/looking_for_smd_network_termination_devices/ about terminating a cable with some inductance (to counteract the capacitance of the cable I guess) in parallel with some resistance, and how that helps preserve the fast edges of a digital signal. How do you properly calculate the inductor value and parallel resistor value? I wouldn't think you'd want to exactly cancel out the cable capacitance, do you? Do you end up with a resonance problem at certain frequencies? Because a sharp rising edge essentially has all frequencies, doesn't this lead to ringing at the resonance frequency?
I need to push a digital signal down a long cable, and I'm having problems with my rising edges looking very "capacitive" at the far end of the cable - of course it gets worse as the cable gets longer, until the edges are rising so slowly that they're not reaching the threshold to be detected as a high before the bit time is over. Putting a pull-up resistor, higher than the "characteristic impedance" of the cable (to avoid the 1/2 voltage drop at the actual characteristic impedance) at the far end helps, but only so much.
I feel like I can never find good information on this topic, either in books, or on the internet, yet it seems like a very important and fundamental topic to get anything practical that needs a long cable to work correctly.
