Roland Jollivet wrote: > A twisted pair, or any long piece of wire coming out of the back of a PC, is > first and foremost, a transmission line, and needs to be terminated > properly. This has nothing to do with the DC bias. The impedance of such a > line is generally 50R to 75R. > Actually, twised pairs usually come out to about 110 Ohms. The ratio of wire diameter to wire spacing changes it somewhat, but for normal signal wires just twisted together without additional spacer between, the differential impedance is almost always going to be between 100 and 120 Ohms. > In the case of a proper balanced line, there is usually a single low value > resistor (50R) across the line as it enters the differntial input of the > amplifier, but thats not the case here. > You will sometimes see a split termination, with two 50 Ohm resistors to ground or other voltage supply, the differential impedance would then be 100 Ohms. > Trying to satisfy both AC and DC requirements with a single resistor, may > either overload the port line if it is too low, or allow signal reflections > if it too high. > > The line should be terminated with a pull-up according to the power output > of the driver chip, then a scope used to select the correct RC combination. > The RC connects the line to ground. Use a 100R resistor, and start with cap > order of 50pF and increase it until you see a nice square wave. > Yes, scoping the final result to make sure all the transient energy is completely absorbed is a good idea, if you are so equipped. Because of inductive and capacitive effects of all components, the "ideal" resistor value may not perform the best, and some adjustment to get the minimum ringing may be needed.
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