It is worth noting that skipping the end termination is probably a bad idea when daisy-chaining a signal from one output to more than one device input. The input at the end of the cable will see a clean rise from zero to 5 V (or whatever the driver's open-circuit voltage is), but the other inputs along the length of the cable will not. They will see an initial rise from 0 to 2.5 V as the series termination at the driver and the cable impedance act as a voltage divider while the cable is being charged. Later, they will see another step change from 2.5 V to 5 V as the reflection returns from the open-circuit far end of the cable. If the input threshold is automatically set at half the input voltage swing, the input could trigger on the outbound or the reflected pulse, or even somewhere in between.
This is in contrast to having a 50 ohm termination at the end of the cable (plus the 50 ohm series termination at the source), where all inputs along the length of the cable see a single edge transition from 0 to 2.5 V. They will each see the edge at a different time due to propagation delay, but all will see a clean edge. Dave On Tue, May 15, 2012 at 4:00 PM, <saidj...@aol.com> wrote: > > To make this work without the unnecessary power consumption simply remove > the end-termination resistor, and use it as the series termination resistor > (R1 in your schematic)! Done. > > Attached are two plots of a series terminated (~55 Ohms) high-speed 1PPS > transmission from our CSAC GPSDO board zoomed-in and zoomed-out to show > the > actual rise-time, and a longer time frame view. > > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.