David Woolley wrote: > David J Taylor wrote: > >> >> In the RS-232 implementations I have seen, the line is /never/ >> correctly terminated, as I use the term. Correct termination >> implies, if the > > I'm not disputing that. I was really pointing out that by using RS232 > you couldn't follow the earlier advice to use terminated lines. RS232 > is designed to be used on bandwidth limited, electrically short lines, > both driven and terminated above the characteristic impedance, so that > the line behaviour approximates a discrete capacitor.
Thanks for clarifying that. Agreed - that's why David Lord is proposing line drivers for his PPS signal over co-ax. Had I the equipment and the time, I would like to see how well the GPS 18x LVC drives that amount of capacitive load. > >> cable impedance 50 ohms (such as the often used Belden 8777), that: > > RS232 cables tend to have a high frequency impedance closer to 100 > ohms, although at the frequencies involved, the impedance is variable > and far from the high frequency limit. Yes, that's what I thought until I looked again at the specs. for the cable we used to use! >> >> - the driver output impedance is 50-ohms > > Commonly not true, even for controlled impedance systems. Radio > transmitters rarely reverse terminate the line properly. But there, the power loss inside the transmitter is important. Take a look at fast data systems, sending video over cables etc. - they will be matched. >> - the cable impedance is 50-ohms >> - the receiver input impedance is 50-ohms > > Whilst optimum for data transmission, for low noise radio receivers it > can be the wrong thing. Agreed. David _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
