On 2012-01-08, Mike S <[email protected]> wrote: > On 1/8/2012 5:19 PM, unruh wrote: >> On 2012-01-08, Chris Albertson<[email protected]> wrote: >>> I have a UT+ It does 5V logic level pulses but I connect the TTL >>> level PPS to a MAX232 chip which converts the levels to +/- 8 volts >>> >>> I think the problem is my long cable. > ... >> >> Make sure you properly terminate the cable with 100 ohms. Otherwise you >> will get reflections back and forth along the cable, and the cable will >> slowly fill with charge, broadening the pulse by a huge amount. It may >> also be that the signal is decreasing due to resistance in the line. > > The proper termination for EIA-232 is 3000-7000 ohms.
He said he was using 80 ft of cat 5 cable. That is 100 ohm cable. To get rid of reflections he needs a 100 ohm termination. This has nothing to do with 232. If the serial input requires some other termination, then you need to have a buffer between them. (Actually, an interesting question is what the input impedence of the DCD line on an RS232 port is. It is an input line so should not have to be pulled to anything. Ie it could have a very high input impedence. But does it?) Also, I hope that the output of the PPS is able to drive 100ohm, since that is what it will see on an 80 ft line, at least for the first .3usec or so. _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
