[ntp:questions] Oddities in termination of cable from gps18.

Chris Albertson albertson.chris at gmail.com
Thu Feb 23 16:21:59 UTC 2012


On Wed, Feb 22, 2012 at 10:35 PM, David J Taylor
<david-taylor at blueyonder.co.uk.invalid> wrote:
>> I could not get good result using cat-5 either.  My "fix" was to move
>> the GPS and computer closer so I would not need a long cable.
>>
>> I sort of understand coax but cat-5 is a bundle of pairs.  If you use
>> the cat-5 for both RS232 and PPS then the twisted pairs are
>> un-balanced because the return current path is shared by the RX, TX
>> and PPS signals.   I think that might have been my problem
>>
>> I was thinking about use balance line driver and receiver chips but
>> then figured moving the equipment would be better
>
> []
>>
>> Chris Albertson
>> Redondo Beach, California
>
>
> It's all guess work unless you actually /measure/ the signals.  For the sort
> of cable lengths being discussed, the cable acts mostly as a capacitive load
> on the output signal, and terminating in its characteristic impedance is
> /not/ the correct thing to do unless you add line drivers at the sending
> end.  The GPS 18x LVC signal is TTL level, and not line driver level.
>
> Again, for the sorts of distance being discussed, cat-5 cable should be fine
> for the GPS 18x LVC, using the four pairs as:
>
> - ground and +5V
> - ground and RX
> - ground and TX
> - ground and PPS/DCD
pairs.
That is what I used and then got poor result.  (Yes I have a scope)  I
figured the reason has that all the conductors called "ground" above
where tied to gather at the DB9 shells pin-5 at each end of the cable
 So the return current divides.   You then don't have balanced twisted
pairs.    You are right about it being a capacitor but really it's
several caps that all interact.

I think the better way to use cat-5 is like this
RX, TX, DCD, ground
With each of the above in a "solid" color wire. I think you get less
capacitance that way

Also while many RS232 ports will accept TTL signals, if they do they
are operating way off spec and you are just lucky it works.  You
really should be sure the PPS, TX and RX signals are at rs232 voltage
levels.  out of spec voltages and long cables are a bad combination.

I've converted to rg58 drives with 50R driver for the PPS and then
convert is very close to the computer to RS232.  No ringing.

Chris Albertson
Redondo Beach, California


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