David J Taylor wrote:
My Oncore docs give a suggested circuit with a 74HC132:
GPS-6_1PPS --+-----------------+ +--- MAX232_T2in
| | |
| 4 +-- |
1 +-- 3 & o--+
| & o-- R=10k --+-- 6
2 +-- 5 |
| +-- C=470p -- +5V
|
9 +-- 8
| & o-- R=820 -- D=PPS LED -- +5V
10 +-- K A
I think that the generated pulse is only about 5 usec and was
doubtful it would work for me over a 15-20m cable run. I've had
regular rs232 work without problem over the same distance in
the past.
I've not started on this yet but will test with a short cable
run first and increase the pulse width as needed.
David
David,
Difficult to read your diagram, even in a fixed spacing font.
My point was that the individual active lines should be paired with
ground lines for transmission.
If the pulse is only 5 microseconds, I would definitely want Chris to
see how mangled (or otherwise) it was at the end of the run he has. It
may well be pushing the limits, especially if it's not using the cables
as twisted pairs. Perhaps your circuit could be modified to produce
something nearer 100 ms by replacing the C with something about 20K
times bigger? A 1 uF or so?
The circuit is from the "Oncore QuickStart Guide" and I
guessed that the pulse shaper was to get the 5 usec pulse
otherwise output from pin 3 of the 74HC132 could have been
used for default pulse width.
When I get around to assembly of my 2-to-6 rs232 expander
I'll scope with both short and long cables and I've also
rs422 MC3486/7 to try.
I'm doubtful my modern scope can catch and display a 5 us
pulse with period of 1 sec.
Intention is to feed PPS and NMEA direct to each of my
pool.ntp.org servers and also servers on the lan.
I keep getting kicked off the pool rotation and it then
takes up to a day before the servers are being polled
again. An extra GPS source will keep the servers in sync
for that much longer so mostly the servers will still be
in sync when internet is back (BT upgrading the system).
David
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