David Lord wrote:
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've not put a scope on the pps but I added an extra section
of cable from my Sure GPS, 18 m of 4 x pair telephone cable.
I will need that length to reach from the GPS down to my PCs.
In the process the offset jumped from 0.004ms to about 12ms
but fairly quickly went back to a few usec.
PC = me6000 (VIA ME6000), NetBSD-5, ntpd 4.2.6p5
From my ntp-stat file:
time offset(ms)
15:06 -0.004
15:12 0.000
15:18 0.001
18:24 0.006 extra 18 m cable added
15:30 -12.938
15:36 -3.672
15:42 -0.814
15:48 -0.186
15:54 -0.054
16:00 -0.012
16:06 -0.006
16:12 -0.002
16:18 -0.002
16:24 -0.001
David
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