[ntp:questions] Oddities in termination of cable from gps18.
David J Taylor
david-taylor at blueyonder.co.uk.invalid
Thu Feb 23 16:32:38 UTC 2012
"unruh" <unruh at invalid.ca> wrote in message
news:4ut1r.6712$VR6.850 at newsfe14.iad...
> The edge is much faster than 200ns. It is at the 20-50ns level which is
> less than a wavelength away. From the
> Sure, it is at the 5ns level.
You might be better using the RS-232 output, not the TTL, because RS-232
is intended for driving longer cables which TTL is not. I've not measured
it here but, for example, the Texas Max232 datasheets suggests a rise-time
around 400 ns, and that's without a heavily capacitive load. This helps
avoid the wider-bandwidth problems which you may be seeing.
> The receiver is the ACK pin on a parallel port, not on a serial port.
> And 1 V is just on the hairy edge seemingly without hysteresis.
Using something which is intended for RS-232 reception would be a better
choice.. If you /are/ seeing reflections on the signal, try adding a small
resistor (say 47-82 ohms) in series with the /sending/ end, so that the
cable is slightly better terminated and you don't get multiple
reflections. Adjust with the 'scope for the cleanest edge. Even better,
use the RS-232 signals.
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