[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.


More information about the questions mailing list