[time-nuts] FE5680 missing PPS - hallelujah!

John Beale beale at bealecorner.com
Fri Feb 3 04:41:46 UTC 2012


Thank you Bob!  I just tried removing the "lock" indicator LED on my three 
5680A units and sure enough, in every case, there was the 1 PPS (+5V, 1 
usec, rise/fall time ~100 ns) just as if it had always been there.

As a reminder, this and much else useful lore is now collected at the 
ongoing FE-5680 FAQ at 
http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:fe5680a_faq

(and if the answer to any of your nagging FE-5680A questions is missing 
from the FAQ, please feel free to add it!)

-John

On 2/2/2012 12:46 PM, bob grant wrote:
> Some info...
>
> Its tempting to attach an LED to the /LOCK signal on the DB9.
> However this signal is very weak and the LED does not seem very bright
> and PPS signal does not pulse...Hmm
>
> Internally the /LOCK pin is connected the 74AC240 buffer, but with an
> LED helping to keep the signal voltage high (2.3V) a logic low is never
> asserted.
> This logic low is needed to enable one half of the 74AC240 buffer (pin
> 1) that gates the PPS signal.
>
> Don't directly drive LEDs from the LOCK signal on the DB9 and, voila,
> the PPS signal reappears.
>
> Bob




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