[time-nuts] verifying synchronization with PPS

[Hal Murray]

scs at eskimo.com said:
> My specific question concerns the case that one of the boxes is a computer,
> for example, a Linux box with time kept using ntpd or chrony (perhaps also
> listening to a PPS signal coming in on a serial port).  But I've never seen a
> computer with a PPS *output*. 

There is code in the Linux kernel to generate a PPS output.  I've never tried 
it.  I think it uses the printer port which isn't available on modern systems. 
 You can probably get a PCI plugin card.  You will probably have to build your 
own kernel.

If I wanted to try something, I would write a user-level hack to flap a modem 
line and compare that to a PPS signal.

The code would look something like this:
  t1 = time()
  set line low
  t2 = time()
  set line high
  t3 = time()
  log t2 and t3
  wait 1/2 second
  set line low
  wait 1/2 second
  loop

The first time/set is to warm out the cache.


> (Perhaps PPS is overkill for this situation; perhaps it's the case that given
> the inherent inaccuracies of software-based timekeeping, a software-based
> query mechanism -- perhaps using ordinary NTP or PTP protocols -- would be
> sufficient for 

That just gets time from one computer to another.  Now you have the same "is 
it right?" question on another box.



-- 
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