[time-nuts] Thunderbolt PPS Modification

Ed Palmer ed_palmer at sasktel.net
Sun Sep 26 03:46:57 UTC 2010

Hal Murray wrote:
>> I was thinking of modifying my Tbolt to put the 1 PPS on one of the  unused
>> RS-232 leads.  It just seems like it would be really convenient  to have
>> that signal available to import into a computer. 
> NTP expects the PPS signal on pin 1, DCD.
>> It looks like it's not going to be quite as simple as I'd hoped.  So  before
>> I reinvent the wheel, has anyone done this mod and could you  offer any
>> suggestions or warnings? 
> What's not simple about it?
> I just turned the board over and ran a wire from the BNC pin over to pin 1 on 
> the RS-232 connector.  It wasn't connected to anything.  (There may have been 
> a trace, but it went to an empty pad.)
> The problem for me was that the PPS pulse wasn't wide enough for one of my 
> PCs to catch.  (Another one does catch it.)
> I changed the wire into a diode, resistor, and cap.  It's a kludge, but it 
> works.
Problem 1:  As you found, the pulse was too narrow.  I was thinking 
about doing something very similar to your idea.

Problem 2:  Because you connected the TTL-level 1 PPS to the RS-232, you 
may find that not all computers will recognize it.  A computer that 
actually followed the RS-232 rules wouldn't see it at all.  I don't know 
if any modern computer actually follows those rules.

Problem 3:  The fix to Problem 2 is to use the spare converter in the 
TTL / RS-232 converter chip.  However, it already has data on it that 
goes absolutely nowhere so which trace should I cut to free the input 
without affecting anything else?

Problem 4:  Who knows?  That's why I'm asking.


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