[time-nuts] Thunderbolt PPS Modification
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
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.
More information about the time-nuts