[ntp:questions] Re: PPS from TrueTime 468-DC GOES Time Receiver (with GPS antenna)

simonpg paul at thesimonet.org
Fri Aug 25 20:47:15 UTC 2006


Thank you for responding.  Please excuse me while I learn to work with
this interface.  My comments are interspersed below:

Hal Murray wrote:
> In article <1156450631.842127.105780 at 74g2000cwt.googlegroups.com>,
>  "simonpg" <paul at thesimonet.org> writes:
> >So I have this fine receiver and have been a member of pool.ntp.org for
> >several years.
> Have you found the web page for the driver?
>   http://www.cis.udel.edu/~mills/ntp/html/drivers/driver5.html
> (That may not be the latest one, it's just what google found for me.)
>

I guess I didn't make myself clear enough.  The clock works great:

ntpq> peer
     remote           refid      st t when poll reach   delay   offset
jitter
==============================================================================
*TRUETIME(0)     .TRUE.           0 l   29   64  377    0.000    1.233
 1.901
-clepsydra.dec.c .GPS.            1 u  504 1024  377   56.294   16.420
 0.023
-clock.xmission. .GPS.            1 u  567 1024  377   81.726   15.580
 1.204
+bigben.cac.wash .USNO.           1 u  553 1024  377   46.023   15.031
 2.000
-time-nw.nist.go .ACTS.           1 u  566 1024  377   36.026   20.802
 0.702
+clock.sjc.he.ne .CDMA.           1 u  509 1024  377   54.394   15.218
 0.100
 LOCAL(0)        .LOCL.          10 l   35   64  377    0.000    0.000
 0.004

> Lots of info here:
>   http://www.leapsecond.com/museum/468-dc/theory.htm
> That doesn't mention a PPS signal.  It says the signal is 468 MHz
> so I wouldn't be surprised if a GPS antenna filtered that out.

Yup, it does, "4-5 A phase locked receiver is used to receive and
recover raw data from the satellite signal. The raw data is processed
by analog circuitry and then passed to a microprocessor for conversion
to useful outputs; among which are a visual display, one Hertz and one
kilo Hertz timing pulses."  It's just a little hidden.

>
> The usual approach with a PPS signal is to connect it to
> one of the RS-232 modem control signals.  (I forget which one, but
> it should be easy to find.)  Most of the PPS signals are TTL levels

DCD (Data Carrier Detect)

> rather than RS-232.  If you want to get fancy you can wire up a
> level converter (MAX-232) type chip but it will almost-for-sure
> work with a direct connection.  You do have to get the inversion
> right though - MAX-232 type chips are inverters.  Software can
> normaly use either.
>

So, both the 1 and the 1K Hz pulses are available in the form of a BNC
connector.  I know that TTL stands for Transistor To Transistor Logic,
but I don't know if that looks like a BNC connector.

> Once you get the signal into your box, try the ATOM (aka PPS) driver.
>
> Is GEOS still working?  I found this note from June 2005 saying it
> was going off the air:
>   http://www.febo.com/pipermail/time-nuts/2005-June/001050.html
>

I guess GOES is indeed dead, but we have GPS now.  Truetime came out
with a replacement antenna that converted the GPS signal to something
that makes the 486-DC happy, so I am happy.

>
> --
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