[time-nuts] Advice on Synergy M12+ adapter and SYMTRIK SYM-RFT-XX.

[Chris Albertson]

On Thu, Apr 19, 2012 at 6:18 AM, Andrew Back <andrew at carrierdetect.com>wrote:

>
> So, I have a Motorola M12+ GPS receiver in a Synergy Systems adapter
> board, and one of those little Symtrik MSF receiver boards. I'd like
> to try and get both working under Linux with ntpd (not at the same
> time!), and was hoping that someone might be able to provide some
> pointers. Namely:
>
> - Pinout for the P1 adapter on the Synergy Systems board. From the NTP
> Oncore driver documentation it looks as though this should be well
> supported.
>
> - Advice on how to hook up the Symtrik board and to get that working with
> NTP
>
> As to the host, I'm looking to use a Raspberry Pi board running Linux:
>
> http://elinux.org/RaspberryPiBoard
>
> This does have a UART, but I think I'll only have TXD and RXD with the
> USB<->3.3v serial adapter that I'm using. Though the board does also
> have GPIO pins, which I'm hoping I could somehow use for the PPS with
> GPS, or with the Symtrik.
>
>
Here is what you need to do...

1) Get NTP working on the Linux system using Internet peers as the
reference clock.  Use "pool servers".   This is very easy to do and don't
skip this step.  It verifies NTP works and you will need the pool servers
to verify you GPS is working

2) I hate to say this but it is POINTLESS to connect a GPS vis a
Serial->USB adaptor.  Yes it will work but you loose maybe three orders of
magnitude accuracy.  Theat means when you could have micro-seconds you will
be gets milliseconds.   and you can get that germ the Internet pool
servers.   So if you don't have a "real" serial port there is not need to
connect a GPS except as an exercise.    This problem come up all the time
on the NTP email lists, people complain that their GPS is giving very poor
persomance and the reason is always they it is connected via USB.

3) OK now that you have a computer with a hardware RS-232 port and an GPS
that runs in 3 volts you have to convert the 3 volt logic level signals to
rs-232  level.   Some rs-232 ports will accept 5V "TTL level" signals (TTL
level is out of spec but many times works) but I doubt many rs-232 ports
will take 3V level inputs reliably.  Look at the MAX232 chip.  You can also
do the level conversion with transistors but the max232 is foolproof..
 If you look on eBay you can buy a MAX232 chip soldered to a DB9 connecter
with all the required caps and a 0.1 inch header all for the price of a DB9
connecter.   On my systemI used one of those.

One more detail.   RS232 data is defined as "1" is low and "0"  is high.
 Backwards from TTL.  But the control signals like DCD are the other way
around.   So you will likely need "hex inverter" type chip to invert some
of the signals.   On my system ALL signals go through this inverter either
an odd or even number of times.  The inverter can drive a small LED so you
can visually see the pulse per second.  (The GPS can't directly drive
anLED.)   Verify the RS232 connection works buy just using "cat" to send
gps data from the serial port to the screen and see if the expected
gibberish is printed to the screen.


At this point you have (1) a verified working NTP running under Linux and
(2) RS-232 level output from the GPS with the PPS pulse tied to "DCD" pin.
  All you need to do is add the Oncore driver to NTP and cable the gps to
the computer and re-start ntpd.

Possible problems are that you have the signals inverted.

Chris Albertson
Redondo Beach, California