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

[Andrew Back]

Hi Chris,

On 19 April 2012 19:54, Chris Albertson <albertson.chris at gmail.com> wrote:

> 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.

Understood. Might try this via the laptop, if only as an exercise, though.

> 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.

The target machine — a Raspberry Pi — has a serial port with 3.3v levels.

> 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.

Right, so the Synergy Systems Adapter Board that the M12+ is plugged
in to has a 74ACT04 hex inverter on it, so maybe this is taken care
of...

> 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.

I don't have DCD on the Raspberry Pi — only GPIO pins. So it's looks
as though the driver would need hacking.

I'm also yet to find details of the Synergy Systems board, and looks
like I might have to just sketch it out. Thankfully, there doesn't
look to be much to it.

Many thanks, and also to Hal for the advice!

Regards,

Andrew

-- 
Andrew Back
http://carrierdetect.com