Hi Chris, On 19 April 2012 19:54, Chris Albertson <albertson.ch...@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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
