[cctalk] Re: PDP-11 hardware clock

GPS modules can now be had from various outlets for $20 or so, perhaps less depending on specs. I got one from Adafruit recently that will track 15 or more satellites, and deliver time to a modest number of nanoseconds. Some have embedded antennas that work quite well even indoors (though I would assume not if you have solid metal layers in the ceiling). The common standard is NMEA, which is well enough described in product data sheets (the ones from u-blox are particularly good, and they also make good GPS modules at prices somewhat higher than Chinese competitors) even though the standard itself is annoyingly secret. It's a simple serial protocol on a 9600 baud link, so a PDP-11 can easily process it. If you want time to the nearest second, just parsing the incoming NMEA message is sufficient. If you want to do better, hook up the GPS module's PPS (pulse per second) line to a convenient logic level input, perhaps a modem control signal of a serial port or an input line of a GPIO digital controller. That will get you second signals to far better than the interrupt latency of a PDP-11. paul