> On May 4, 2025, at 9:43 PM, Ethan Dicks via cctalk <[email protected]>
> wrote:
>
> On Sun, May 4, 2025 at 8:03 PM Bill Gunshannon via cctalk
> <[email protected]> wrote:
>> If you have a free serial port there is always an old GPS receiver
>> like A Delorme Tripmate. They show up on eBay all the time.
>
> Sure. We used to use rackmount GPS as time standards all the time for
> scientific experiments in the 90s and 00s. For those, though, we had
> to run antennas to the outside of the building.
>
> If you have a spare serial port (easy with a DLV11-J) a Hayes
> Chronograph can work, but those are hard to find and expensive now.
> Easier to emulate that with a modern MCU and a DS1307 where the MCU
> implements some parsable serial command scheme like the Chronograph
> does.
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
