> NMEA (probably ZDA only)
Mimicking a NMEA ZDA sentence is fine. It's both simple and familiar. It
even includes an optional checksum.
> IRIG timecode
This would be a fun Arduino project. And it would allow you to drive all
those NASA-era IRIG displays that you've bought on eBay ;-) On the PC
side you would probably use a sound card to receive the signal. Read the
NTP docs. Also google Arduino IRIG for a list of existing projects.
> SMPTE timecode (this too?)
This would be fine too. There's some old PC code to generate SMPTE in my
www.leapsecond.com/tools/ directory.
> Are there any other obvious candidates I missed?
If it were me I would just use a 9600 baud ascii string in ISO 8601
format. For example, right now it would output "2019-08-07 16:29:56\n".
Very readable, for both human and computer.
Another method I've seen used by telecom GPS receivers is to output unix
integer time; right now it would output "1565195396\n". That's even
simpler (for computer) but not as self evident (for human).
Note that in any of these solutions you have to decide if the timestamp
refers to the previous or the next 1PPS. It's not always a simple decision.
> How did e.g. HP atomic clocks tell their time to connected devices
They didn't. Almost all "atomic clocks" are just precise frequency
standards that output, for example, 10 MHz. As such they don't know what
the date and time is. Some other external h/w or s/w deals with all that.
Some exceptions are hp 5061 (Cs) and 5065 (Rb) with option 01. That adds
a 1PPS output and a 24h clock display to the front panel. But it's up to
the user to manually set it (knobs or push-buttons) and there's no
computer interface to output the time; it's visual only. The 1PPS
alignment is done with a BNC input.
Another exception is the 5071A. It too has a 1PPS output and a 24h time
display. Serial / SCPI commands or the front panel keypad are used to
set the date (MJD) and time and to enable the BNC sync input.
/tvb
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