Hi,
I've just bought two displays ("clocks") for a central clock system, you know the kind that was popular in schools and other facilities. They're supposed to be driven from a central clock giving a 24V DC pulse at the start of every minute, with the added twist that the polarity is flipped every time (ie. minute 1, a +ve DC pulse, minute 2 a -ve pulse, etc).
The simple idea is a R
π with two GPIOs pulling two different transistors depending on even or odd minute, and running from NTP. That's OK. Probably better than 90% of such systems in their heyday. (We still have one at work, though..)
Now, I've got a Meinberg GPS receiver, with 1PPS out. Obviously I want it to trigger the pulse, without being fuzzed by non-real-time control in a multi-user OS. (that the clock displays are mechanical and likely have significant delay from trig to movement, is beside the point!)
If I really want to do this the hardcore way, I'd do a /60 divider in logic circuits, top it off with a state storage that selects +/- and that would be it. That is all fine and dandy, certainly has a certain
je ne sais pas to it, but there's a problem here, and it of course is that such a circuit can't in and of itself know when a new minute begins: It would, even if manually timed at startup, not be safe from leap seconds.
My current idea is to combine the two approaches;
- The Rπ is responsible for knowing when we're in a window of say +/- 400ms from a minute start. It also remembers if it is time for a positive or negative pulse. It will be timed to the NTP server in the Meinberg. It will select polarity and enable the pulse from the Meinberg during the window around the full minute, but not decide further than that.
(To keep things suitably vintage, part of me wants to use relays to do the enabling) - The Meinberg will still have final say over when to trigger, by a combination of opto-couplers and transistors.
So, how far out am I? Has anyone done something similar? Suggestions? (The argument of diminishing returns is of course relevant, but the process in itself is interesting enough, IMNSHO.)