[time-nuts] HP Z3801A info needed - 1PPS interface

[ed breya]

My plan has been to go with it being a 50 ohm source. My go to driver 
setup for these sorts of things is a 74HC/AC 500 series octal bus 
device, so you get all the ins and outs across each row of pins - easy 
to parallel.

For now, it's looking like a 74HCT541, with the outputs ballasted with 
series 220 R, contained in a SIP R package. Depending on how many outs 
will be used, this gets it into the 30 ohm ballpark, not including the 
device output resistances. The rest of the 50 ohms is a PTC, about 10 
ohms, plus a tweak resistance selected to set it about right overall. 
So, it will deliver about +2.5 V into 50 ohms, or 100 mA into a short. 
Any external fault input applied will get clamped behind the PTC and 
tweak R, and the PTC will limit the current. The HC541 will be in a 
socket too - just in case - and allow for experimenting with AC541. 
Also, because of the possible high output demand, this section will run 
on a shunt regulator. like I did long ago for the digital clock LED 
supply to eliminate pattern dependent loading. The same goes for the 
circulation fan ripple current - I just finished that part yesterday.

Presuming this "pseudo-ECL" is just PECL, I'll be using a comparator to 
shift it to TTL/CMOS. I have some nice high speed ones like LT1016, but 
unfortunately it won't quite work out, PS-wise, without more complexity. 
After looking around, I determined that the good old LM311 will do the 
job directly, but maybe ten times slower, unfortunately. I'll probably 
end up using an HCT541, just to get the threshold down a bit more for 
better matching with the LM311. I'm really cramped for board space, so 
simple and compact is the goal, even if it could be much better. I'm 
still thinking about a possible discrete circuit, but it's hard to beat 
just one comparator IC for space.


BTW, this unit does have the RS-232 mod, which I did many years ago. I 
think the last time I had it hooked to a PC and talked to it was around 
2008. Sometime around then is when I decided to rebuild it into the 
5065A carcass (I think I've mentioned this before). The original Z3801A 
layout is hardly recognizable, since I had to make extreme mechanical 
changes to cram it in there. I have to plan carefully which things I do 
together, to minimize the number of times I have to take things apart 
for access.

So anyway, this 1 PPS thing is up now, next will be the clock, then 
communication, then the battery system (two 12 V 12 A-hr SLAs built in), 
then the control panel, clock setting system, instrumentation, thermal 
control, etc - it goes on and on. Every so often, I get a burst of 
enthusiasm and get a bunch of things done on it - but certainly not all, 
so it will wait until next time. At least this time, I think I'll be 
able to shove all the loose ends inside, and put the covers on.

Ed