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

[Magnus Danielson]

Hi,

Yes, you need to modify it to operate with RS232 levels. If one goes to 
the now more acient annals one find the instructions. Some of the 
resources may be on the brink of dropping of the web.

It would be good if the many scraps of material was supplied on a common 
place, such as a common Wiki where we could cover the many topics and 
update common resources.

I remember I had to modify up my Z3801A to get RS232 levels and then 
solder up a suitable cable. I now have one with a failed power-supply 
and have not attended to that. Maybe in due time.

Cheers,
Magnus

On 2021-10-10 18:08, paul swed wrote:
> I believe it is as thats the alternate strap on the z3801. Its also the way
> z3801s arrives as RS 422.
> Regards
> Paul
>
> On Sun, Oct 10, 2021 at 11:14 AM Joseph Gwinn <joegwinn at comcast.net> wrote:
>
>> On Sun, 10 Oct 2021 03:30:24 -0400, time-nuts-request at lists.febo.com
>> wrote:
>> time-nuts Digest, Vol 210, Issue 7
>>
>>> Date: Sat, 9 Oct 2021 15:18:08 -0700
>>> From: ed breya <eb at telight.com>
>>> Subject: [time-nuts] Re: HP Z3801A info needed - 1PPS interface
>>> To: time-nuts at lists.febo.com
>>> Message-ID: <61621520.9050200 at telight.com>
>>> Content-Type: text/plain; charset=utf-8; format=flowed
>>>
>>> I have the 1 PPS circuit working just fine. The pulse width is around 27
>>> uSec, nice and flat and strong regardless of the termination. I can't
>>> discern the rise time or prop delay yet.
>>>
>>> I discovered an interesting thing about the 1 PPS signals from the DB-25
>>> connector. They are (or rather, one of them is) rather odd in voltage -
>>> not PECL, except under certain conditions.
>>>
>>> I hooked up one of the 1 PPS outputs to the circuit, just with a pair of
>>> wires. This gave me a chance to make some measurements out in the open.
>>> The comparator circuit worked fine, and once I got a good view of the
>>> result, I started looking into the details. The first thing I found is
>>> that the quiescent "low" value of the "1 PPS_1-" (J3 P17) rests at about
>>> 2.5 VDC - not PECL at all. The high side "1 PPS_1+" (J3 P9) seemed about
>>> right, near 3.9 V. Uh oh - I thought maybe the port is damaged. I double
>>> and triple checked the connections (they were right), then tacked some
>>> wires on the number two port, pins 8 and 21.
>>>
>>> They behaved exactly the same, so probably normal - or both burned out
>>> the same way. So, I figured there must be some logic to this big
>>> asymmetry. It couldn't be terminations to ground, since the 2.5 V one
>>> could only go lower, so differential is the only kind that makes sense.
>>> I tried various values across the lines, and sure enough, the 2.5 V
>>> level rose substantially with decreasing R, but did not reach a "proper"
>>> PECL low level until the differential load was around 50 ohms. The high
>>> side changed only a little, indicating it goes right to the output of an
>>> ECL part - if it was reverse terminated it would have dropped much more
>>> with the loading.
>>>
>>> So, it looks like these lines are connected to the outputs of ECL parts
>>> (run as PECL), or maybe a simulation from some other kind of circuit. If
>>> you picture each line being the emitter output, the high one is on most
>>> of the time, and of proper level, You'd think the low one should still
>>> hold at PECL low, at some current into its load, but it doesn't. It
>>> could be that its load is made heavier, and to ground, on purpose,
>>> drawing it down more. If it were terminated into a proper terminator
>>> supply, it should be 2 V below Vcc, or 3 V in this case, so it couldn't
>>> go to 2.5 V. Anyway, I understand what it's doing, but don't see why it
>>> was made this way.
>>>
>>> Just in case, I checked these levels under different conditions - fresh
>>> power-up, locked, and hold modes, to make sure the common-mode levels
>>> aren't changed for external signalling of conditions. They were constant
>>> in all conditions.
>>>
>>> Then I checked the signals on all the lines with a scope, directly
>>> through coax. I tried a few different termination Rs, as shown below,
>>> with the results.
>>>
>>> When the pulse goes active, the high side drops, and the low side rises,
>>> to roughly the same as the DC levels, so only the terminator value and
>>> end levels are needed to get the picture. Remember, these are
>>> approximate, from eyeballing a scope trace flash once a second.
>>>
>>> Open circuit 3.9/2.5
>>> 221 R 3.8/2.5
>>> 100 R 3.7/2.6
>>> 75 R 3.7/2.8
>>> 47 R 3.7/3.2
>>>
>>> So, there's plenty of signal under all conditions, and I think it's just
>>> a matter of picking a termination for whatever cable is used. I was
>>> quite surprised by this oddity, but it seems to work fine with my
>>> circuit no matter what.
>>>
>>> BTW the two 10 MHz outputs there are also described as "pseudo-ECL," so
>>> I'd imagine they have the same characteristics. I'll take a look when I
>>> get a chance.
>>>
>>> Ed
>> Can this be RS-422 from a 5-volt source?
>>
>> .<https://en.wikipedia.org/wiki/RS-422>
>>
>> Joe Gwinn
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