Looking for a Readable Version of Cromemco ZPU Schematic REV 4

[Joe Corda]

Greetings

I'm look for a readable copy of Cromemco ZPU Schematic Rev 4 that I believe is a close match based on parts lists to my board I attached pictures of.

So far all the schematics I found are unreadable or readable but for incorrect revision board.
Thanks
Joe

[Martin Eberhard]

The schematic in this manual is split in half, but is pretty readable:

-Martin E

[Joe Corda]

Martin .. 
Thanks for the reply !
This doc is labeled exactly as I requested  and is readable but sadly it doesn't match my board.   So the manual I was working from must have the wrong schematic attached.
My board doesn't have items R18/R19 in the parts list or on the board.. If you look at the parts list (page 9/10) in this document I link you it stops at R17 (just like my board. 
I also just found on back of my board label REV C
Joe

[Roger Hanscom]

Hi Joe,

I have a very clear schematic, but it is marked rev.E (6/1981).

Be aware -- I've had trouble in the past with revs. prior to "E" not working in some backplanes.  They seem to work OK in Cromemco boxes, but not so well in ones such as Integrand.

Roger

[Joe Corda]

Roger...
If you don't mind please send me a copy of your schematic and I can compare to my board .. it would nice to have it just in case I come across that version of ZPU in the future. Also I will keep in mind your comment concerning ZPU and back planes..  

As for my back plane options I'm currently testing the ZPU in a N8VEM (9) slot back plane I soldered up (nice open setup easy to move stuff around). But ZPU final home will be in a Cromemco Z-2 with 21 slot Blitzbus which I just completed a basic repair and restore on.

So the reason I requested a readable schematic was I have found an issue with the ZPU not resetting when reset is pushed.  Since you mentioned  back plane issues,  tomorrow I will give the ZPU a try in the Z-2 and see if it resets properly there.

Thanks for you help !
Joe

d

[Roger Hanscom]

Here 'ya go, Joe --

Hope you can read it.  My scanner (HP) is kind of crappy.  *frown*

If your ZPU is going to work at all, it ought to work just fine in your Z-2!

Roger

[Mike Stein]

Hi Roger, 

interesting that your schematic is Rev, 5 but also for the Rev. E board like Joe's Rev. 4; two different schematics for the same board? I'll have to look into this when i get back home.

There are at least three different manuals; I wonder where Joe got his Rev. 4 schematic?

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[Mike Stein]

Yeah, that's what it looks like, that Rev 5 is just a correction to Rev 4 of the Rev E schematic; I wonder if it includes any of the mods.

In any case I think Joe is looking for a Rev C board schematic; there is a manual older than the Sep/78 version that included the Rev 5 schematic but I don't see a schematic in that older one. Definitely confusing.

On Sat, Aug 30, 2025 at 1:03 PM Ken Lesniak <kjle...@gmail.com> wrote:

In my opinion, the schematic Rev's are just corrections to the schematic
and not board changes. I can't prove that though, so take it for what
it's worth.

Ken

On 8/30/2025 12:51 PM, Mike Stein wrote:
> Hi Roger,
>
> interesting that your schematic is Rev, 5 but also for the Rev. E board
> like Joe's Rev. 4; two different schematics for the same board? I'll
> have to look into this when i get back home.
>
> There are at least three different manuals; I wonder where Joe got his
> Rev. 4 schematic?
>
>
> On Sat, Aug 30, 2025 at 11:54 AM Roger Hanscom <norwe...@gmail.com
> <mailto:norwe...@gmail.com>> wrote:
>
>     Here 'ya go, Joe --
>
>     Hope you can read it.  My scanner (HP) is kind of crappy.  *frown*
>
>     If your ZPU is going to work at all, it ought to work just fine in
>     your Z-2!
>
>     Roger
>
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[Joseph Corda]

Thanks Roger 

The scan is perfect..  btw the board did reset properly in the Z-2

Still trying to get it to access the ROM.. but step closer 

Joe 

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[Joseph Corda]

Mike

Here is the link to the Zpu manual I was looking using 

http://www.bitsavers.org/pdf/cromemco/Cromemco_ZPU_Rev_E_1976.pdf

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[Mike Stein]

Don't know how I missed that; I did look at that manual but somehow missed the schematic.

Thanks!

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[Roger Hanscom]

Hi Mike!

>> In any case I think Joe is looking for a Rev C board schematic; there is a manual older than the Sep/78

>>  version that included the Rev 5 schematic but I don't see a schematic in that older one.

Yep.  I've got a much older version of the manual (not the slick cover -- yellow instead) dated July 1980.  BUT, unfortunately it doesn't include a schematic either.  *frown*

 >> Definitely confusing.

You can say that again!  So many rev.s, but which ones match the manual schematic?  Or are there any schematics for them at all?

Roger

[Peter Higgins]

Joe - your rev C board might have been the first series of production boards, and perhaps was soon replaced by revisions that incorporated fixes identified after the rev C board came out hence few of them exist. Since the schematic for your rev C board seems to be unavailable, could you find some time to trace out the circuitry leading from the S100 pRESET input through IC11, where your board is missing R18 and R19 so it is available for future interest? Likely R18 and R19 were added to improve the responsiveness of the reset circuit. I also note that R18 (initially spec'd as 1K) was changed to a higher value resistor in the final "F" revision of the board, and according to Cromemco's documentation this was done to reduce an "overheating" problem in IC11.

[Joseph Corda]

Peter 

I will see if I can trace/ dcument the reset flow  and share .. I’m kinda busy these dayd so it will take me some time to get to it 

Joe

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