Technical questions about the cpc6128 schematics

[orzel]

What i'm really trying to understand is the address signalling in the amstrad cpc 6128 (not +). That seems the most difficult part of the schematics to me.
 
1) I understand the pal has no memory. It also has as input data bits 0,1,2, 6and7. I suppose for the MMR "register". I understand 6and7 is used to make the difference with
 the gatearray. But how is the information about this "register" kept once written ? The gatearray surely doesn't store this, and anyway doesn't route any relevant signal.
 
2) (might be related?) the pal also outputs x1,x2,x3, not connected in the cpc. Are they unused ? or used in some other hardware ? Or only here to be used as (feedback) inputs for the other outputs ? (outputs can be used as input in a pal).
 
3) At first it seems we do have cpc 6128 schematics. But after checking, i've only found two such schematics :
 
- (bad) photos of what is found on the service manual, which covers only one version, and is known to be incomplete and/or with errors.  
- the amazing work of 'pelrun' recreating the version 2, which is different.
 
Seeing how much work has been done in understanding the hardware otherwise, i'm really surprised not to find anything else. Not even a 'corrected' or 'annotated', or just
 'better resolution' of the one in service manual.
 
Did I miss something ??
 

[McArti0]

x1, x2, and x3 are signals that are the outputs of the D-latch flip-flops implemented in the PAL. They store the mixer settings for the 64KB main and 64KB secondary memory.

x1, x2, and x3 are used inside the PAL.

D6&D7 are signals that establish the PAL setting as byte &C0+b2,b1,b0. The PAL output produces new values AD15 and AD14, which are sent to the LS153 multiplexers.

[orzel]

Oh ? So there's actually some memory in the pal ?

I didn't think it was possible to have a flip-flop, i thought the pal was only combinatorial, mixing inputs with and/or to create outputs.

My understanding was that pal16l8 are only combinatorial, while pal16r8 have an added flip-flop on each output. But the CPC uses the pal16l8.

[McArti0]

The three D-latch flip-flops are MADE of simple gates.

You have feedback acting as a latch

Q0    = (  D7ANDD6 & nRESET & D0 & !A15 & !nIOWR
        #  !D7 AND D6 & nRESET & Q0
        #  nRESET & A15 & Q0
        #  nRESET & nIOWR & Q0 );

[pelrun]

My understanding was that pal16l8 are only combinatorial,

While there are no discrete "flip-flop blocks" in a PAL16L8, that doesn't mean you can't make them. All that's needed is the ability to feed one or more outputs back in as inputs, and even the non-registered parts have that.

[orzel]

Ok, I understand now, thanks a lot.

[orzel]

What about point 3 ? No more/better information about the original schematics ?

[pelrun]

The official schematics may be incomplete, but they're basically correct. And the things they're missing have to be functionally compatible with the original versions, so alongside the appropriate datasheets and looking at a real board is enough to understand what is going on. It's not like the CPC is particularly complex.

[Bryce]

What about point 3 ? No more/better information about the original schematics ?

The information available in the Service manuals and amendments document ( https://www.cpcwiki.eu/index.php/Schematics ) is enough for any required diagnosis or repair. What benefit would you get from a few known annotation mistakes being corrected and a slightly less blurred picture?

Bryce.

[orzel]

Complete schematics: I would get something reliable, that i can be confident with. I would save time searching the net for fixes or missing information.
Better quality image: there are some text i can not even read, especially on the disk interface schematics.

There's a lot of knowledge, but it's spread over, not gathered in one place.

I'm certain it's enough for people "in the know", who does repairing, and have done so for twenty years. But i'm new to this, and repairing is not what i'm interested in.

[McArti0]

Is resistor R112 working?
What do you see on your screen?
Does the computer with a different ROM work?

[orzel]

(answered on the other topic)