I found it! A very sorry looking MK14.

[SiriusHardware]

That mod is for the addition of the single-step feature (as you have said) because the revised / new OS has the support firmware for it built in. The memory hole mod may coincidentally have needed similar mods. If you don't fit the SS hardware but you do fit the 'new' OS, you need to take SENSE-A to 0V otherwise programs will not run.

Right back at the beginning of Slothie's epic 'Schematic Revisions' thread I posted the instructions SOC sent to me when I enquired about how to add extra memory - they don't explain how to add extra memory but they do explain how to free up the memory hole. Not sure if it has also been posted in this thread, if so I could not find it.

https://vintage-radio.net/forum/show...13&postcount=6

Problem is they only explain it from a practical / hardware level (cut this, join that) rather than at circuit diagram level, so I can't say whether the IC pin numbers referred to are correct for an issue II PCB. I actually never did the mod, instead, I removed the VDU which was chewing up nearly all of my memory and didn't use it again until Slothie's issue VI PCB rode to its rescue.

Agghh, just noticed Tim reposted it a few moments ago.

[SiriusHardware]

I do have an actual issue II here of course - I'm not going to be removing anything from the PCB at this stage in its life but happy to follow or buzz out any connections you may need to know about. - It's in storage at the moment though so would have to wait until tomorrow now.

[Buzby123]

I think I duplicated the required gate arrangement using new gates on the daughter board, not re-using chips on the main board. This would mean I didn't need to make the reset switch mod.

Looking at my notes the new memory map was :

0700 Extra RAM on daughter board
0600 Extra RAM on daughter board
0500 Extra RAM on daughter board
0400 Extra RAM on daughter board
0300 'OLD' ROM on daughter board
0200 'OLD' ROM on daughter board
0100 ROM on main board
0000 ROM on main board

( It looks like I made no changes from 0800 upwards, well, not intentionally !. )

Cheers,

Buzby

[SiriusHardware]

What was the rationale behind having the old OS at 0200-03FF, do you remember? I had imagined you must have had them both mapped at 0000-01FF with the chip enables switched either-or in some way.

[Buzby123]

The idea was that I would use the 'old' ROMs to check the circuit worked, then replace them with custom ROMs at some time in the future, but that never happened !.

[SiriusHardware]

You've never mentioned having the official PROM programmer, I've never come across anyone who did - it must be one of the rarest items of original MK14 hardware around.

Ironically I do now possess both a capable programmer and a small stash of MK14 compatible PROMs, but even now I can't think of any application which I really need to commit to PROM. I think the other idea you implemented, battery backed RAM, must have made the need for custom PROMs more or less redundant. That's one mod which probably would be worth reinstating eventually.

[Buzby123]

I never mention the official PROM programmer, 'cos I never had one !,

By the time I'd got round to programming EPROMS I'd left Micky behind, and moved onto ZX81.

( I built an ultra-simple programmer for the ZX, I just wish I'd kept that as well. )

[Buzby123]

Back to Micky.

I know SoC didn't put pin numbers on their circuit diagram, but did they also scramble the ABCD notation of gates in a package ?.

Just checking address lines A8 and A11 to IC 16.

On the board A8 is to pin 13, A11 to pin 12. SoC have shown these as inputs to gate A.

Usually these pins are labeled belonging to gate D.

SoC are not making it easy !

[SiriusHardware]

If you feel like re-drawing the circuit diagram of the issue II the way it really is with a modern circuit diagram package, I for one would find that useful. Maybe you could start with Slothie's issue VI and roll it back to issue II.

It would be interesting to know what proportion of the total output of MK14s were issue II, III, IV, V respectively. I don't count issues 0 or 1 as I have never seen one.

Issues II, IV and V each seem to have had respectable runs, issue III not so much.

[ortek_service]

Well it's normal convention to letter-label gates in ascending pins used order. But it does seem SoC have just assigned letters from top to bottom on (Iss.4? / 5? at least) schematic, that does have pin numbers (unlike earlier 'Remove from Book' Issue 1? / 2? one in the V1 manual).
I've not found any other versions on the 'net so far (But I presume SoC wouldn't have changed the connections of the existing circuit unnecessarily, and just added the odd extra).

And it seems SoC had chosen which gate to use on PCB connections, based on what made tracking to it easier, so often made their schematics difficult to tie-up with the PCB if they weren't clear with actual pin numbers and standard labels.


Chris Oddy's replica (based on Issue 5, with PCB tracking kept as close to original as possible), schematic also has A8 is to pin 13, A11 to pin 12 on IC16 - which he's labelled IC16.4, seemingly using ICxx.(1-4) to avoid confusion with SoC's letter allocations:
http://theoddys.com/acorn/Replica%20...0Schematic.pdf

[Slothie]

When I was working out a schematic for my replica I came to the conclusion that SoC's schematics where more of a general guide than an actual representation of the circuit.

[ortek_service]

Originally Posted by SiriusHardware

If you feel like re-drawing the circuit diagram of the issue II the way it really is with a modern circuit diagram package, I for one would find that useful. Maybe you could start with Slothie's issue VI and roll it back to issue II.

It would be interesting to know what proportion of the total output of MK14s were issue II, III, IV, V respectively. I don't count issues 0 or 1 as I have never seen one.

Issues II, IV and V each seem to have had respectable runs, issue III not so much.

A few people now have re-drawn the MK14 (although mainly latest versions) on a modern CAD system, to make it more readable. With people like Chris Oddy trying to keep it looking the same as the original, with all the same designators as well as overall style of the schematic - but sometimes with some extra labels to make it clearer.
So could probably edit one of these, to make older versions. However it does seem that it's not very clear how many version did actually exist originally / what version they actually were, if PCB (and schematic) wasn't labelled with this.
I've only found two versions of the schematic on the 'net so far, and later one is rather confusing in notes that say spare gates are only used on issue V, but they don't connect them, so possibly indicating it is only for previous issue 4.
The other main one is from the V1 manual, so might have presumed that was Issue 1.

Then there is the issue that there seems to be uncertainty on how many in total were actually made. This: https://en.wikipedia.org/wiki/MK14
gives either 15,000 or a rather high 50,000 (half that of the ZX80)
Although ZX80's seem to be about as rare (I've only got the PCB from one, that someone has sawn the keyboard part off!) , compared to the 1.5m made ZX81's which were commonplace - I've got quite a few, and still often see others for sale at rallies etc.)

What would be very useful, is a list of all the exact changes between the various issues of schematic / PCB - I've not seen a definitive list, so far.

[ortek_service]

Originally Posted by Buzby123

I never mention the official PROM programmer, 'cos I never had one !,

By the time I'd got round to programming EPROMS I'd left Micky behind, and moved onto ZX81.

( I built an ultra-simple programmer for the ZX, I just wish I'd kept that as well. )

I'm surprised no-one seems to have done a replica of the PROM-Programmer, if they are even rarer than original MK14's itself.
There wasn't too much to one / hadn't cost too much, from original photo of one in advert here: http://theoddys.com/acorn/Acorn%20Sy...4%20Advert.jpg

And there is a copy of the manual for it, here: http://theoddys.com/acorn/Acorn%20Sy...M%20Blower.pdf

Although it would be nice to have a detailed dimensioned photo of PCB with tracking etc.

However, as it only programs relatively expensive & getting rare and one-time fusible-link PROM's, then one that also did EPROM's (like Acorn System one, that came from the same original MK14 designer?) and a converter for these back to PROM sockets might be more useful.

[Timbucus]

MK14 Man has an original (photo attached) and Martin Lukasec did a repro Board - I have all the components to build one here but, as yet no board. As you say a bit of a curiosity due to the difficulty of getting the 571 PROMS.

[Timbucus]

As regards the versions - this is the best images of Issue 3 I have located so far from keeping an eye on very blurred bay sales... I had an issue 4 from the same source but, a friend has an original and is looking at getting a better scan from the badly damaged image he has.

[Timbucus]

In fact scratch that we should switch to the Versions thread as I have just found a mislabeled one that seems to be the same as the blurred one - no idea on source my note taking was poor at the start.

[ortek_service]

Originally Posted by Timbucus

MK14 Man has an original (photo attached) and Martin Lukasec did a repro Board - I have all the components to build one here but, as yet no board. As you say a bit of a curiosity due to the difficulty of getting the 571 PROMS.

Thanks for info - particularly the good top-side photo.

I see the repro one has double-side tracking, with quite a bit each side. Whereas the original one only has a few top-side wire-links.
So I would have thought that it would have been easier to go for a single-sided PCB that was originally used for minimal cost / could probably do one at home without the silkscreen - especially if there's a photo if the original PCB's underside tracking (although should be able to work out something similar, from component positions)

I recall seeing VDU boards being around (and info for a replica one?)

But I've not seen much about the cassette interface. It's simply enough to knock-up on a bit of veroboard, but would be nice to have somethng a bit closer to the original, so any details on these would be useful).

[Timbucus]

Again Martin's site is the source of the VDU - here is a link to one of the articles with his repro Issue II - be careful to read as he ended up doing an Issue V but the top side picture may be useful.

https://www.8bity.cz/replika-mk14/ja...sinclair-mk14/

There is an English flag on the right had side which is pretty good translation.

The cassette interface page is harder to find as it is not on the index:

https://www.8bity.cz/2018/science-of...rface-replica/

The documents above are from 2015 in his blog that cover the early work on main board, VDU, cassette and programmer:

https://www.8bity.cz/2015/sinclair-s...om-programmer/

[Buzby123]

... But I've not seen much about the cassette interface. It's simply enough to knock-up on a bit of veroboard, but would be nice to have somethng a bit closer to the original, so any details on these would be useful).

You only have to ask .....

[ortek_service]

Originally Posted by Timbucus

As regards the versions - this is the best images of Issue 3 I have located so far from keeping an eye on very blurred bay sales... I had an issue 4 from the same source but, a friend has an original and is looking at getting a better scan from the badly damaged image he has.

Thanks - That 'Remove from Book' one, with 'Issue 3' hand-written on it, looks at first glance to be the same as one I already had found elsewhere and had come from 'V1' manual so I'd thought to be an earlier version.

One scanned-version I have, has IC12 & IC13 part numbers and function hand-written on it and I think I may have printed from a scanned V1 Manual, as another I found with blue pen writing of some IC types on it, is rather low-resolution (might have been due to site like this compressing it)
There is a version of just the schematic, amongst the many bits of canned info you can view here: http://www.computinghistory.org.uk/d...otype-of-MK14/
(But that copy has some punched-holes at the top)

Chris Oddy only refers to the 2 he has as early and later, here:
http://theoddys.com/acorn/Acorn%20Sy...Mk14/mk14.html
- Although it seems the links are currently broken, so I will let him know.