UK Vintage Radio Repair and Restoration Discussion Forum

UK Vintage Radio Repair and Restoration Discussion Forum (
-   Vintage Computers (
-   -   MK14 schematic revisions (

Slothie 13th Apr 2018 1:40 pm

MK14 schematic revisions
I'm in the process of making a replica MK14 (I know its been done before, but I've always wanted one and I'm using the exercise to learn/practice how to use Kicad). I've been comparing the schematics for the V1 and V5 boards I found on the interweb, and the main difference seems to be the circuit driving the PROM enable pin. Originally it was tied to A11, but now it's using the once spare NAND gate of IC17 to NAND the "RDS or WDS" line with the inverted A11 signal. This means that a Write to PROM space will enable the PROM outputs causing both the SC/MP and the PROM to drive the data bus.

OK, so before any time the A11 line was 0 this happened too, so I don't see how this is an improvement. I could understand if they gated with the RDS only, or am I missing something obvious?

Anyhow as a newbie on the forum I'd like to say hi and I'm enjoying reading through posts here!


SiriusHardware 13th Apr 2018 6:37 pm

Re: MK14 schematic revisions
Do you have a link to the late version of the diagram? I'm only familiar with the older version.

From what you say it sounds as though the mod partly addresses an earlier oversight, in so far as the PROMs should never have been activated solely when A11 was in one state or another, it should always have involved at least the RDS signal, just as you say.

It is slightly baffling that the design allows a write to PROM space which ends up activating the PROM outputs. Maybe the thinking was just that nobody would ever try to write to a PROM address, as that would be an illogical (!) thing to do.

I've read somewhere recently that the late(st?) issue MK14 board included some kind of improvement to the address decoding - On early through middle issues there are multiple images of the RAM, the PROMs, etc throughout the address range because the address decoding is only partial. In particular, the PROMs appear no less than 4 times in the first part of the address range.

So maybe this change - adding WDS and RDS to the enable signal for the PROM block - is used in conjunction with a change somewhere else which stops the PROMs from appearing as multiple images throughout the first 2K of the address range and frees up some of that range so that more RAM can be mapped into it - in which case it does make sense to have the WDS signal involved as well, as the chip select for RAM will obviously need to be activated by a write signal as well as a read signal.

Slothie 14th Apr 2018 4:22 am

Re: MK14 schematic revisions
I can't find where I got it from but heres a link to an image hosting site (its just too big to attach to a post!)....

SiriusHardware 14th Apr 2018 10:44 am

Re: MK14 schematic revisions
...I'm away from base just at the moment, but just before I came away I had a hunt through my archive of MK14 related stuff and found a single sheet document, sent at my request from S.O.C. describing fairly extensive modifications for early issue MK14s like mine, the purpose of which were to remove the unwanted PROM images from the address range 0200H to 07FFH.

If these mods were done, it was then possible for the user to map 1.5K of added-on offboard memory into the address block no longer occupied by the PROM.

On reflection, this is probably one of the rarest original MK14 documents around, so I'll scan it and post it here - my scanner is so old that it only works with Win XP, so I'll have to drag out another semi antique in order to do that.

My thought is that the late(est) issue of MK14 PCB may actually have incorporated these mods. The simplest way to find out is for someone with an issue V machine to have a look through 0200H-07FFH and see if there are PROM images present there.

Slothie 14th Apr 2018 5:40 pm

Re: MK14 schematic revisions

...On reflection, this is probably one of the rarest original MK14 documents around, so I'll scan it and post it here - my scanner is so old that it only works with Win XP, so I'll have to drag out another semi antique in order to do that..
I'd certainly like to see that when you get the time!

I think you are right that this difference is to prevent the PROM from being enabled every time A11 goes low, possibly because it might interfere with the VDU board, and they used the "read or write" line because it was there and the inverted RDS wasn't! I also noticed that the two spare AND gates are marked "These gates are used on rev 5 boards" so maybe the schematic is of rev 4 and the 2 gates were used to more completely decode the PROM address as you described. Its odd they didn't show them on the schematic when they added this remark but it was Science of Cambridge after all ;D

I think that clears up the mystery for me at the moment but as I said I'd love to see that letter when you have time.

Ian aka Slothie

SiriusHardware 16th Apr 2018 1:07 am

Re: MK14 schematic revisions
1 Attachment(s)
Hi Slothie - thanks for the link to the later version of the MK14 diagram, which is a new one for my collection. I didn't see your post #3 until after I had posted #4, due to your posts currently being delayed by new member moderation.

So here it is (Merry Christmas...) a scanned copy of Science Of Cambridge's official modification to early issue MK14s to remove the unwanted PROM images from the 0200-07FF address range.

I think the background to this was that I had written to them asking for details of how to expand the memory beyond its already maxed out 640 bytes, and they sent me this sheet with the handwritten comment scribbled across the top.

The first drawback (for anyone hoping to incorporate these mods into a replica) is that they are not shown in circuit diagram form, so reverse engineering the final diagram of the PCB as modified will involve looking at these physical mods and applying them to the early version MK14 circuit diagram until you have, in effect, the issue V circuit diagram.

The document does not go so far as to describe how to fit extra memory, it only describes how to free up some of the memory range so that extra RAM could be mapped into that range.

I never actually applied these mods to my own MK14: Intead, I uninstalled the VDU which was chewing up nearly all of the memory.

Slothie 16th Apr 2018 11:30 am

Re: MK14 schematic revisions
1 Attachment(s)
An additional difficulty is that SOC changed which gates within packages were used, so the pin numbers in the v2 schematic are not the same as in the note... perhaps they were referring to a v1 board!
However I've worked it out (please excuse the rubbish image editing!)
Basically the reset circuit is reworked to free up a NOT gate and the spare gates used to NAND the inverted A9,10,11 signals with RDS or WDS.

SiriusHardware 16th Apr 2018 1:06 pm

Re: MK14 schematic revisions
Fast work :)

I didn't have time to try to relate the mod info to the issue II diagram but it doesn't surprise me that the gate pin numbering does not match the mod document.


S.O.C. were fond of other stunts as well, such as ignoring the D0-D7 and A0-A7 order of the data / address pins on RAM ICs and tracking / using them in whatever order it was most convenient to do so.

This didn't affect the operation of the circuit (it doesn't matter if bit 0 of your data is stored in physical bit 6 of a completely different physical RAM location, as long as it is also recovered from bit 6 of the same RAM location) but it made tracing / faultfinding bus faults tiresome in the extreme.

So, will you now work these changes into your replica?

Slothie 16th Apr 2018 1:31 pm

Re: MK14 schematic revisions

So, will you now work these changes into your replica?
To be honest I'd worked out how they might have done it whilst not getting to sleep (insomnia isnt all bad!) before I read your document, but it confirmed my thoughts.

I'd like to roll these changes in, as I might as well build a rev 5, but I might need one of the AND gates for the RAM enable line, actual MM2111 ram chips are proving hard to find and the pin-compatible(ish) IM65X62 that I have managed to acquire from Poland requires the enable pin to be pulled low in read and write cycles. Unless I can liberate another of the inverters from the reset logic and invert the 'rds or wds' line, but I'm not sure the SCMP will like a slow rising reset pulse!
I might have to breadboard up a test circuit to check it can be made to work timing wise!

SiriusHardware 16th Apr 2018 2:17 pm

Re: MK14 schematic revisions
I see P2111-1 available for $6 each in the USA just now, but the postage is, as always, prohibitive.

Slothie 16th Apr 2018 2:40 pm

Re: MK14 schematic revisions
Yes, and he only seems to have one... I need at least a pair :)

It might just be a waiting game, it depends how patient I feel :)

SiriusHardware 16th Apr 2018 2:50 pm

Re: MK14 schematic revisions
Sorry, didn't realise that source only had one.

I also see 4 * AM9111, second hand and two different speeds. Not sure if these are fully compatible with MM2111 / P2111, although the AM9111 devices often have 'P2111' marked on them alongside the AM9111 number, as if to say 'equivalent to P2111'.

Slothie 16th Apr 2018 2:58 pm

Re: MK14 schematic revisions
The AM9111 has only one chip select, so would require the extra logic anyway, and its a 16 pin chip so wouldnt leave the opportunity to make the board compatible with MM2111 chips.

SiriusHardware 16th Apr 2018 3:14 pm

Re: MK14 schematic revisions
Sorry again... good luck with the project, anyway.

Edit: the AM9111s I've just been looking at are 18-pin, but I suppose the issue with the CE pins is a deal breaker anyway.

Edit #2 : Also two chip selects on the AM9111 datasheet I have just looked at. Was this chip available in two outlines?

Slothie 16th Apr 2018 3:39 pm

Re: MK14 schematic revisions
My fault, I looked at the wrong section in the data book, I saw the outline for the AM9112 :-[

Looks like the AM9111 would be fine, just need to get a reasonable price. Thanks for pointing this out!

Slothie 19th Apr 2018 2:18 am

Re: MK14 schematic revisions
1 Attachment(s)
Well I've got the schematic finalised and PCB layout made, and ordered the PCB's! Heres the 3D model kicad generated....

SiriusHardware 20th Apr 2018 1:02 pm

Re: MK14 schematic revisions
Nice! It's not so long ago that there were people wistfully wishing that someone would make some MK14 reproduction PCBs and now like buses, they all seem to be coming along at once. (You haven't said that you plan to sell any of your PCBs but at least two other sources recently started to sell replica MK14 PCBs).

Like yours, my (original) MK14 has a male pin-row connector fitted where the display would be and the display has been reconfigured that it can be plugged into / unplugged from that. The flat 0.1" spaced 'stiff' ribbon cable originally used for the display seems quite hard to come by now in any case.

I think I would have allowed myself the luxury of nicer keyswitches, but I can tell you that any kind of keyswitch at all is a huge improvement over anything which S.O.C. originally fitted to the MK14. That's why so few original MK14s have their original keypads.

Slothie 22nd Apr 2018 6:27 am

Re: MK14 schematic revisions
I hadn't really thought about selling PCBs but if it works (!) I'll probably release the design files on Bitbucket in case anyone wants to make alterations or improvements.
As for the key switches I remember well the original keyboard on the MK14 I used at school, and since I actually would like to use it I thought the key switches where a good idea, especially since I have loads of them I bought on eBay a few years ago! I also have a lot of those round metal dome.switch contacts so I have made a footprint that will allow tbose to be used if I want something more authentic looking that might actually be usable, assuming I can work out how to make a flexible overlay and the black plastic cover the keys had!
As for the display connector I confess I shamelessly stole that idea from you :) it also leaves a row of holes in the PCB that could be used for ribbon cable etc. Strangely enough this weekend I was clearing out the loft of my mothers house as she's moving to a retirement home, and in a plastic tub of ancient transistors there was an eight inch length of ten way .1" solid core ribbon cable that I'd stripped out of something years ago so my hoarding instinct has paid off again!

SiriusHardware 22nd Apr 2018 11:02 am

Re: MK14 schematic revisions
By 'nicer' keyswitches I didn't mean 'more authentic' as in dome switches, I meant 'nicer to use', like the high-end full travel 'Cherry' type used in the older computer keyboards. Those modern tact-switches tend to have a rather 'hard' and unyielding feel, with limited travel. If you take one apart you'll find it is just a self contained dome switch.

If you did want to try making the keypad more authentic by using dome switches, some of the hard work has already been done for you as one of the other individuals making replica PCBs has gone so far as to have some keypad overlays similar to those used on 'dome switch' MK14s made. (This was already discussed in other recent threads regarding the MK14, so I refer you to those).

Pretty much all of us here are incurable hoarders, so we all share in your triumph over finding that bit of cable. You've gone to a lot of trouble to keep the machine reasonably authentic by using hard to get RAM when it might have been easier to use a slightly later, larger 8-bit wide SRAM, so perhaps you should also install your display on ribbon cable to keep it looking original.

Back in the day, we often used to put our MK14s in enclosures and for that reason it was common, some time after the machine had been built, for the display to be removed and extended on wires / ribbon cable to a more ideal location on the enclosure. Mine went through several phases of being installed in various enclosures until about ten years ago, when I decided to revert it back as far as possible to its original self contained single-board format. The only component still mounted off the board (and on a heatsink) is the regulator. A fully populated MK14 typically draws (from 5V) around 0.5A, nearly half of which is taken by the PROMs containing the operating system.

Slothie 22nd Apr 2018 8:42 pm

Re: MK14 schematic revisions
Well I can try out several different ideas because the minimum order on the PCBs was 5 (which I was pleased to find cost just over 18 inc shipping from JLCPCB, I think I got first order discount on shipping). I've only got 1 SC/MP and set of PROMS but I can (carefully!) swap the components from board if I'm experimenting. I was looking at "proper" keyswitches but couldn't find any (in an admittedly brief search) that looked nice enough to warrant the extra cost. Another option would be to "recycle" an old cash register or similar, but finding one with separate keyswitches or a 5x4 block would be a challenge as would relabelling them.... In the first instance I'll try the tact switches since I have them, they have clear plastic covers I can slip a printed label under, and I'll be impatient to test it!
Since I didn't have an original pcb to measure, I ended up measuring from photo's and using the IC pins for scale, so its probable that any "off the shelf" overlays etc wont quite fit.. although that doesnt rule that out should I make a second version of the pcb!!

All times are GMT +1. The time now is 10:25 pm.

Powered by vBulletin®
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Copyright ©2002 - 2020, Paul Stenning.