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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
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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
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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
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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!!

SiriusHardware 23rd Apr 2018 1:15 am

Re: MK14 schematic revisions
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I have to admit that sounds fantastically reasonable. Last time I looked at the possibility of having a PCB made in low quantities the cost was easily up into three figures.

This CAD package you've used to produce your project, is that freeware or commercial software, and for what OS(es) is it available?

Unfortunately I can't easily measure the key spacing on my MK14 as the replacement keypad is mounted millimetres above the main PCB and the wiring from it is tack-soldered onto appropriate points on the original keypad tracks and all hidden carefully underneath the keypad - which makes it difficult to remove the keypad far enough to perform measurements. (See attached image #1 - note the fairly haphazard alignment of some of the key legends, which were laid down using rub-down transfers and sealed in place with clear varnish).

I did this sometime early on in the life of the MK14. The keypad is not built on a PCB, but on ordinary stripboard / veroboard. To disguise that and make it look a bit nicer I placed a sheet of black modeller's 'plasticard' over the top of it and cut it to the same size before drilling holes to allow the pins of the switches to pass through it.

For the (as yet incomplete) Karen-O PIC14 emulator I used the same method of lettering the keytops but made a slightly better job of it (image #2), as I'm now older and rather more more patient.

As it happens the keyswitches I found for that emulator are nicer, with a better action than those currently fitted on the MK14 so I may consider swapping the keyswitches over to the keypad on the actual MK14 at some point.

Slothie 24th Apr 2018 4:10 pm

Re: MK14 schematic revisions
In the last few years prices for PCB manufacture have dropped hugely as China has become more accessible to the "hobbyist".
The CAD software I am using is KICAD, its a bit quirky and has a few irritating but not show stopping bugs, but its unrestricted and its used and developed by CERN so it has reasonable support. It is supported on Linux (which I use), Windows and MacOS. I used to use Eagle CAD but its 80mm x 50mm board size limit for the free/cheap versions made it unusable for any substantial circuit with 40 pin DIPs! To get bigger boards you needed to buy a 1000+ licence which was a non-starter. All I can say is that having climbed the steep learning curve and the quirky interface its been worth it. There are possibly better free or cheap options but I didn't have time to evaluate too many ;)
Thanks for even thinking of measuring the keyboard! I'm constantly conflicted between making an exact replica and a functional one. I suppose I'm trying to recreate the "look and feel" with perhaps period appropriate modifications to make it more usable (keyboard being the obvious example!) To be honest I'm hugely impressed with Karen Orton's PIC14 and it would have been good enough to satisfy my nostalgia until I stumbled across a SC/MP on eBay and this whole crazy project started... I love your PIC14, looks like you'll be able to take it on the bus or train to while away those boring trips!

SiriusHardware 24th Apr 2018 6:49 pm

Re: MK14 schematic revisions
Thanks for the info about KICAD. I run a mixture of Windows and Linux here so it's good to have the choice.

Actually the reason I asked is that I have for years been interested in making an MK14 -keypad- PCB, as the task of wiring up a bit of veroboard full of switches is incredibly tedious, especially given the weird nonstandard keypad matrix adopted by S.O.C. It would make sense for it to be laid out for at least two alternative types of switches, maybe cheap 'tact' switches and more luxurious full-travel switches - if it is even still possible to buy those now, and at sensible prices.

As you've probably realised keypads are the bane of the original MK14, with many original MK14s having faulty or missing original keypads. I would like to be able to make an easily replicable version of the 'floating' keypad I have on mine, with 12 blind pads on the underside at the right hand edge - these could either be taken directly to the PCB via descending wires underneath the keypad for a neater look as they are on mine, or alternatively they could be carried out to the right hand edge of the PCB via a 12-way 0.1" spaced ribbon cable going to a 12 way edge connector for a 'fast fit' solution, in which case you'd just fit the keypad with four bolts and spacers, plug the edge connector in on the right hand edge and <Bam> you have a working MK14 keypad again.

As brilliant as Karen's PIC14 absolutely is, I'm not sure that it has onboard nonvolatile program storage, nor have I measured the current drain - probably a bit too heavy for a PP3 (9V) battery - so I'm not sure that it would be truly practical for portable use.

I'm informed (although I have not seen it) that there is a very faithful MK14 emulator for Android - that might be more practical for the commute.

Slothie 24th Apr 2018 7:50 pm

Re: MK14 schematic revisions
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I've seen "Cherry" MX keyswitches for sale for ~$0.95 each, but as pricey as that is they don't include a keycap and those seem hard to find....

For my board I created a footprint for 12mm square tact switches (because I have some) and 9mm domes (ditto) - the latter would be stick down with kapton tape or similar and then covered with a silicone overlay/metal frame like the originals. The footprint was easy to make although its not possible to tell Kicad that the pairs of pins are internally connected which means the "design rule check" pulls up "unconnected pin" errors if you make use of this feature, irritating but as I said, not a show-stopper.

Where did you get the keys for the emulator?

SiriusHardware 24th Apr 2018 10:24 pm

Re: MK14 schematic revisions
Ah, unfortunately they were bought as part of a full QWERTY legended set for a couple of pounds at a radio rally (an amateur radio junk fair) in the 1990s, so they are not a repeatable purchase. I wish I had bought more sets at the price. They have a lovely light action, are proper switches and are designed to fit snugly, side by side, into a 0.1" matrix grid (like veroboard or similar).

If you look at original dome switch keypads like those on a cheap late 1970s calculator, the areas where the three 'feet' of the domes rest are quite broad / wide, smooth and heavily plated without any depressions or holes in them. The tape alone keeps the domes centred where they are supposed to be.

I think this is because when you press the centre of the dome it flattens and the three feet skid outwards a little bit each time. If that also happens on your keypad the relatively small contact areas surrounding the three holes will tend to suffer chew damage / wear and tear.

I have the keypad for a late 70s Texet 880 calculator which, sod's law, I can't find just now, otherwise I'd take a photo to show you how dome switch PCB pads typically looked back then. I think I have another at work, I'll try to remember to bring it home.

In any case, if you can fit actual switches (even tact switches) rather than PCB mounted dome switches, I would definitely do that. I would have been very grateful for that option back in the day, but S.O.C didn't include the option to fit proper switches until around issue IV or V of the MK14. The fact that they eventually did means it's perfectly OK / perfectly authentic for your replica to have the option to fit proper switches as well.

I have (hoarded, naturally) a set of 20 grey keycaps with a large square hollow in the base of each cap which would very likely fit those cherry switches if they have a square topped (rather than cruciform topped) actuator: I just don't have the switches to put them on. :)

Mine are all blank, but until quite recently (sometime in the last ten years or so) RS sold sets of 20 of the same or similar keycaps legended with 0-9, A-F plus four blanks, ideal for an MK14 keypad. I only found that out about a year after they were discontinued.

Slothie 25th Apr 2018 3:03 am

Re: MK14 schematic revisions
The dome switch part of the footprint was mostly experimental, since I had them after buying them on a whim from china for next to nothing... I took apart an old calculator as a teenager and saw how they were put together, but I dont recall how the PCB looked. The dome switches I have have small "legs" that locate in the hole,and (hopefully) the hole is big enough to allow them to skid enough, but its highly probable wear will be an issue on reflection.
In reality I'm going to use the 12mm switches unless the dome switches work incredibly well :)
I have discovered Rapid Electronics sell the D6 style switches (like the reset switch on the original MK14) which I believe from photo's I've seen are the type later MK14's were drilled for, I used the round-topped variant in an alarm clock and they were very positive, although not as good as a cherry key! They have the square top type SOC used, but there used to be clip on caps which they no longer sell. If I'd known that at the time I'd probably used that for the second footprint (all 3 would be a bit of a squeeze!)
If I ever invent a time machine I'll add those RS key switches to my (long!) list of things to buy and get you a couple of sets too!

Slothie 25th Apr 2018 2:45 pm

Re: MK14 schematic revisions
The PCBs arrived! They look great, first news is that the dome switches are not an option bcause they don't fit :-] Still the tact ones fit nicely so its resolved my dilemma and saved me having to make keyboard parts!
I'm still waiting for some parts to arrive and I need to get some 74 logic chips when I get paid in a few weeks (this months budget spent !) but at least I can get started. Just need to remember what I did with all those IC sockets....

SiriusHardware 26th Apr 2018 8:58 am

Re: MK14 schematic revisions
That was fast! I look forward to seeing a completed machine.

When my MK14 kit arrived I tried everywhere locally for 18-pin sockets and could not find any, at which point my patience ran out and I built it anyway - to this day, it still has a mixture of socketed and not-socketed ICs.

I took a look at that ex-calculator keypad I mentioned - as I recalled the pads for the three 'feet' of each metal dome were broad, smooth and heavily plated but I also noticed that the feet don't actually move or skid when the domes are pressed- instead the shape of the main part of the dome just flips from convex to concave. Anyway, this is of limited interest now that your domes aren't going to fit anyway. To be honest, you had a lucky escape there.

I did go looking at keyswitches such as the Cherry MX series - to my surprise there seems to be some kind of renaissance surrounding PC keyboards with proper mechanical switches, with people custom-building their own keyboards. You can even get a 'sample kit' of the nine or so subtly different versions of Cherry MX series switch (distinguished by the colour of the actuator) so you can try them all out and decide which one you most prefer the feel of.

As to keytops, the way to get those is to buy a full PC keyboard set of keycaps which will obviously include some rectangular and other odd shaped keys you won't want, but will include plenty of the square ones you would need. It doesn't take much searching to find a full blank (unlegended) set of black or grey PC keyboard keytops (designed to fit the Cherry MX series and clones) for very reasonable money.

But anyway: Good luck with finding the ICs you need. If you have trouble finding any let me know and I'll have a hunt around, I do have a few 'original' 74 series ICs, although most of what I have will be 74LS... series.

KeithsTV 26th Apr 2018 4:48 pm

Re: MK14 schematic revisions
What 74 ICs do you need as I also have a number of 74 series ICs.


Slothie 26th Apr 2018 6:14 pm

Re: MK14 schematic revisions
I'm still tryimg to decide if the 74LS08 will be ok to replace the 7408 as a display driver, I need to look through the data sheets. I expect they will, its probably worth just trying, its just odd that SOC specifies the 'LS version for the third 7408.

The 80L95 looks the same as the 74LS365A which Farnell stocks,

I don't have the 7445, but Farnell seems to have some and I've seen them on ebay, All the other gates I have in my (small!) collection.
So I'm not really stuck, just need to get them. I'm going to see what other bits I need for this and other projects and order them all together so I don't get wiped out with P&P charges!

BTW, I just got a delivery of 50 4.433619MHz crystals, so if anyone needs some, I have a few to spare!!

TonyDuell 26th Apr 2018 6:58 pm

Re: MK14 schematic revisions
I seem to remember that the 7408s are used as the anode drivers for the display. As such they are expected to source more current than the data sheet suggests, and you really do want the plain 7408 here.

The 7445 is the open-collector version of the 7442. Again it was chosen for a good reason, you do not want to be shorting totem-pole outputs together if you happen to press 2 keys at once.

The 80L95 is equivalent to one of the 74LS365/6/7/8 ICs (I forget which), so if the 74LS365 looks to be the same, it will work.

The latches for the display anode (segment) signals are 74LS157s. Yes, multplexers. The rumour at the time was that they should have been specified as 74LS175s (D type latches) but somebody ordered the wrong part and the design was modified to use them. My experience is that not all 74LS157s work, you may need to select some.

Slothie 26th Apr 2018 7:22 pm

Re: MK14 schematic revisions
I've been looking at data sheets and the 74LS08 can only drive 0.4 mA compared to the 7408's .8mA, so you are right the LS part probably would die or make the display dim. If I get stuck then a 74HC08 could be used as the outputs clamp at 20mA.. although I might need to put in some current limiting resistors for the display as SOC clearly relied on the current limiting of the 7408 and I dont want to destroy my rare 7seg displays! So I really need to find some real 7408's.

kan_turk 26th Apr 2018 8:29 pm

Re: MK14 schematic revisions
Just been following this thread - I'm sure I have some 7408s but I'm away until middle of next week & will have a look when I return - you're welcome to them FOC
Will post on here when I find them


Slothie 26th Apr 2018 8:52 pm

Re: MK14 schematic revisions
Thank you, that will be great. The displays haven't arrived from Russia (!) yet so there's plenty of time :)

SiriusHardware 26th Apr 2018 11:17 pm

Re: MK14 schematic revisions
I think the excess / unlimited anode current sourced by the 7408s is mitigated by the fact it's a multiplexed display, so each display cell is only actually 'on' for approximately 1/8th of the time, and not every segment of every cell is lit.

If you hold down the reset button on a working MK14 the display scanning is paused and you get a single very, very bright digit: Probably unwise to hold it in that state for long otherwise it might possibly cook the lit segments in that one display cell.

You appear to have your sources for 7408s and 7445s. Anything else missing?

Did you manage to find at least one pair of 2111 / AM9111 RAM ICs?

Slothie 27th Apr 2018 12:50 am

Re: MK14 schematic revisions
1 Attachment(s)
Yes. a guy in California was selling them (I think it may have been the one you pointed out) so I bought 4. They arrived Thursday! So I have those and some 65X61 chips, and there is a link on the bottom of the board to select which type (they all have to be the same type).

If John can find his 7408s then I have a source for all the chips!
I just need to get my PROMs programmed; I have the schematic for the MK14 prom programmer, which I could hook up to an arduino. I just need to be sure it works first time because I only have the two!! Did I see you say you had a 74S571 programmer? could I ask you to program them for me if I sent you the PROMS and return postage?

The 7408 has a 130 ohm resistor in the output (see schematic) which would limits the current through the LED to about 25ma, which multiplexed 8 ways is an average of ~3ma which is the correct sort of average current for the LEDs in the bubble display.

SiriusHardware 27th Apr 2018 9:21 am

Re: MK14 schematic revisions

Originally Posted by Slothie (Post 1038551)
Did I see you say you had a 74S571 programmer? could I ask you to program them for me if I sent you the PROMS and return postage?

I have a Hi-Lo Systems ALL07A programmer which I know can handle those PROMs because I have used it in the past to read my MK14 PROMs and back them up - bipolar PROMs are among the small, annoying subset of historic devices known to suffer from 'tin whisker' disease.

If you sent them to me for programming you'd also get them back tested, since I have an actual MK14 to test them in. :)

You'd need to make a decision about which version of the OS you want - original or improved.

The original version matches the manual, so if you are primarily wanting to make a working museum piece you might prefer that version.

If you're actually going to use the machine then the later 'improved' OS is much to be preferred as it takes far fewer keystrokes to enter each byte of code and it contains the support routines for the optional cassette tape interface plus a jump offset calculator and the support routines for the optional single-step hardware, all of which the original OS did not.

Slothie 27th Apr 2018 1:14 pm

Re: MK14 schematic revisions
I'd definitely want the revised rom, if only for the tape routines.

The single step function is intriguing, I have often heard it mentioned (usually in reference to needing to pull SenseA down for the new rom to work) but have never seen a circuit. I presume it involves latching the "I" status from the bus during NADS and then raising an interrupt on the sense input if it is high.

When you have a moment, PM me your address and I'll post them out to you, It'll be in a week or two because I'm still in the throes of clearing out mums house and finding places for all the "treasures" I'm finding!

SiriusHardware 27th Apr 2018 1:21 pm

Re: MK14 schematic revisions
Sure thing - I'm at work at the moment supposed to be working, so I'll send a PM over tonight.

I have to confess it's easier for me if you choose the revised monitor as I already have those saved two files (low nibble / high nibble) - didn't want to pressure you either way though.

For details about Single Step, have a look in the main MK14 manual for the program of the same name in the 'System' section. I think the hardware is described there.

Slothie 27th Apr 2018 1:45 pm

Re: MK14 schematic revisions
Ah! hidden in plain view :)

SiriusHardware 27th Apr 2018 6:12 pm

Re: MK14 schematic revisions
Slothie, PM Sent.

Slothie 30th Apr 2018 4:47 pm

Re: MK14 schematic revisions
PROMs in the post! There's a sticky address label with stamps on in the envelope.The edge of the label has 'high' and 'low' stickers for the proms!

SiriusHardware 30th Apr 2018 5:00 pm

Re: MK14 schematic revisions
OK, I'll look out for them and have everything juiced up and ready.

gertk64 3rd May 2018 7:08 pm

Re: MK14 schematic revisions
1 Attachment(s)
Just managed to get first signs of life out of my MK14 replica,
now waiting for the 80L95, well 80C95 or 74LS365 and the RAM chips (ordered the AM9111 also).

I did have a fight with the new 74LS157 multiplexers (did not work as latch) and the fact that my temporary substitute RAM (HM6116) seemed to dislike the floating read, write and address lines of the CPU.

With 4 pullups on the top address lines and the read and write lines from the CPU the display suddenly got a lot more stable but still no 0000 00. I swapped the 74LS157 with an old set of 74LS257 (tri-state version of the LS157) and tada, steady display.

For testing the keyboard I used a DIL plug with a small PCB on it which carried a LS32 and a LS367 I had lying around and I could enter some data.

SiriusHardware 3rd May 2018 7:57 pm

Re: MK14 schematic revisions
Slothie's PROMs arrived, but I was surprised to find that they were 'Tesla' branded MH74S571 Proms, not the expected National Semiconductor DM74S571s which I know my ALL07A programmer can program. My fault for just assuming they would be National Semiconductor 'DM' prefixed devices.

The All07A programmer only specifically supports the Nat Semi part, and does not (as programmers sometimes do) have a slow-but-sure 'Generic' 74S571 option. So the dilemma is, whether to try to program them as DM74S571s and hope that the algorithm, programming voltages and so on are the same or similar enough.

Several comments in forums suggest not: On Mattieu Benoit's site someone has made a one - off .exe for for the ALL03 (sadly not my ALL07) programmer, which already natively supports the DM74S571, to allow it to program the MH74S571 as well. If someone went to all that trouble it seems likely that programming these devices as per DM74S571 does not work, or does not work reliably.

I don't want to ruin Slothie's expensive one-shot PROMs. Slothie has already said that it's OK to return them unprogrammed, but I'm open to last-minute persuasion from anyone who has, for example, successfully programmed MH74S571s 'as DM74S571s'

I have found one currently available programmer, the Elnec Beeprog2 - also available badged as Dataman - which explicitly lists support for the MH device.

It is not cheap, and on the only marginally cheaper cut-down version, support for BPROMs is dropped altogether.

Another option would be to buy a set of programmed PROMs from this Czech site,

...which is selling replica MK14 PCBs, Keypad overlays and programmed PROMs. I'm sure the site owner would sell a set of programmed PROMs only, or even program a set sent to him, if asked. It seems likely that his PROMs are the MH... flavour, so he must have the means to program those.

Or, this well documented Arduino project, clearly meant to program the 'original' DM74S571, could be adapted and the timing modified if there is sufficient data available about the MH74S571...

You could always also build the original Science Of Cambridge PROM programmer add-on for the MK14, the paradox being that on the face of it, it requires a working MK14 to operate it: Of course an Arduino or PIC could be used more conveniently nowadays. MK14man's definitive website concerning all things MK14 seems unavailable at the moment, so I can't link to the manual which contains details of the circuit, connections and software support routines - but I think Slothie probably has it anyway.

Slothie, give it a couple of days and we'll see if anything interesting comes up, or just say the word and I'll send them off back to you sooner.

SiriusHardware 3rd May 2018 8:04 pm

Re: MK14 schematic revisions

Originally Posted by gertk64 (Post 1040609)
Just managed to get first signs of life out of my MK14 replica.

Well done! May we ask which type of PROMs you are using, and whether you bought them programmed or programmed them yourself?

SiriusHardware 3rd May 2018 8:32 pm

Re: MK14 schematic revisions
Further, this website

An excerpt from which is here:


A6 |1  16| Vcc
A5 |2  15| A7
A4 |3  14| A8
A3 |4  13| CE/
A0 |5  12| O1
A1 |6  11| O2
A2 |7  10| O3
GND|8    9| O4

  Signetics    MMI    TI      Harris  Raytheon  AMD      National  Intel
  ---------    ---    --      ------  --------  ---      --------  -----
TS 82S131 (50ns) 6306-1  -        7621-5  29611    27S13    74S571    3622
  82S131A(30ns) 63S241  -        7621A  -        27S13A  74S571A  3622A

OC 82S130 (50ns) 6305-1  -        7620-5  29610    27S12    74S570    3602
  82S130A(33ns) 63S240  -        7620A-5 -        27S12A  74S570A  3602A

..Suggests that the Philips / Signetics 82S131 and the AMD 27S13 are drop-in equivalents for the National Semiconductor DM74S571. My programmer can program all three of these types.

Slothie 3rd May 2018 9:15 pm

Re: MK14 schematic revisions
1 Attachment(s)

Originally Posted by SiriusHardware (Post 1040627)

You could always also build the original Science Of Cambridge PROM programmer add-on for the MK14, the paradox being that on the face of it, it requires a working MK14 to operate it: Of course an Arduino or PIC could be used more conveniently nowadays. MK14man's definitive website concerning all things MK14 seems unavailable at the moment, so I can't link to the manual which contains details of the circuit, connections and software support routines - but I think Slothie probably has it anyway.

Slothie, give it a couple of days and we'll see if anything interesting comes up, or just say the word and I'll send them off back to you sooner.

I've seen the hackaday link and the mk14 design and sketched out a design for one, using an ATMEGA328 to drive it rather than bootstrapping the mk14!
Hang onto the proms for a while in case we get any response on here. I'm still waiting for stuff to arrive, and to get some free time! As i said in my PM if it comes to it I can program a 2716 and make an adaptor on stripboard so I can plug it into the mk14 until a prom solution appears. If I want to keep it "retro" I found an eprom programmer for my PET in a box from the loft so I could use that!

The idea of making a programmer is growing on me since one day I'd like to make an Acorn 1, and that uses 74S571s too! I'd be able to tweak the timing capacitors for the Tesla chips

I'll query the guy in the Czech republic too, looking at pics of his replica he is clearly using Tesla 74S571 chips, not surprisingly since I imagine the fall of communism there probably revealed warehouses full of vintage electronics. Tesla are now a mojor supplier of valves nowadays I understand!

SiriusHardware 4th May 2018 10:51 am

Re: MK14 schematic revisions
I'm informed by M. Benoit, who kindly replied to my query by email, that I definitely should not try to program Tesla 'MH' devices using the 'DM' algorithm, unfortunately.

The Hi-Lo ALL03 can do the MH series chips if you utilise a special bit of software which Matthieu has on his site, but that software does not run on the ALL07 as it stands.

gertk64 5th May 2018 4:24 pm

Re: MK14 schematic revisions

Originally Posted by SiriusHardware (Post 1040632)
Well done! May we ask which type of PROMs you are using, and whether you bought them programmed or programmed them yourself?

I got them pre-programmed. With the latter version with the tape routines and the "0000 00" startup display.

Got the AM9111 RAM chips today, will put them in this weekend.

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