Re: MK14 schematic revisions
 

I will pass on the thanks - I gave him the link to here so he may appear at sometime hopefully...

Indeed - although the review for the MK14 did rather unfairly say the most fun you would have with it was building the kit... so perhaps by extension debugging the kit should be even more fun?

On a more serious note, I hope to have some time (when I am not tired) this coming Sunday to look over the design in more detail just as a double check for you. As yes it starts to gets scary when others want to use your stuff - so then I will only have myself to blame for any mods we need to get them running. Let us know if you need some cash to chip in for the prototype run.

Reading through CAA in Issue 5 the review of the VDU suggests hooking up B13 to Flag 1 , B14 to Flag 2 and B16 to flag 0 - so we are breaking canon! Using the IO chip is better though I think.

It also hints at how the memory board was configured to allow sockets for 2111, 2x2114 and some 74S571 PROM - not sure if that is the proposed SoC one or their own. I may find out in the next instalment - exciting, not sure I could have waited a month for a chain letter to arrive or not! Right back to archive.org.

Re: MK14 schematic revisions
 

The original instructions for the VDU say to do that (use the Flag outputs), otherwise it would have excluded people who did not have a RAM I/O fitted - bearing in mind that the RAM I/O was an option, not standard.

People generally did not use a 'hard' one to one connection between the VDU and the MK14, so they were able to choose which flags or I/O lines to connect for these purposes.

One minor problem I forsee (I have not thought it through, but...) you will want the VDU to be disabled on MK14 reset, because when it is enabled it constantly halts the CPU and drags the overall system speed right down, meaning that the cassette interface or any scheme which uses it indirectly (ie, Serial download from PC as per Karen O's idea) and for that matter the Pi uploader will not work, unless of course you slow the latter down to the speed of mud.

The I/O device I/O ports start off in high impedance input mode so a pull up or pull down resistor (can't remember which) will be needed on the 'display blank' connection to hold the VDU off until such time as the user puts that I/O port pin into output mode and intentionally turns the VDU on.

Re: MK14 schematic revisions
 

One final idea for Slothie before he sends his design off to be made into PCBs - How about putting a copper trace 'Issue VI' in the appropriate place on the track side of the PCB?

With the buses tracked to the rear edge and support for alternative RAM types, it really will be the most advanced 'legacy' MK14 PCB, original or replica, ever made*. I think it deserves that finishing touch.

*MK14man's noughties version was a contemporary redesign, rather than a 'what-might-have been'.

Re: MK14 schematic revisions
 

Yes I think that would be lovely - the MK14 ISS V is above the extra keyboard fingers (which appear on the bottom as well as the top) of Martin's board so his is of course a perfect replica except for that which is an extra - so he is probably 5.1!

Re: MK14 schematic revisions
 

Thats an Idea!

Re: MK14 schematic revisions
 

Actually I had a thought that what might have been next for SoC would have been a way to disable any activity on the board in PROM, MEMORY or IO so that extra 'pages' could be added with a latching circuit for D0-D3 which holds the high Address lines A12-A15 during NADS - i.e. the BoardEnable would only trigger on four zeros for Page 0. This would have been needed for the NIBL BASIC expansion card or larger PROMS etc. I must study the circuit and see if there is a simple way that could be added - probably by routing the RD/WR enables to an off board option with a pair of links for a bare board. Break the links and add the extra circuit for pages to work is on the expansion card.

Re: MK14 schematic revisions
 

I tried something like that about 35 years ago. A hand-wired (on Veroboard) SC/MP system with the low 4 data lines latched on NADS to give the full 16 bit address bus. And I tried running the MK14 monitor ROM on it.

I discovered (the hard way!) that said monitor does not put everything in page 0. I seem to remember it addressed the keyboard and display in a higher page -- page 9????

Re: MK14 schematic revisions
 

Ah interesting - I had read about the reliance on the wrap around at the end of the page for the variables at FF9-FFF which I thought might initially prevent the use. But after further study (especially the Elektor articles on adapting NIBL for page 1 onwards) I suppose that the above must be a bug in the monitor, as the pointers do not increment beyond Bit 11 supposedly so only XPAH and XPPC can change the page, something using one of those instructions must set those bits incorrectly.

Re: MK14 schematic revisions
 

As there have been some questions about the connectors on the PCB, and I admit that the schematic doesn't really help, here are some pictures of the connectors as rendered in the 3d view. I have marked what each connection is on the silkscreen, since I personally will find this helpful, and I am sure that others will too.

Re: MK14 schematic revisions
 

Certainly will, rather nicer than having to scrabble for my threadbare piece of paper which describes the upper rear edge connector, and of course especially helpful to have the rear lower side connections labelled since your PCB is unique in having those already routed so there is no existing documentation which describes those connections.

I don't know whether SOC would have ever have gone so far as to implement memory paging as suggested by Tim / tried by Tony, but they might and ought to have got to the point where the bus connections were all available at the edge connector (they did, after all, do this on the ZX80 onwards from the very start). They should have done it on the MK14 on the first issue which had contact fingers for the lower side of the rear edge connector, whichever issue that was.

If they'd done that (and with the unwanted PROM images already removed from issue V onwards) they could have introduced a 'RAM pack' PCB which plugged into the rear edge connector in the best tradition of Sinclair machines going forwards from that point, perhaps with an ongoing connector on its own rear edge, appropriately pinned out for the VDU via a DIN connector of the same type which the VDU was obviously laid out for. (Your third or fourth next project, perhaps...)

Incidentally the same forum member who confirmed that the issue V did have those unwanted PROM images removed 'out of the box' also told me that there are still multiple images of the RAM I/O and display in the issue V memory map, so he then further modified his issue V machine to remove those images as well. I invited him to post details of the mods (in the area of IC15 through to IC17 on issue V PCBs) to one of these threads but I think he prefers to remain a read-only member.

Re: MK14 schematic revisions
 

I deduce that they must have from the promised BASIC board in the manual update letter as NIBL would have needed paging.

http://www.computinghistory.org.uk/d...Update-Letter/

The fingers are not in the Issue IV as I found a photograph of the bottom of one in the "Story of the Commodore 64 in Pixels". In the article on Archer Maclean he incorrectly attributes his to 1976 but the photo is quite clear:

Attachment 185979

I had always intended getting to a memory board, again as described in the Update Letter, that perhaps even turned a right angle as well to make the final layout of the VDU go to the right so there was room for a 40 Key keyboard to the right of the board and below the now horizontal VDU putting the UHF/Composite out on the right. This could then contain a second connector for the NIBL board perhaps directly above it. Making the whole thing close to a single board machine of the time... and manageable on a normal desk!

Attachment 185978

Maybe they can provide them privately for you to present?

Re: MK14 schematic revisions
 

Given that the MK14 issue 1 came out in 1977 and the ZX80 came out in 1980 I imagine any uogrades or further perripherals that SOC may have planned for the MK14 were canned in favour of the ZX80 which was a superior prospect for volume sales than the MK14. The Z80 processor had just dropped in price due to its widescale adoption in S100 machines, and even without the ZX81's ULA the chip count was only a little higher than the MK14!
While, from our perspective, it is a shame that SOC didn't produce the RAM upgrades, NIBL, ASCII keyboard etc it was clearly a wise decision because in one year of production before the arrival of the ZX81 it easily sold more than twice the number of units than the MK14 did in 3 years.
And then SOC/Sinclair took over the world with the Spectrum and looked unstoppable untill they produced the QL and... well the rest is history!!

Re: MK14 schematic revisions
 

From sources that sounds likely although according to this reference:

https://nosher.net/archives/computers/adve_075

The ZX80 was a U-turn where essentially Clive canned the BASIC version of the MK14, triggered the setup of Acorn who DID do a machine with BASIC on... and eventually lost the BBC contract

Indeed it was the correct path for them (eventually) - in fact if you look at the ZX80 with the keyboard built onto the single board it is very much the missing link between the MK14 and the ZX81 and those that followed.

I quite liked the QL, it was very strange sitting on a train from Cambridge with a padded envelope with one in, among others with similar padded envelops not saying anything to each other, as we were all in fear of the massive NDA we had just had to sign!

Re: MK14 schematic revisions
 

Actually, yes they are according to the evidence of the photo you attached :)

Maybe we aren't talking about the same thing, but I am referring to the contacts on the track / print side of the PCB for the rear edge connector. That issue IV PCB does have them although, as with the issue V, they aren't tracked to anywhere. My issue II PCB has no copper at all on the track side in that area, there are only contact fingers on the component side for the power, I/O port connections etc.

The manual for the VDU also says that on issue IV and V PCbs the connections for the VDU can be picked up from the rear edge connector: What it doesn't say is that you have to link the necessary signals to those contact fingers yourself.

Based on this, my assumption has always been that the track / print side contacts appeared from issue IV onwards, but I've never seen a good shot of the underside of an issue IV until now.

There is an added complication with respect to the forum member who provided the info about PROM images on the issue V - as far as I can make out he only visits the site incredibly infrequently. He may be the forum equivalent of Halley's comet.

Re: MK14 schematic revisions
 

:-D :-D

Re: MK14 schematic revisions
 

Ha ha yes you are correct - Thank you for that. I was looking and thinking of the ones by the keyboard - which don’t appear in issue V either only Martin’s. Thanks for that - we just need an issue I and III image now.

Re: MK14 schematic revisions
 

I also have to issue another correction - the picture was not from Archer's article in that book just checked - it is actually the one from:

http://www.old-computers.com/history...l.asp?n=37&t=3

Apologies I am slipping.

Re: MK14 schematic revisions
 

Since you can solder a DIN 41612 a+b connector to the edge of the MK14, I'm very tempted to make a 3 or 4 slot "Motherboard" to plug into the top of the MK14 that will allow cards to be plugged in vertically. Thus you can have, for instance, the VDU, a 1.5k RAM/ROM module and other cards connected all at once. it might be required to have some kind of frame to support the cards, but in my experience the 41612 style connectors are pretty secure unless the card has some heavy components on it.
Just a thought.

Re: MK14 schematic revisions
 

I may have a small number of the card support rails - if you look at any recent image or video clip of my MK14 you may notice that that is what holds my MK14 PCB in place in its plywood box by gripping the left and right edges.

The original PCBs gave no thought to how the board might ever be mounted in any sort of receptacle or on any surface, so the only holes through the PCB were those intended for the keypad rivets.

That's a thought, actually: Maybe the PCB could do with some strategically placed 3mm or 4mm holes with mounting posts in mind. Any nearby tracks would have to be steered away from them, though, to avoid the possibility of metal posts or screw heads making contact with tracks. Even if they are green coated, metalwork will bite through the coating eventually.

Re: MK14 schematic revisions
 

Well today's my birthday so I've treated myself to 5 new PCB's from JLCPCB. I did get a quote from PCBWAY and it was twice the cost! Maybe I didn't get some of the options checked right, but I found the UI confusing at first glance.
The new quote is remarkably cheap!

Re: MK14 schematic revisions
 

Happy Birthday!

That does seem astoundingly cheap, it's a shame around half of it is for carriage. It's not that long ago that to have a one-off or small run of PCBs made would have cost well into three figures, that's why nothing I make ever gets further than veroboard.

I'll drop you a PM about this later.

Re: MK14 schematic revisions
 

Happy Birthday to you... thanks for sharing that as at least I know what to expect in the way of costs when I eventually get to a PCB with the things I am working on... I I stand by my earlier offer to chip in with the cost and build one if you will permit me...

Re: MK14 schematic revisions
 

Hi! Sorry about the radio silence. The boards have arrived and look great, but I managed to hurt my back and have found it painful to sit up let alone scrabble about looking for components :)
I've offered a couple of you some of my spare components and I'm working on a "price list" as I discover components but it may be a while before I recover enough to find the rest.

Re: MK14 schematic revisions
 

Who knew working on MK14 projects would be so dangerous?

(I'm thinking foot / ankle problems last year, eye problems and now back problems). Maybe you need to take up a gentler pastime?

Will look out for a parts list from you presumably via PM or email - anything you can't currently find or reach, I'm sure I can have fun trying to unearth for myself. None of this is super urgent, and certainly not worth you wrenching your back (again) for.

Re: MK14 schematic revisions
 

I actually did my back in by lifting shopping bags out of a cart and into my car. Who knew milk and vegetables could be so heavy! I should go back to eating junk food, thats a lot safer!

Re: MK14 schematic revisions
 

Owch - had a week in bed once getting a sock out of the tumble dryer... hope it is better soon. I have quite a lot of odd parts already as well as I bought several of some chips and passives.

Re: MK14 schematic revisions
 

Well I've given this a thorough inspect and aside for a couple of trivial silkscreen errors (Dated 2018 not 2019, U11 label overprinting part no) all seems good.
(Sorry the images are a bit fuzzy I had to crop and compress them to death to get them under the 100k limit. If you want so see bigger images try http://www.ianrolfe.com/page/mk14-rev-vi-boards/)

Re: MK14 schematic revisions
 

In time, that minor screen print error may make these ones especially collectable, like banknotes or stamps with similar small errors. :)

Re: MK14 schematic revisions
 

Looking good, looks like an MK14 so it is an MK14 - over 40 years later - impressive achievement since that first advert in January 1978... in the Feb 1978 issue of Practical Wireless

I was going to link to American Radio but, the PDF links are broken for that magazine at the moment. So here is a capture...

Attachment 186864Attachment 186865

Re: MK14 schematic revisions
 

Been quiet on the MK14 front in here - just thought you may all be interested in the fact that one of Martin's reproduction Tape Interface PCB's with a BC107 rather than a 2N2926 (hfe 120 rather than 100 so I took a chance) works well (once I found the read LED was faulty...) on my JM Precision repro.

Seems that either the SCIOS save or the load routine do not handle the last byte of a 256 byte block... Unlike a Spectrum the system is happier at a much lower volume. It is also very sensitive to any clicks or noise so important you don't press GO until after the mechanism settles, the first bit is corrupt or the whole data stream slips.

Using the Sanyo M1104X on batteries as recommended by SOC - on this repro board there is no R11 (27k) in series with D1 - I used a 1N4148 as it is not named in the manual. The clip on the resistor is just a handy place to pick up 5V and still allow the expansion connector to be removed easily. The white wire loops SIN and SENSEB. GROUND, SENSEB and FLAG0 are soldered on the connector.

Attachment 188671

Re: MK14 schematic revisions
 

There's a fine line between taking a keen interest and outright hassling someone so I've resisted the temptation to ask Slothie how he is faring with his latest repro PCBs.

On my (original) cassette interface I see that I changed C1, the audio input capacitor, to a lowish value polystyrene type which would have been more at home in a radio project. The obvious intention was to attenuate the input and shave off the low frequency end of the incoming, so maybe I had problems with mains hum and low frequency rumble coming out of the particular tape recorder that I used. SOC recommended using battery powered recorders, as you have pointed out.

One other thing to watch for on original MK14 cassette interfaces, the PCB screen printing for the orientation of the diode D1 was originally the wrong way around, although I believe the circuit diagram always showed it correctly oriented - but if anyone happens to buy an original example it might be worth checking the orientation of that diode just in case the diode was inserted as per the screen printing. Later runs of the cassette interface PCB may have had that error corrected, as well.

Incidentally the (original) diode on my interface is ... a 1N4148, identified by rings in the resistor colour code rather than with printed numbers as per more modern versions.

Re: MK14 schematic revisions
 

Hi folks, my back is recovered now but it effectively knocked me out for 5 weeks and I'm having to catch up with everything I couldnt do while laid up in bed. I haven't so much as soldered a single joint but I have come across some of the missing parts (including the displays!) while doing other things so I'll be finishing off the parts lists and getting them out soon.

Re: MK14 schematic revisions
 

Glad you are on the mend... I still have not found my 'missing' spare SC/MP and 8154, although I have not really looked very hard yet. My own fault really, I've known exactly where they have been for the past 10 years or so (at work) and I only recently thought I had better move them to a 'safe' place more under my direct control. No idea where I've put them.

Re: MK14 schematic revisions
 

Ah yes, I'm always putting things in a "safe place" that I've been tripping over for years then a few weeks later needing them and not being able to remember where I moved them to. That's why tidying is always a dangerous move!

Re: MK14 schematic revisions
 

Exactly what I keep telling SWMBO.

Still haven't found them, though. I will mount a concerted attack on this problem once I've done all those other things I still haven't done.

Re: MK14 schematic revisions
 

Slothie glad to hear about the Back being better. I have been absent due to a significant stint in hospital and I am still laid up and find it difficult to use a computer - or sort through the chaos of my study for anything - I like you both, know where everything is in the mess until I dig it out and then remember I moved it last time I found it but, cant remember where too.. Speak soon.

Re: MK14 schematic revisions
 

Wow, I hope this is not some sort of Phaerohic MK14 curse which I'm about to become the third victim of - best wishes and a speedy recovery for you both.

Re: MK14 schematic revisions
 

Well I had just finished getting my MK14 tape interface to work and save/load successfully... Hmmm

Thanks Sirius... maybe I should delay building the VDU until I fully recover.

Re: MK14 schematic revisions
 

Tim, you mentioned somewhere (I think) that you thought the cassette routines were failing to work with (Save? Load?) the 256th byte of 256 saved bytes.

I know there isn't much information that you don't absorb, but did you notice the bit in the tape interface manual where it says that if you want to save a full 256 bytes, you have to enter '00' as the number of bytes to be saved?

Re: MK14 schematic revisions
 

Its worse than that, ive been in hospital for the last 3 months which is why Ive been silent. Ive been diagnosed with miloma, s type of bone marrow cancer whi h lucky for me is treatable. However i have had to move out of my flat because its unsuitable for me on my return because of the stairrs and because i won't. Be able to work for some time, so everything is going into storage. When I have access to my laptop i will upload the gerbers etc for tbr pcb in case someone wants to give it a try.

Re: MK14 schematic revisions
 

I'm so sorry to hear that, and obviously the absolute priority is for you to regain your health, everything else fades to insignificance. Take care, and stay in touch as and when you can.

Re: MK14 schematic revisions
 

Indeed that is horrible for you - here is hoping for a speedy recovery and successful treatments that return you to full health soon.

Re: MK14 schematic revisions
 

Than ks guys! Progress is slow but in the right direction.

Re: MK14 schematic revisions
 

Hi all.

It's been interesting reading all the posts in this thread and it brought back some memories and as such I decided to try and resurrect my MK14. The only bit I had left was the processor chip the rest having been used to make various "computers" in the years afterwards and the PCB finally getting so messed up it ended in the bin.

Thanks for the various idea of where to get parts and I have bought the PCB plus other bits from Martin in the Czech Republic, his web site is very helpful. Most of the other chips I've managed to find in my and my friends chip store. He also had a chip checker from Bang good that shows that the old chips are still working ( well enough of them).

I'll post to let you know how it goes.

Cheers David

Re: MK14 schematic revisions
 

Hello David,

Welcome to the fold. Let us know how you get on. As a new forum user your posts will be subject to time delayed moderation, but after a few posts from you that will wear off and you will be able to post in real time.

There are several other large threads concerning the MK14 on this forum, some now closed, but any closed thread can be re-opened by a polite request to the mods if you find one which seems more directly relevant to your present situation than one which is still open.

Re: MK14 schematic revisions
 

Welcome fellow MK14 enthusiast - I have yet to build my one from Martin but, still enjoy my JS Precision one...

Re: MK14 schematic revisions
 

Hi Timbucus

Did you mean the JM Precision PCB.
I got one of the JM Precision ones as well as one from Martin - took the opportunity whilst collecting bits.

Do you know which switches I need to use the JM Precision PCB?

I have most of the chips - or at least ones that should do the trick - but have drawn a blank on the small seven segment Led display.

Cheers David (aka DeltaAlpha52 )

Re: MK14 schematic revisions
 

There are a couple of people here who have already gone down the same road as you and may possibly have bought more displays than they really needed, but if not, one dodge which you can use is to harvest the display from a contemporary basic calculator, which you can find in the usual places.

I have found that the 'Texet 880', sold in large numbers by Woolworths etc in the late seventies, has exactly the right sort of display - I have actually had a Texet 880 display connected to my original MK14 so I can say for sure that it definitely works.

I believe Tim has mentioned the exact switches which fit the JM PCB, as he had some initial difficulty locating them as well.

Edit: Yes, back in post #148 of this thread, Tim mentioned the switches...

Re: MK14 schematic revisions
 

Just checked and blimey, Texet 880s seem to have gone up a bit.

Last time I was looking they were typically between £6-£7 for a nice one and £2-£3 for a scruffy / broken one, the latter obviously being the one you would choose to buy as a parts donor.

If no-one else here has any displays they feel they can spare, hopefully they can at least tell you where they got theirs from.

Re: MK14 schematic revisions
 

I got mine from a Texet (It had a faulty key but, I have kept it so it could be restored if needed) and I also bought a Russian equivalent but, have not been able to get it to work... My JM precision has one of the large reproduction ones as it is clearer when demonstrating to people - it was from a listing by trev-ham, he was doing keypad covers and display enhancers as well but, seems to not be listing at the moment.