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Old 29th Jun 2015, 1:48 pm   #1
TonyDuell
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Default The Sinclair / Science of Cambridge MK14

I've started this based on the last messages in the 'Favourite Microprocessor' thread so as not to get too confusing.

The MK14 was of course Sinclair's first microcomputer. Based on the SC/MP, with 256 bytes of RAM and 512 bytes of ROM containing a monitor program. I got one many years ago, I still have it somewhere, although I raided the CPU chip for a project and then wrecked it when the +5V line in that project went sky-high,

Anyway, back to the MK14. There were many things to dislike about it. The first was the keyboard, which was typically horrible. Early versions had a conducive rubber sheet over PCB tracks. There was an 'upgrade' to metal dome contacts and plastic buttons which was supposed to be better but which I found to be as unusable. The good thing was that the keyboard area on the PCB had some undocumented sets of holes in it. It turned out you could solder a keyboard switch that Maplin then sold into each group of 4 holes and get an actually useable keyboard. I remember labelling the keycaps with Letraset, eventually it all rubbed off but by then I knew where the keys were. There was a set of edge connector fingers at the front right of the PCB which carried the keyboard signals. For some unknown reason this was not documented anywhere, It wasn't hard to trace them out, though.

This leads me on to the other main dislike. The manual. In my opinion it was terrible. Remember that for many people this was the first time they had ever used a computer. A program listing like

0F12 C4 05
0F14 07
0F15 90 FE

may be obvious now to mean store C4 in location 0F12, 05 in location 0F13, etc, but it took me many months to work that out. I could not get any of the demo programs to run until I realised that. There should have been explicit instrucitons on how to type in one of the programs, exactly what to type.

One bit of the hardware sticks in my mind. The display output port used 74157s as the data latches. Now a 74157 is a multiplexer, but one of the data inputs was tied to the output to make a sort of transparent latch. In my experience it worked with some 74157s and not others. The story is that they intended to use 74175s (which are 4 bit D type registers) and there was a typo in the order. This led to a redesign to use the wrong part. And I am afraid that really sums the machine up to me.

Oh well.. The good points outweighed those, though. It was my first computer, I learnt a lot from figuring it all out and I have thus got to work on many more interesting machines since.

I really must try to find a spare SC/MP (I have plenty in other devices, but not one I want to raid at this point) and get it going again.
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Old 29th Jun 2015, 7:57 pm   #2
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Default Re: The Sinclair / Science of Cambridge MK14

Sinclair made some quiet progress throughout the life of the MK14 - I believe it eventually reached at least issue IV, possibly a couple of issues higher than that by the end. One of the improvements on the late marks was to (at last) bring out the address, data and control signals on the rear edge connector (the new contacts were added on the underside of the PCB, making what had been a single sided edge connector on earlier marks into a double sided one).

Prior to this, it had been virtually impossible to neatly install one of SOC's own main peripherals for the MK14, the 'VDU'. I had / still have this item as well. I have to say, it was not well thought out.

Apart from there being no official port or connector to connect it to on the issue II MK14, it also had no onboard memory of its own, stealing 512 bytes of the MK14's official maximum of 256+256+128 bytes of memory, leaving just 128 bytes in which to write your graphics-intensive power station control program.

I used my MK14 + VDU together only for a very short while, then restored the MK14 to largely original stand-alone condition (except for the keypad, which I had replaced). In 2013 I wired a PIC microprocessor PCB to the VDU card with the aim of at least being able to demonstrate it working without having to wire it back up to the MK14. (See attached image #1). Although I could theoretically use the down-time when the PIC is not serving data to the VDU to manipulate (animate) the screen data and make it do something interesting, it's one of those things I still haven't got around to doing.

One genuinely worthwhile improvement was the revised monitor (the one with the '0000 00' reset prompt, as opposed to the original one with the '---- --' prompt) - as well as needing far less individual keypresses to enter each data byte, it also incorporated the essential code to run the optional cassette tape interface, without which it was impossible to save any of your laboriously written code. You could use the tape interface with the original monitor, but you had to type some code in first!

I didn't have your original difficulty with working out how to enter code, but only because my MK14 took so long to arrive that in the meantime my similarly electronics mad friend had built the 'Elektor' SC/MP system - that was even more crude, with address and data entry by binary switches, but it introduced me to the basic concept so that I understood what I was supposed to do by the time the MK14 actually arrived. And after the Elektor system, the MK14's hex keypad entry and 7-segment display seemed like luxury beyond compare... until I realised how bad the keypad was.

The mounting holes for proper switches aren't present on my issue II PCB. I believe they were another improvement incorporated into the issue IV and I have a dim memory that suitable switches were made available as an optional upgrade kit by SOC themselves. I've also seen pictures of MK14s with clear plastic buttons fitted under the original black upper keypad frame and I'm not sure if this was also an official SOC improvement or something which was sold by a third party as an add-on.

Although the MK14's circuit diagram does show the diagram of the keypad it does not (as you rightly said) illustrate the connections to the keypad edge connector in sufficient detail. I have a crude self-drawn keypad diagram (attached image #2) - note that this specifically applies to the keypad edge connector on my issue II, it may be that later marks made use of the connections which I have marked 'N/C'. I always felt that the reset line should have been brought out to this connector as well, so you could mount an off board reset switch alongside your off-board keypad without having to clag the offboard reset switch connections onto the PCB itself.
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Old 29th Jun 2015, 8:02 pm   #3
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Default Re: The Sinclair / Science of Cambridge MK14

I quickly grew fed up with my MK14 because it didn't have any storage.....and entering a complete routine (one could hardly call them 'programs') on that keypad was to say the least, a tad awkward.

I swapped mine for a mini synthesizer and later bought a NASCOM which did have a cassette interface. But that's another story!
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Old 29th Jun 2015, 8:08 pm   #4
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Default Re: The Sinclair / Science of Cambridge MK14

I have no idea which issue mine is. AFAIK it doesn't have the address and data bus brought out on the rear edge connector, but does have the holes for keyboard switches.

The clear buttons in the original key frame are part of the 'metal dome' upgrade offered by Science of Cambridge. A machine without them has the rubber sheet which was totally unusable. I remember buying that upgrade, it was not much better.

The revised monitor was almost essential, but as ever there was a minor problem with it. IIRC it used the Sense A input for something (single-step related) and if you didn't realise what the cryptic comment in the instructions meant you ended up with a machine that wouldn't run user programs. I fitted the new monitor ROMs, found nothing would run and actually put the old monitor back for a time until I realised what was going on. IIRC tying Sense A either high or low (I forget which) cured things.
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Old 29th Jun 2015, 8:37 pm   #5
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Default Re: The Sinclair / Science of Cambridge MK14

Thanks for the info about the 'metal dome' upgrade - I have never seen one up close.

My knowledge(?) about the address and control lines being available on the rear edge on late marks comes from the VDU board 'manual', which states that the connections were available there on issue IV onwards. If you can lay hands on yours at some point it will be interesting to see what issue it is and what improvements that issue did or did not incorporate.

On earlier marks you just had to find the connections anywhere you could, typically on the tracks surrounding the CPU. It looked horrible.

If I were considering reconnecting it now I'd solder a 40 pin turned pin socket to the CPU socket pads on the UNDERSIDE of the PCB, then use a 40 - pin DIL header on some wide ribbon cable to pick up and convey all the required connections rearwards, beyond the rear edge of the MK14 PCB. Doing it that way, the whole VDU could be unhooked in a second or two. (Although, I suspect there may have been one or two required track cuts and mods to undo as well - not including the need to change the main crystal to 4.00Mhz when the VDU was fitted).

Yes... with the new monitor fitted you had to tie Sense-A to 0V either directly (in which case it was effectively lost to you as a user input) or with a resistor, say 1K.

I believe it was because the revised monitor also incorporated support routines for the 'Single Step' feature, but if you didn't add the little bit of hardware required to make that work, the pulldown resistor was needed to maintain normal system operation when the single-step hardware was absent.

I don't think even the MK14's most ardent fans would claim it was a great machine, but for someone on my budget (I was in my early teens) it made the difference between owning and programming a 'computer' or not owning one. Even then, it took me the best part of three months to save up half the cost, at which point my parents realised I was serious and donated the other half.
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Old 29th Jun 2015, 8:49 pm   #6
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Default Re: The Sinclair / Science of Cambridge MK14

Next time I dig mine out I will see what version I have. I am sure that the CPU buses are not brought out on mine, I remember thinking that was rather silly. I never had the VDU(or any of the add-ons other than the cassette interface) though.

I may still have some bits of the metal dome upgrade around. Obvously it's not fitted to
the machine as I soldered all the switches in but I think I came across at least some of the buttons, domes, metal panel, etc when packing up for the house move. In which case I will have kept them.

The problem with needing the pull-down resistor was that it wasn't totally obvious what to do (not for a newbie anyway). A clear 'solder a 1k resistor between here and here' would have solved a lot of problems

Like you, I got one because it was all I could afford. Thinking back I wish I'd saved a little more and bought an Acorn System 1. Having used later Acorn Systems it is obvious they were much better designed machines.
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Old 29th Jun 2015, 10:16 pm   #7
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Default Re: The Sinclair / Science of Cambridge MK14

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Originally Posted by mole42uk View Post
I quickly grew fed up with my MK14 because it didn't have any storage.....and entering a complete routine (one could hardly call them 'programs') on that keypad was to say the least, a tad awkward.

I swapped mine for a mini synthesizer and later bought a NASCOM which did have a cassette interface. But that's another story!
What kind of synth? One of the Casio CZs, or something more exotic like a Roland SH101? (Just trying to imagine what would have equivalent value to an MK14 at the time).

When a computer expert (lecturer) from the nearby Newcastle Polytechnic came in to see us with a view to persuading us to take up a career in computer engineering, he brought with him a fully expanded Nascom built into a wooden suitcase. As we'd had prior warning of his visit, I proudly took my MK14, at the time really neatly installed in a hard briefcase with a concealed chunky battery supply (6 * D cells!) into school to show him.

I thought he'd be delighted to encounter someone with a genuine interest but he just made a bit of a face and said he thought I'd wasted my money and should have bought something like the Nascom. I asked him how much money he thought a fourteen year old schoolboy had, and pointed out that if I didn't have the MK14 I wouldn't have a computer at all.

My later progress was through the ZX81, Spectrum, and then the first really expensive computer I bought was an Atari ST, but by then I had been working for a few years. I was never able to afford any of the Acorn machines when they were contemporary, although I do now own two complete, working BBC model Bs complete with disc drives - I bought them for about 5 each when their price had hit rock bottom in the mid nineties.

Much like your first car or what have you, the MK14, the first computer I ever owned, remains my firm favourite, and I'm very glad that I did keep it. I continue to do odds and ends with it so it doesn't feel too neglected. Several years ago, I made a hex downloader / programming interface for it (which makes playing with it a lot easier) and more recently - since my original manual was in terrible shape with schoolboy pen and pencil writing all over it - I managed to buy a nice clean replacement manual for it.

Ultimately, it would be nice to find a complete replacement set of original rubber-mat keypad parts for it, but that is extremely unlikely given that most people got rid of them at the earliest opportunity.
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Old 30th Jun 2015, 7:13 am   #8
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Default Re: The Sinclair / Science of Cambridge MK14

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What kind of synth?
Oh, it was a home-built affair, based on one of the sound generator chips (I don't remember which), lots of switched controls, lots of pots and a small keyboard. It didn't stay with me, I was playing music in a band by then and needed a Fender guitar!

I was in my early 20s and working by the time I could afford an MK14. The NASCOM came to me as surplus from my employer, we were building video games at the time and used NASCOMs as development platforms.
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Old 1st Jul 2015, 4:45 pm   #9
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Default Re: The Sinclair / Science of Cambridge MK14

I have just looked at my MK14 board again.

It clearly says 'Issue IV' on the solder side. It does have the holes for the Maplin-type keyswitches (same as the reset button). It does not have any connections to the fingers of the rear edge connector on the solder side. The top side carries the SC/MP I/O lines (the 3 flags, the 2 sense inputs and the bit-banged serial lines) and the ports of the (optional) 8154. The data and address buses are not brought out as standard on this issue board.
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Old 1st Jul 2015, 5:11 pm   #10
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Default Re: The Sinclair / Science of Cambridge MK14

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Originally Posted by TonyDuell View Post
It clearly says 'Issue IV' on the solder side. It does not have any connections to the fingers of the rear edge connector on the solder side.
While you were writing that, I was reading this, from the top of page 3 of the VDU card manual (actually a folded piece of paper).

"First of all make the following connections. If you have an issue 4 or 5 board the connections can be made through a double sided connector at the top of the MK14 board". (This is followed by a long list of required signal connections including the address bus, data bus and control lines).

I've attached the whole document, which includes the VDU card circuit diagram, for your information / interest.

..And yet you say your issue IV PCB has no such connections available at the top edge. This is pretty typical of the chaotic nature of Sinclair's documentation in the early days.

Now we need to find someone who has an issue V to find out whether the 'V' did or did not have the double sided connector as the VDU document claimed it did.
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Old 1st Jul 2015, 5:24 pm   #11
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Default Re: The Sinclair / Science of Cambridge MK14

My issue IV board has a complete set of edge fingers on the solder side of the PCB, they just don't go anywhere. I wonder if the VDU manual is suggesting you could wire those fingers to the address and data pins of the SC/MP and then use a double-sided connector to link up the VDU, rather than some hardwared mess or another connector.

Does your issue II board have the fingers present?

Thanks for the VDU manual. I might even have a go at making one if I have nothing better to do. Better get the MK14 running again first, though.
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Old 1st Jul 2015, 6:49 pm   #12
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Default Re: The Sinclair / Science of Cambridge MK14

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Does your issue II board have the fingers present?
Aaaah... No it does not, the edge connector on the issue II is resolutely single sided, with nothing on the solder side of the edge connector at all.

So that's what they meant?

Just had a look around for relevant info and found this photo on the owner's flickr page:-

https://www.flickr.com/photos/retrophilepix/4193484807/

The comments make it clear that he did this in order to bring the necessary connections to the rear edge for the VDU (there are more pictures of the same MK14 and other equally interesting bits of retro hardware on the same flickr page). So, for the issue IV at least, that's how it was.

It wouldn't have occurred to me in a million years that SOC would have added the lower side PCB fingers for a double sided connector but then NOT tracked them to the system buses.

If you decide to build the VDU, I think the optional (!) character generator IC is actually a bipolar PROM programmed with a basic alphanumeric character set, so they wouldn't be available to buy anywhere (at least, not with the character set already programmed). Without the CG IC, the VDU can only work in 'Bitmap Graphics' mode.

I could read the character code out of my CG IC and upload it for you, but I would suggest that you redesign the circuit a little bit to use a more modern memory IC (eprom or eeprom) rather than the PROM originally used.

If you were determined to use it with your MK14 I would also suggest you provide it with its own RAM, such as can be found in a single-chip solution in the 48-pin DIP version of the IDT7132 2K dual-port RAM IC, details here:

http://www.idt.com/products/memory-l...-dual-port-ram

(EDIT: Sorry, that link doesn't seem to work even though I cut and pasted it from the page I was viewing. Try going to idt.com and putting the part number IDT7132 into the query box).

As originally conceived and directly connected, the VDU absolutely cripples the MK14 not only by stealing most of its RAM but by halting the CPU whenever it is reading a line of video data from the RAM, which amounts to a considerable portion of each video frame time.

This is why my VDU is not and never again will be directly connected to the MK14, and why I decided to create an independent, dedicated method of driving the VDU for demo purposes. The VDU just messes up the MK14 too much, both physically and from a system point of view. You've mentioned briefly that you have other SC/MP based systems - maybe some of those have considerably more RAM and can afford to donate some of it to the VDU without compromising their ability to run code.

I don't say that the VDU was completely useless, it taught me about the relationship between bits in screen memory and pixels on the display in a simple-to understand way. I'm still hoping to create a slightly more action packed demo for the VDU than the static information message it currently displays.

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Old 1st Jul 2015, 6:57 pm   #13
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Default Re: The Sinclair / Science of Cambridge MK14

If you're not careful, though, you end up with a VDU which is nothing like SOC designed. Now, I've designed, repaired, etc all sorts of VDU systems from the simple (did one in 3 standard ICs once) to the very complex (one I have here has over 3000 DRAM chips and about the same number of logic ICs). So yet another doesn't really interest me.

But a logic-wise copy of the original MK14 one might do.

As for the character generator, it can't be hard to work out a suitable bit pattern for a character font and blow it into some kind of PROM or EPROM.
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Old 1st Jul 2015, 7:23 pm   #14
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Default Re: The Sinclair / Science of Cambridge MK14

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Originally Posted by TonyDuell View Post
If you're not careful, though, you end up with a VDU which is nothing like SOC designed.
Not in this case. You use the MK14 VDU hardware exactly as designed and the MK14 hardware exactly as designed: You just put a dual-port RAM between them so that the VDU reads continually from that, leaving the MK14 with all its RAM available and running at full speed, writing data into the other port of the dual port RAM every now and then.

Quote:
Originally Posted by TonyDuell View Post
As for the character generator, it can't be hard to work out a suitable bit pattern for a character font and blow it into some kind of PROM or EPROM.
Not hard, but unnecessary. You could find the data sheet for a HD44780 LCD display driver IC and pinch the already designed character set from that: I was assuming the aim was to make an exact clone of the MK14 VDU, in which case it would need to have the same character set with the same codes to access them (it only has 64 characters, no lower case, and many of the miscellaneous ASCII characters are missing).
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Old 2nd Jul 2015, 4:41 am   #15
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Default Re: The Sinclair / Science of Cambridge MK14

Sure, but the original desgin didn't have the dual port RAM. Admittedly it could be fitted on another PCB between the MK14 and the VDU, but it still makes a device that is different in operation and performance than the Sinclair one.

As a couple of asides, I think Practical Electronics did a design for a VDU board with its own RAM at about this time and that could be interfaced to the MK14 fairly easily (it might even have been shown in the magazine how to do it.).

And if I was going to do this now, I'd probably get a SAA5243 chip out of an old teletext TV, add a static RAM chip and a 6MHz clock and bit-bang I2C through the flag and sense lines of the SC/MP. Now that would be totally different to the original but would not take up any SC/MP RAM or even address space.

As for the character generator, a dump of the original would be useful to get a perfect copy of the font, but to start with anything with almost the right chracters would work. Anyway, I am not likely to be doing this any time soon (too many projects, too little time...)
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Old 2nd Jul 2015, 6:09 pm   #16
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Default Re: The Sinclair / Science of Cambridge MK14

My problem as well.

I vaguely remember the PE VDU as well. I never had that one. PE were quite strong supporters of the MK14 and published many software routines including a demo for the SOC VDU which showed an animation of four parachutists descending and landing. Not a bad achievement in 128 bytes of RAM.

When playing with retro kit, my mantra is, as far as possible 'never modify the original hardware' - so my scheme with the dual-port RAM would indeed have the DPR chip on an additional board acting as the bridge between the unmodified MK14 and the unmodified VDU card.

(That my MK14 no longer has its original keypad is not something I can really do anything about now: The original pad went the distance long before the machine ever came close to being 'retro').

But nowadays, I aim to disturb the original hardware and firmware as little as possible. For example, the recently built hex downloader / programmer works by 'typing' code into the machine via the keypad edge connector, and it takes that approach so that it:

1) - Requires no modification to the content of the monitor PROMs, which are the original SOC supplied parts.

2) - Requires no permanently installed support hardware such as a MAX232, which would be required for serial hex download directly through the SIO pin. Nor are any other hardware mods to the MK14 required.

3) - Uses no system RAM which is not already used by the standard monitor, therefore all user RAM remains available for user programs. (firmware based serial download routines would probably need to ring-fence some RAM as a buffer area).

Another decision I made was that the interface must be able to be connected or disconnected safely when the MK14 was powered and running, so that a demo program could be uploaded to it and the interface then disconnected to leave the MK14 in its clean standalone form, still running the demo.

For that, the interface connections to the keypad edge connector had to be completely isolated. 20 relays connected to the edge connector would have been one way to do this (it would have been interesting to hear that working!) but for higher speed, zero keybounce and less required power, I went for 20 optocouplers instead, with their outputs connected to the keypad edge connector in the same way as the 20 switches of an external keypad would be.

Another possibility, with the revised monitor fitted, is to read a prepared MK14 .bin program file into a modern computing device (Raspberry Pi?) and have it transmit that file to the MK14 in the form of the digital input stream that the MK14 expects to receive from the optional cassette interface. (The data format is described in the manual for the cassette interface, attached). The interface between the computer and the MK14 cassette data input could again, for isolation purposes, take the form of a single optocoupler which could be disconnected from the MK14 once the software has been loaded into it. The main reasons not to do it this way are (a) speed - only four characters per second - and (b) a 256 byte limitation on upload file size. (Plus, you have to set up the 'Load in' address manually every time).
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Old 2nd Jul 2015, 6:23 pm   #17
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Default Re: The Sinclair / Science of Cambridge MK14

I have a slightly looser view of what I allow (and to be honest I work on things a lot rarer than MK14s). Basically, no permanent irrevesable modifications, use technology that was contemporary with the machine (so if I am making an interface for a PDP11 it gets built from TTL and microcontroller do not appear anywhere near it!), and when repairing keep the original design, even of things like PSUs.

Adding the dual port RAM violates the last of those. While a VDU with a dual port RAM would be an add-on I would consider for the MK14 (I think dual port RAMs existed at the time, albeit they were very expensive), I would not consider it to be in any way related to the original SOC one.

For your keyboard add-on, why 20 opto-isolators. Can't you put 2 in series, and get away with just 12 (8 columns and 4 rows)?
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Old 2nd Jul 2015, 6:40 pm   #18
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Default Re: The Sinclair / Science of Cambridge MK14

That baffled me for a few seconds, but I get it now: Where were you when I was originally putting this together?

At the time, I just visualised substituting electronically controlled switches for the twenty physical switches of an external keypad. The input LEDS of the optocouplers are indeed connected in a row / column matrix, driven by tables of values chosen so that only one optocoupler LED is ever turned on.

The optocouplers came from the obsolete part bins at work so they didn't cost me anything, but I do admire the elegance of your pared - down solution.
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Old 2nd Jul 2015, 6:46 pm   #19
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Default Re: The Sinclair / Science of Cambridge MK14

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When playing with retro kit, my mantra is, as far as possible 'never modify the original hardware' - so my scheme with the dual-port RAM would indeed have the DPR chip on an additional board acting as the bridge between the unmodified MK14 and the unmodified VDU card.
Ironically, when such hardware was current, it probably would have been modified in all sorts of ways in the name of experiment.
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Old 2nd Jul 2015, 7:06 pm   #20
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Default Re: The Sinclair / Science of Cambridge MK14

Absolutely - although the VDU card remains intact and original as built, the MK14 bears many a solder blob and several repaired track cuts as a result of having been used in various projects at various times.
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