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SiriusHardware 23rd Apr 2018 1:15 am

Re: MK14 schematic revisions
2 Attachment(s)
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
1 Attachment(s)
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 :)

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