Re: MK14 schematic revisions
 

In that case I think yours will be MH74S571 PROMs rather than original DM74S571s?

Let us know how you get on. I should think it will go OK because you have already proved the system using substitute RAM.

Until TonyDuell mentioned it, I didn't know the 74LS157s could be so troublesome. I should look into sourcing some good working spares for my MK14 while the originals are still working.

Re: MK14 schematic revisions
 

My proms are the TESLA brand MH74S571. I also have a set unprogrammed TESLA proms which I bought earlier.

The AM9111 RAM chips have arrived from Israel and are working fine.

I also got two 80C95 (instead of the hard to find 80L95) from littlediode in the UK and it works fine too.

From Martin (the Czech guy) I got the tip I could also have used the 74LS365 instead of the 80L95. Ah well, my MK14 is complete now :-)

Re: MK14 schematic revisions
 

Congratulations! Whats next, an Oric Atmos? ;D

Re: MK14 schematic revisions
 

Well done, Gert.

The primary address range of the OS PROMS is from 0000H-01FFH. I believe the Czech replicas (one of which I think you must have) are of the issue V MK14. Could you look at address 0200H onwards to see if there is a copy (image) of the PROM code there? I would be interested to know if the address decoding modification (discussed earlier in this thread) has been incorporated into those replicas.

What was the first program you ran on your replica? 'Message', from the original manual, is often popular as you can see something happening and then have some fun trying to devise your own scrolling message without using 'K', 'M', 'V', 'W', 'X', or 'Z'. :)

You mentioned that you had to use 74LS257s instead of the 74LS157s you had originally. What prefix / brand were the 74LS157s which did NOT work?

I have found a UK source for original Nat-Semi branded DM74S571 PROMs, a pair of which arrived today. At almost 12 GBP for the pair they weren't especially cheap but in light of the discovery that I can not program the Tesla MH74S571 devices, I thought I had better obtain a pair of National Semiconductor 'DM' prefix devices to program up and keep as spares for my MK14.

I've asked a one-off favour from someone who believes they can program Slothie's Tesla MH74S571 PROMS, and we are awaiting the outcome from that.

For anyone else, I'm happy to program (and test) PROMs for an MK14 if they are any of the following devices:

National Semiconductor DM74S571 (but not DM74S571A)
Philips / Signetics 82S131 -or- 82S131A
AMD AM27S13 (but not 27S13A)

Obviously, they must be unused blanks because this type of PROM can not be erased and reprogrammed.

I am NOT able to program the Tesla MH74S571. I believe Slothie has a medium-term plan to make a specific programmer for those...

Re: MK14 schematic revisions
 

When I've finished my MK14 and refurbished my PET and nixie calculator I'll be on it!

Re: MK14 schematic revisions
 

Hi
Just to let you know - after a bit of archaeological excavation in what I laughingly call my stores, I located the 7808s
If you pm me your postal address and let me know how many you require I'll send them on their way - I'm still surprised that I found them as well as other long forgotten 'treasures'

Kindest regards
John

Re: MK14 schematic revisions
 

That should be 7408s of course

J

Re: MK14 schematic revisions
 

Oops when I didn't hear anything I thought you'd not been able to find them, and got some off eBay for about 60p each! Sorry if I've put you to unnecessary trouble. When everything arrives I should have everything now, assuming a pair of the 74LS571's I have will work. I have ordered some new ones, and if they don't work in the MK14 I have some in various old computers that I can swap them out for.

Re: MK14 schematic revisions
 

74LS157s?

Re: MK14 schematic revisions
 

No trouble - sorry but just took a while to find them - it gave me a chance to put a bit of order on my stores and I now have a better handle on what I have.
Best of luck with the project - I will be following thread with interest

Rgds
John

Re: MK14 schematic revisions
 

Yes! I have proms on the brain :D
I'd love to know why some '157s work and others don't. I assume its due to the circuit relying on the propagation delays through the multiplexer. It's odd because they used a proper latch for the column decoder, I suppose the multiplexer was just cheaper!

Re: MK14 schematic revisions
 

Slothie's gone a bit quiet lately - last I heard he had managed (via a pan-European co-operative effort) to get a couple of PROMs programmed, and somehow hurt his foot in the process.

Hope all is well, Ian, don't forget to keep us posted on progress...

Re: MK14 schematic revisions
 

The proms arrived in the post this morning! Thanks to all involved in getting them done.

I've still got some soldering left to do (well, lots really!) but I have all the components now, with only the question of the 74ls157s I have and if they'll work.

My foot/ankle is much improved, but put me out of action for over a week and so a number of things have been delayed, but I'll be back on the project asap and will report any progress/problems here!

Ian

Re: MK14 schematic revisions
 

If you need to resolve the question of compatibility of your 74LS157s you can always send them to me and I can tell you if they work in an original MK14.

The original parts are National Semiconductor branded parts, as that was who supplied the kits of parts to Science Of Cambridge. Anyone attempting an MK14 new-build should probably therefore aim to find the NS branded type if possible.

Gertk64, who posted in this thread a while back, said he had had to replace his original choice of 74LS157 parts with 74LS257s, so that might be another option if you have problems sourcing parts which work.

Re: MK14 schematic revisions
 

I think you have a point.. I just looked on eBay and there's someone selling NatSem DM74LS157 chips for cheap so I've ordered some of those, I don't mind having even more as they are a generally useful IC in many circuits and I'd rather not start pulling chips off of my vintage PCs if I can help it, especially since my desoldering iron committed self immolation last year and has yet to be replaced.
If I've any question I might take you up your offer Sirius!

Re: MK14 schematic revisions
 

:-[ D'oh! Those were SMD parts... oh well perhaps they'll come in handy sometime.

Re: MK14 schematic revisions
 

As you already have some DIP 74LS157s which (I deduce) must not be Nat Semi branded you might want to give those a chance to work - or not work - first.

If you can demonstrate that some other brand of 74LS157 either works or doesn't work then that would be helpful information for anyone following in your wake.

Contact the seller ASAP and explain - I've made the same snap error myself in the past, and as long as the parts are not already in the post the seller is usually happy to amend the order - if they don't have Nat Semi DIP parts to replace them with, they may at least have some other brand which is not the same brand as the DIP parts you already have.

Re: MK14 schematic revisions
 

To those following this thread I've not given up, I've just had a lot on and a period of illness so I've had only a little time to spend on this project. I certainly am making some progress but its slow!
I'm close to getting the board finished and the few wiring errors on the pcb thar I know about fixed (oops!) so I've just got to finish that and find a suitable power supply and do some testing.

Re: MK14 schematic revisions
 

I think this may have been done to introduce a small delay, so Tsu and Thold weren't violated (Setup and Hold times).

Re: MK14 schematic revisions
 

I loved my MK14. I used the SC/MP to decode MSF rugby fast code tine (which they've stopped now - just the slow code is in use).
Hated that damn keyboard! Difficult to knock the program in with it.
I still have my pencil-written program for the clock I made, if anyone is interested...I can photo. it, or scan it and publish a link to it.
I used the MK14 to learn, but the clock I made used NO RAM(!), just the registers, and I used HP latching displays. It worked second time! I'd swapped the 10's and 1's of the hours over, so 12:34 read as 21:34. As prom's were expensive then, (and I'd had to make my own prom programmer - which had switches for the address and data, and a button to program that 'nibble' - 4 bits at time) I just swapped the wire-wrapped pins over. Taught me more about computing than kids today can ever learn. 1978, IIRC.

Re: MK14 schematic revisions
 

My replica is having tact switches rather than the conductive rubber things on the original MK14.
The tact switches aren't as good as proper keyswitches but they're orders of magnitude better than the original!!
Your clock sounds interesting, as is your inventive solution to the digits!

Re: MK14 schematic revisions
 

(Brought across from the 'Tesla MH74S571 Programming' thread, as it was veering well away from that specific topic)

I think those 'clickswitches' are what my MK14 has on the keyboard currently fitted to it, but, being an issue II it has no provision for them to be fitted on the MK14 itself.

On mine, they are fitted on a separate PCB (actually a well disguised bit of veroboard / stripboard) which is mounted directly over the original keypad position with a gap of about 3-4 millmetres between the main PCB and the keypad PCB. I really must get off my lazy rear and design a proper generic MK14 keypad PCB. If Slothie can come up with a whole MK14, I'm sure I should be able to do that one little thing. You would think nowadays that it would be trivial to get a set of keytops custom 3D printed with legends embedded in them in a secondary colour, but I don't think we are quite there yet, so it's still down to plain keytops and good old Letraset.

As I imagine you (Tony) know, but others may not, the command key names came from the development system from which the MK14 was derived, the earlier National Introkit which I believe is what we see here in this Youtube Video.

https://www.youtube.com/watch?v=fK-IZuIonnY

If you pause during close ups of the keypad you can clearly see the same Go, Mem, Abort, Term command keys.

As far as I know the monitor / OS in the National Introkit was byte for byte identical to the one subsequently used in earlier issue MK14s.

That still doesn't explain the choice of the actual words, though....

Re: MK14 schematic revisions
 

A quick question regarding the 2111 RAM - The best price I can find is around £9 each for AM9111BPC. Is that a good price? Or can you recommend a supplier?

Thanks

Re: MK14 schematic revisions
 

I paid about £29 for 4 chips from an eBay seller in the USA (almost half of that was postage...) There are part factors that advertise in various places and the price you quote is typical of them.
There is a part IM65X61 which turns up cheaper from time to time that is pin compatible and usually much cheaper, but requires you use one of the spare gates to generate chip select during write cycles, so you'd have to modify your design. I put a jumper on my board to select one or the other depending on availability. I haven't tried it yet so I cant guarantee it will work!

Re: MK14 schematic revisions
 

Graham, you could try dropping a line to LittleDiode, who are at least in the UK (Surrey) so even if they quote you a little bit more for the devices you may win back on postage.

There's also Donberg over in Eire (The 'o' should have an umlaut over it, but I don't know how to generate that on a UK keyboard).

Re: MK14 schematic revisions
 

I thought about printing the legends backwards onto Laser OHP film then cutting them out and glue-ing them to the keycaps printed side down, so the film protected them from wear. I would have to experiment with what glue is best but you might be able to get very good results.

Re: MK14 schematic revisions
 

It used to be possible to buy keyswitches with plain white flat keycaps which had snap-on clear covers, the idea being that you could print your key legends onto plain or transparent paper, cut them out to the exact size of the keytop, place it on the keytop and then snap the clear cover on over the top.

There's a video here

https://www.youtube.com/watch?v=Ed3zSPnodhE

of an MK14 with an added on keypad using that type of switch, although the ones used in that example are perhaps over-scale. It also illustrates an inherent problem, that the originally clear plastic covers tend to go yellow over time.

It may still be possible to buy smaller switches and keytops which use the same principle.

Re: MK14 schematic revisions
 

You can, I've got a load of 12mm tact switches with coloured keycaps and clear covers from china. They were cheap but I've not seen them since (not that I've been looking much) and the quality is not huge. And of course they are tact switches and not proper keyboard switches.

Re: MK14 schematic revisions
 

The 'floating' keypad on my own MK14 - made only a few years after I originally got the MK14 - actually uses very low profile tact switches and low profile keytops, mainly to keep the height of the keys as low as possible. Full travel keys like cherry switches would be the ideal of course but they would be disproportionately tall on what is otherwise a fairly low profile PCB.

Although tact switches do feel very 'hard' and unyielding compared to proper full travel keyswitches they have the considerable advantage over PCB mounted dome switches or 'rubber squash' membrane keypads that they work every single time you press them.

Believe me, that is a big step forward from what the MK14 originally came with.

If you were going to do what people often did do back in the day, bury the MK14 PCB inside an enclosure and mount the keypad and the (often enlarged / upscaled) display on the front panel of the enclosure then a 'real' keypad with full size, full travel keys like the one on that Youtube example I linked to would be much preferred for sheer comfort and ease of use.

Re: MK14 schematic revisions
 

I tried Littlediode, but they are more expensive than the USA ones. I have, though, managed to find some (hopefully) equivalent chips (SAB2111A) from a dealer in Germany at just 4 euros each (plus shipping), so I've ordered these.

Re: MK14 schematic revisions
 

That sounds like a good find for the price. I think the SAB prefix (Siemens?) is a prefix not as widely known as, say, the Intel devices (P2111) so you did well to come across those.

The programmer I'll be using to program your PROMs with also has a handy IC testing feature which includes once-common static and dynamic RAM ICs, so if you'd like those tested feel free to include them with the PROMs. However, I see no reason to assume that any of them will be faulty.

Unfortunately I wouldn't be able to test those in the MK14 as they are one of the few ICs which are soldered rather than socketed in my particular machine - when I built it I happened not to have any IC sockets with the right number of pins, and I was impatient to get it built and up and running.

There must, somewhere, be large old redundant PCBs full of 2111 RAM ICs just lying around waiting to be found. I used to see a lot of that sort of thing in cardboard boxes at the larger radio rallies during the nineties.

Re: MK14 schematic revisions
 

They are coming from Germany, so I expect a week or two - I will send them with the PROMs if I may as there is no rush on the build and it will be good to be sure they work!

From looking around the internet, the RAM seems to have been commonly used in old Atari arcade games - unfortunately arcade boards generally fetch high prices these days, so not really a viable option! (I did scour the internet looking for cheap boards or spares from the arcade spares suppliers, but no luck)

And yes - I understand these are Siemens.

Re: MK14 schematic revisions
 

Sounds like a bargain. I wasnt able to find a datasheet for the SAB2111A but it would be surprised if its not compatible, 2111s were used in all kinds of products so it would be bzarre to make a device rhat wasnt 100% compatible, and the MK14 hardly stresses them timing wise.

Re: MK14 schematic revisions
 

Information on these is so scarce that I was worried they might not actually be RAMs at all, just some other IC which had 2111 in the part number.

However, I did find this one page referring to the use of SAB2111s as substitute 2111 RAMs for an old development system.

https://www.***********/photos/51699638@N06/26956276139

Re: MK14 schematic revisions
 

This is the only data I could find - the actual part number of the devices I ordered is SAB2111A-D4 which is listed as static RAM 256k x 4 @ 450ns.

Re: MK14 schematic revisions
 

I have a single step circuit that can switch single step on and off, I believe. It is currently on a piece of Vero board but I could reverse engineer it if any one is interested. I last used it in the 80's you see.

Re: MK14 schematic revisions
 

No need, Pepper, that circuit is in the manual which is available from various places on the 'net'. It goes with the 'Single Step' program in the 'System' section in the original manual. The improved 'New' version of the MK14 OS has the Single Step code built in.

One word of warning, if you use the new OS and you don't add the Single Step hardware, it is necessary to tie the Sense-A input low, otherwise you'll find you can't run programs.

Re: MK14 schematic revisions
 

My circuit is not the same as the one in the manual and is switched. I am an original OS and PCB guy too.

Re: MK14 schematic revisions
 

The one in the manual is actually switched as well, so you can swap between Single Step and normal running.

If you use your MK14 from time to time (rather than just maintaining it in running order as a museum piece) the 'New' OS is a big improvement over the old, requiring fewer keypresses to enter each byte of data and also incorporates the support routines for the tape interface (easily built on a bit of veroboard) and a handy jump offset calculator.

I'm surprised to hear that an issue V MK14 would have the 'old' OS, which is distinguished by the fact that it starts up with a '---- --' prompt, rather than the '0000 00' prompt of the 'new' OS.

Re: MK14 schematic revisions
 

It is all a learning exercise for me even all these years later, it would see. When I built the kit I never knew that I had anything other than the same version as everyone else, but as my kit starts up with 0000 00 and the board says V on it I must have the later version or the ROM. Hope it works with my replica Version I board...

Re: MK14 schematic revisions
 

It does, you just need to tie Sense-A low if you do not add the optional single step hardware. Otherwise I think you will be able to enter code OK, but you won't be able to run it.

Re: MK14 schematic revisions
 

Well after some head scratching I now know what you mean. The trouble is that the Mk14 I purchased is a Rev V board with a V2 OS, but I was supplied with a manual for V1 OS. That explains why I spent lots of time correcting the manual all those years ago...:-)

Re: MK14 schematic revisions
 

There is an excellent pdf copy of the late version MK14 manual on Martin Lukasek's website here

http://www.8bity.cz/files/MK14/mk14ManualV2_OCR.pdf

That is the version of the manual you need to work with if your MK14, whether early issue or late issue, has the 'new' OS, as it correctly describes the operation of the machine with the new OS.

Re: MK14 schematic revisions
 

Thanks for that, I've been looking for a manual for the v2 rom. Update: My replica mk14 isnt working, I'm not seeing any bus activity so its either a board wiring issue, bad crystal or bad sc/mp.... I've got a spare processor on the way so may be able to report soon!

Re: MK14 schematic revisions
 

Silly question, but is your original SC/MP an SC/MP II? The original SC/MP only operates up to 1MHz as far as I know. SC/MP II is up to 4MHz, although of course Clive chose to overclock it slightly, mainly because 4.43Mhz crystals were widely used in colour TVs and therefore cheap.

If you can tolerate the delay / risk involved in posting chips back and forth - an activity I know you are no stranger to - I'd be happy to test any suspect ICs in my MK14.

The crystal you can test independently by knocking up a simple logic IC based oscillator like this one, attached, found as a web image. Alternatively just fit any crystal you can find in the range from 1MHz to 4MHz in the MK14 - if your original crystal is at fault the machine will run with any other reasonable crystal frequency, just not at the normal speed.

Also definitely check the states of CPU pins which can halt the CPU, such as RESET and other less obvious ones.

The actual SC/MP datasheet seems quite hard to find, here it is presented in unconventional one-page-at-a-time html format, which is better than nothing.

http://searle.hostei.com/grant/SCMP/index.html

SC/MP applications handbook packed with all sorts of SC/MP information here on bitsavers:

http://bitsavers.trailing-edge.com/c...book-Feb77.pdf

Re: MK14 schematic revisions
 

Yes, its a SC/MP II, I just looked at it and its marked ISP-8A/600 which is a SC/MP 2 according to wikipedia. I'll hopefully be able to give it another look with the 'scope soon, and maybe breadboard an oscillator as you suggest.I've also got some 1 mhz & 4mhz oscillator modules which I should be able to rig up as an external clock to test if required.

Re: MK14 schematic revisions
 

Update!
Well, theres good news and bad news....
Good news is the the lack of bus activity I was seeing was because I missed out a track between the SC/MP and the crystal (oops!) and with this fixed all seems to be working well.... except the display is backwards! The keys also map to the wrong functions so I suspect I have something wrong around the 7445 chip.
This does tell me however that the TI 74LS157N multiplexers I bought from Farnell seem to work fine in this application, I am using them with 1977 7408j's to drive the display, so it did occur to me that possibly the older 7408s might have higher input capacitance which holds the signal long enough for the 74LS 157s to latch it. This is pure speculation as I have never seen any plausible explanation of why some 74LS157s might work when others dont.

It also shows my PROMS that so many people got programmed correctly for me are fine, which is another relief! The '8' key seems to working as the 'mem' key and I was able to stem through the first few bytes of the ROM and reading backwards the values are the same as the monitor listing.
It looks like when I have worked out what tracks need cutting and re-wiring I might need at some point get a new board made with all the corrections as I already have 5 "bodge" wires and am l,ooking at adding at least another 10...!

With a 9v power supply the regulator is getting quite hot, so I might have to look inti getting a bigger heatsink for it at the same time.

Slothie

Re: MK14 schematic revisions
 

Well done Slothie - I'm especially glad your PROMs turned out OK since they must literally been to more different countries now than I have in my lifetime.

At first glance it seems you might get away with just cutting and reversing the order of the column / cathode driver outputs from the 7445 to the display and keypad, but I haven't really thought it through so don't take my word for it. You could try it by fitting a socket with the appropriate number of pins onto a bit of veroboard and wiring that into a header plug or another socket with the output pins rearranged in reverse order, then plug that into the original socket for the 7445.

Considering this was just a 'thing you decided to do' completely from scratch, I have been impressed by your dogged determination to succeed, and it finally seems to be paying off.

If the mystery of the 74157s so intrigues you, once you have it working you can knock yourself out trying out all sorts of different brands of 74157 and post the results in a nice table here. ;)

Re: MK14 schematic revisions
 

I've been looking and discovered the version 5 schematic has pin numbers on the 7445, display and IC11 (80L95/74LS365) whereas on the v1 schematic which I was working from they were not marked, and in some cases the labelling is a bit vague.....
It looks like I have the column drives to the display reversed and also the row lines on the keyboard connecting to the wrong data lined via ic11..... if I'd worked from the v5 schematic originally i'd not have made these mistakes. I'm not sure if that accounts for all the symptoms but I like your idea of using a header and jumper wires while I experiment, I might make a veroboard that plugs into IC11 and I can move the connections around to see what makes sense. The display i'll just cut the column drive tracks an run wires between the display and the 7445 because I'm sure why that is wrong. I will definitely be redoing the board once its working and transferring over all the components to get rid of all my bodges!

I've finally persuaded my phone to upload a picture, the flash washed out the display but I'll get a better picture once I've got the display the right way round!

Re: MK14 schematic revisions
 

It looks great for something completely scratch designed!

I suggest that chopping / reversing the 7445 outputs before they diverge towards the display and keypad may fix BOTH of your problems at the same time.

Just for fun, a short program for you to enter and run when you get it working.

With the MK14, it's always a good idea to check through entered code at least once more before running, as just one wrong byte can send the CPU out of control and trash the rest of your carefully entered code in the process.

After entering all the bytes from 0F20-0F42 as below, hit Abort, O F 2 0, GO.

0F20-C4
0F21-0D
0F22-35
0F23-C4
0F24-00
0F25-31
0F26-C4
0F27-0F
0F28-36
0F29-C4
0F2A-3B
0F2B-32
0F2C-C4
0F2D-07
0F2E-01
0F2F-C2
0F30-80
0F31-C9
0F32-80
0F33-C4
0F34-FF
0F35-02
0F36-70
0F37-94
0F38-F5
0F39-90
0F3A-F1
0F3B-00
0F3C 79
0F3D 06
0F3E 74
0F3F-78
0F40-3F
0F41-38
0F42-6D