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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 13th May 2018, 9:48 pm   #61
Slothie
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Default Re: MK14 schematic revisions

Quote:
Originally Posted by SiriusHardware View Post
74LS157s?
Yes! I have proms on the brain
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!
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Old 9th Jun 2018, 10:31 am   #62
SiriusHardware
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Default 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...
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Old 9th Jun 2018, 2:18 pm   #63
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Default 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
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Old 10th Jun 2018, 9:56 am   #64
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Default 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.
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Old 10th Jun 2018, 9:06 pm   #65
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Default 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!
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Old 10th Jun 2018, 9:34 pm   #66
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Default Re: MK14 schematic revisions

D'oh! Those were SMD parts... oh well perhaps they'll come in handy sometime.
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Old 10th Jun 2018, 11:53 pm   #67
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Default 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.
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Old 6th Jul 2018, 2:02 am   #68
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Default 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.
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Old 6th Jul 2018, 8:46 am   #69
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Default 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).
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Old 6th Jul 2018, 9:02 am   #70
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Default 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.
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Old 6th Jul 2018, 11:26 am   #71
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Smile Re: MK14 schematic revisions

Quote:
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.
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!
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Old 25th Jul 2018, 8:41 pm   #72
SiriusHardware
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Default Re: MK14 schematic revisions

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

Quote:
Originally Posted by TonyDuell View Post
...the late version PCBs could take a keyboard switch that Maplin sold (I think they called it a 'clickswitch') and of course that was mentioned in magazines at the time. Many MK14 owners fitted 20 of those with white caps for the digits and some other colour caps for GO/MEM/ABORT/TERM (where did those names come from??). Obviously you then had to label them with Letraset or similar.
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....
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Old 10th Aug 2018, 10:45 am   #73
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Default 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
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Old 10th Aug 2018, 1:25 pm   #74
Slothie
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Default 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!
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Old 10th Aug 2018, 10:44 pm   #75
SiriusHardware
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Default 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).
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Old 10th Aug 2018, 10:47 pm   #76
Slothie
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Default Re: MK14 schematic revisions

Quote:
Originally Posted by SiriusHardware View Post
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..
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.
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Old 11th Aug 2018, 7:31 am   #77
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Default 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.
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Old 11th Aug 2018, 7:37 am   #78
Slothie
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Default Re: MK14 schematic revisions

Quote:
It may still be possible to buy smaller switches and keytops which use the same principle.
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.
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Old 11th Aug 2018, 2:23 pm   #79
SiriusHardware
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Default 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.
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Old 12th Aug 2018, 12:45 pm   #80
GrahamN
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Default 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.
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