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Old 2nd Mar 2023, 5:33 pm   #121
TonyDuell
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Default Re: Idiot building a MK14 thread

These 2 chips are PROMs (programmable read-only memory) which were programmed at the factory (or now by whoever sold them to you) with the monitor program firmware of the MK14. They were identical before they were programmed, so have the same IC manufacturer's number (74S571 I think)

Now this monitor program is 512 bytes long -- 512 8-bit locations. Each of the PROMs stores 512 4-bit words. They are connected up so that one chip stores the high 4 bits of each byte of the monitor program, the other stores the low 4 bits. It obviously matters which way round they are, if the program requires 'C4' it is not going to work if it's been turned into '4C' because the chips are in the wrong sockets. Getting them in the wrong sockets (but with pin 1 in the right place, of course) will do no damage, it just won't work.

The original MK14 monitor, the one that starts ---- --, had one chip with nothing stuck on it and the other one when a white paper dot on it. I forget which had to be IC2 and which IC3. The later monitor, and the one you're using, had one chip with green dot -- that has to go in the IC2 position, the other had a blue dot which meant it went in the IC3 position.
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Old 2nd Mar 2023, 5:35 pm   #122
ScottishColin
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Default Re: Idiot building a MK14 thread

Thanks - so it's a Science of Cambridge standard then?

Colin.

Quote:
Originally Posted by TonyDuell View Post
These 2 chips are PROMs (programmable read-only memory) which were programmed at the factory (or now by whoever sold them to you) with the monitor program firmware of the MK14. They were identical before they were programmed, so have the same IC manufacturer's number (74S571 I think)

Now this monitor program is 512 bytes long -- 512 8-bit locations. Each of the PROMs stores 512 4-bit words. They are connected up so that one chip stores the high 4 bits of each byte of the monitor program, the other stores the low 4 bits. It obviously matters which way round they are, if the program requires 'C4' it is not going to work if it's been turned into '4C' because the chips are in the wrong sockets. Getting them in the wrong sockets (but with pin 1 in the right place, of course) will do no damage, it just won't work.

The original MK14 monitor, the one that starts ---- --, had one chip with nothing stuck on it and the other one when a white paper dot on it. I forget which had to be IC2 and which IC3. The later monitor, and the one you're using, had one chip with green dot -- that has to go in the IC2 position, the other had a blue dot which meant it went in the IC3 position.
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Old 2nd Mar 2023, 5:38 pm   #123
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Default Re: Idiot building a MK14 thread

Yes.

I built an MK14 back in the day, it came with the old monitor PROMs (nothing and white spot). I then bought the replacement monitor PROMs from SoC which had the green and blue spots.

I've not built any of the reproductions that are now available, but I assume the people who are programming the monitor PROMs now are sticking to the same convention to avoid confusion.
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Old 2nd Mar 2023, 5:46 pm   #124
Mark1960
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Default Re: Idiot building a MK14 thread

Green and blue dots are what science of Cambridge used to mark the proms, so Sirius follows the same to avoid confusion.

I guess Sirius would prefer them to rhyme with the IC number. I would have gone with red and orange, following standard colour code, but fading paper dots could cause a problem.
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Old 2nd Mar 2023, 6:07 pm   #125
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Default Re: Idiot building a MK14 thread

It's just the original colour scheme adopted for identification of the 'new OS' PROMs by Science Of Cambridge, we continue to follow it so that the replicas match surviving original machines in that respect. No-one knows why these colours were chosen, I can almost visualise someone being sent out from 6, King's Parade to WH Smiths to 'buy some coloured sticky dots - two different colours'. SOC used to stick the coloured paper dots right over the active centre of the ICs, as if the PROMs didn't already run hot enough. All the ones I do have the dots placed at one end for that reason.

As I mentioned in another current thread it might have made more sense to use red and orange, as those are the resistor colour code colours for (2) and (3) respectively. Any electronics hobbyist at the time would have instinctively understood that red was IC(2) and orange was IC(3) and it would have been easier to remember which one went where.

The PROMs for the earlier version of the MK14 OS were distinguished by one PROM being marked with a paint dot and one not marked at all.

Edit: crossed with Mark and Tony above.
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Old 2nd Mar 2023, 6:24 pm   #126
SiriusHardware
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Default Re: Idiot building a MK14 thread

Is the 74LS173 now the only outstanding part you need? You've got all of the smaller bits, all the resistors and caps etc?
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Old 2nd Mar 2023, 7:42 pm   #127
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Default Re: Idiot building a MK14 thread

Yes thanks - all in now (picture is pre-IC population).

Colin.

Quote:
Originally Posted by SiriusHardware View Post
Is the 74LS173 now the only outstanding part you need? You've got all of the smaller bits, all the resistors and caps etc?
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Old 2nd Mar 2023, 7:52 pm   #128
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Default Re: Idiot building a MK14 thread

That's looking very nice. The only thing I'd point out is that the heatsink is quite large and overhanging the expansion connector fingers. Have you insulated the back of the heatsink so there is no chance if it shorting to any contacts? I know it's got a black coating but you don't want to risk shorting the 7.5V (which is at that end of the connector) to something else.

(Thinking a bit further ahead, when you get to the point of plugging in RealView you'll need to angle the heatsink upwards to make space for the mating connector, but that's one for later).
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Old 2nd Mar 2023, 8:14 pm   #129
Mark1960
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Default Re: Idiot building a MK14 thread

It looks like Colin followed Sirius method of spacer between heatsink and pcb. It might still need extra space depending on the type of connector used for the vdu.
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Old 2nd Mar 2023, 9:55 pm   #130
SiriusHardware
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Default Re: Idiot building a MK14 thread

The heatsink and mounting hardware were among the bits I sent up to Colin and they are identical to the ones on mine.

There are pictures of how it is mounted earlier on in this thread, as Mark says the heatsink is raised off the PCB with an insulated spacer which is high enough to let a traditional edge connector slide right on into the gap between the heatsink and the PCB.

My Realview fits on without any problems, Colin's heatsink arrangement ought to be identical (See post #25).
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Old 3rd Mar 2023, 9:41 am   #131
ScottishColin
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Default Re: Idiot building a MK14 thread

The spacer means it all looks fine to me (see photo with Realview attached).

Colin.
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Old 3rd Mar 2023, 10:02 am   #132
SiriusHardware
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Default Re: Idiot building a MK14 thread

One thing I do notice is that your keyswitches have round actuators (buttons) - normally that would be a potential problem if you plan to fit key tops because there would be nothing to stop the keytops from rotating and it could end up looking quite untidy. On the plus side maybe it is easier to design and 3D print key tops with round receptacles on the bottom than square ones?

How 'tall' are the round actuators on top of the switches? It's hard to tell from the direct overhead view.

There is already an 'alignment aid' in the form of the Slothie-designed keypad frame which, with its carefully sized square cutouts, will keep the keytops all nice and square - Colin did one for me and it makes the keypad look very neat.

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Old 3rd Mar 2023, 10:15 am   #133
ScottishColin
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Default Re: Idiot building a MK14 thread

My plan is to use the Slothie frame to keep the keycaps in place. Something for me to experiment with at the weekend.

There's something close to it here but I've not really looked hard at that design yet.

https://www.thingiverse.com/thing:5339849

Colin.
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Old 3rd Mar 2023, 10:46 am   #134
SiriusHardware
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Default Re: Idiot building a MK14 thread

Oh, I wasn't previously aware of that one.

Interesting project, those are pictures of an original issue IV or issue V MK14 which did have provision to fit the same type of switches for the keys as were used for the reset switch. The problem was that those switches didn't come legended so the aim there is to add legended key tops to the original type switches. The author of that project (pepperm) is an occasional visitor to these threads.

I love the way the arrival of the various replicas from Martin L, JMP, Slothie and Oddy (apologies if I missed anyone out) has spawned a whole cottage industry of add-on accessories, exactly the way it happened back in the day.

With so many original MK14s having missing or non-working keypads, I think there would be interest in a neat replacement keypad PCB which fits directly over the original keypad area, mounted on very short standoffs and either plugged into the keypad edge connector or for a neater look (and to leave the keypad connector available for other purposes) connected by hidden wiring descending straight to the PCB tracks of the keypad matrix below.

Last edited by SiriusHardware; 3rd Mar 2023 at 10:56 am.
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Old 3rd Mar 2023, 11:43 am   #135
Realtime
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Default Re: Idiot building a MK14 thread

Quote:
Originally Posted by ScottishColin View Post
The spacer means it all looks fine to me (see photo with Realview attached).

Colin.
Ah very good, no problems then. The birds-eye view in the other photo lead me astray!
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Old 3rd Mar 2023, 7:07 pm   #136
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Default Re: Idiot building a MK14 thread

Quote:
Originally Posted by ScottishColin View Post
My plan is to use the Slothie frame to keep the keycaps in place. Something for me to experiment with at the weekend.

There's something close to it here but I've not really looked hard at that design yet.

https://www.thingiverse.com/thing:5339849

Colin.
One small point: the position of the mounting holes on the original board and my issue VI board are slightly different because I was basically calculating based on measurements from pictures when I did the original design.... So a keyboard frame for the original boards will probably not fit and issue VI board and vice versa.
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Old 3rd Mar 2023, 7:17 pm   #137
Mark1960
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Default Re: Idiot building a MK14 thread

The round switches would also work with a flat overlay, possibly printed on photo paper or laminated, then a frame to hold it in place.
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Old 3rd Mar 2023, 8:00 pm   #138
SiriusHardware
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Default Re: Idiot building a MK14 thread

The switches Colin has fitted might work with Realtime's ready-baked issue VI keypad bezel and overlay, I'm not sure what type / what height switch that combination is designed to work with though.

I think at the moment Colin plans to 3D print his own legended key caps, basically the same idea as the ones linked to in #133 but redesigned to fit (a) Colin's switches and (b) Slothie's keypad frame, which obviously is designed to fit the issue VI, not original MK14s.
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Old 4th Mar 2023, 12:37 pm   #139
ScottishColin
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Default Re: Idiot building a MK14 thread

Spot on - and hopefully multi-colour.

Colin.

Quote:
Originally Posted by SiriusHardware View Post
The switches Colin has fitted might work with Realtime's ready-baked issue VI keypad bezel and overlay, I'm not sure what type / what height switch that combination is designed to work with though.

I think at the moment Colin plans to 3D print his own legended key caps, basically the same idea as the ones linked to in #133 but redesigned to fit (a) Colin's switches and (b) Slothie's keypad frame, which obviously is designed to fit the issue VI, not original MK14s.
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Old 4th Mar 2023, 1:01 pm   #140
ScottishColin
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Default Re: Idiot building a MK14 thread

I have to go out but the chip has turned up.

A couple of you mentioned to test with some chips removed - can I make sure I have this right:

All chips inserted with the exception of the 8154 (U8) and the two ram chips (U5 and U7) closest to the keyboard.

Have I got this right?

Colin.
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