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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 29th Nov 2018, 7:12 pm   #121
Slothie
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Default Re: MK14 schematic revisions

Hmm thats possible, I've included most of the changes aside from the extended PROM decoding (so it appears multiple copies in the lower 2k) because I needed a gate to allow the use of 6561 ram chips which required a chip select during write cycles. However there is a spare AND gate which might be used to replace the inverter on the reset circuit and allow a standard pull-down reset circuit that could be brought out to the keyboard connector, since +5 isn't on the connector.
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Old 29th Nov 2018, 7:19 pm   #122
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Default Re: MK14 schematic revisions

Here's the current schematic.
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Old 29th Nov 2018, 8:38 pm   #123
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Default Re: MK14 schematic revisions

Nice diagram!

Actually, the simplest way out would be to make the last finger +5V instead of 0V, and the second last finger Reset. There are arguably good reasons not to have unlimited +5V on that edge connector though, as the edge connector is not keyed / position limited and one slip / senior moment could see hard +5V ending up where it should not be.

I don't think there are any pre-existing or historic peripherals which ever used those two 0V connections, because the external keypad doesn't need 0V.
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Old 11th Dec 2018, 10:03 pm   #124
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Default Re: MK14 schematic revisions

I think that I will include reset on the keyboard connector, and make it a pull-down to ground because for the reasons you outline I don't want to add Vcc to the connector even if there was room, and because that's what is the convention is for reset lines both then and now.

I'm also going to add a 2.1mm barrel jack connector for the power, as I've already had problems with wires soldered to the edge connector and having moved the RAM/IO to clear the regulator there is plenty of room for it. Not very "original" but I think the utility of it overrides that objection.

Once I've mulled over other possible improvements I'm not far from ordering rev 2 of my board! Any other ideas are of course welcome. I'm then going to turn my attention to programming Tesla PROMs since I seem to have accumulated a number of them and their seems to be interest in the possibility.
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Old 11th Dec 2018, 11:21 pm   #125
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Default Re: MK14 schematic revisions

Yes please, I would be just as interested in a Tesla programmer since those PROMs are considerably cheaper than any of the pin-compatible ones I can currently program.

Re: The MK14, beware feature creep

Before you know it you will be replacing those hard to get 2111 RAM ICs with a 6116, the PROMs with a common 8-bit wide EPROM, the RAM I/O device with an 8155 or 8255 (Neither has RAM, but the 6116 would make up for that). Basically, you'd end up with an MK15... a bit like mk14man's fine effort a few years ago...

http://mymk14.co.uk/

..but using only conventional components.
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Old 12th Dec 2018, 12:25 am   #126
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Default Re: MK14 schematic revisions

Quote:
Before you know it you will be replacing those hard to get 2111 RAM ICs with a 6116, the PROMs with a common 8-bit wide EPROM, the RAM I/O device with an 8155 or 8255 (Neither has RAM, but the 6116 would make up for that). Basically, you'd end up with an MK15... a bit like mk14man's fine effort a few years ago...
Yes, that is a thing to beware!

When setting out on this project the aim was:
* use the same chips or at least pin compatible
* same layout, look and feel
* include usability modifications like keyboard etc since the original design would be hard to reproduce and pretty useless in practice.

The mods so far are the sort of thing an enthusiast at the time would have made, and although adding a power connector is kind of stretching that a bit I think it wouldn't stick out like a sore thumb and makes using the computer a lot easier, and if I'd had an MK14 back in the day I'd probably have epoxied a power connector to the PCB and wired it in after having the wires break off for the 13th time.....

If I was going to build an "MK15" using more recent, obtainable chips I'd build a SC/MP computer with a NIBL ROM, >4k RAM and maybe built in keyboard and VDU a la ZX80......

Just for the hell of it!

PS: I do have a load of 6116's someone gave me that have been sitting arounf unused.....!
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Old 12th Dec 2018, 1:22 am   #127
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Default Re: MK14 schematic revisions

I would actually be quite interested in an 'MK14.5' PCB which used a slightly later 8-bit wide RAM and EPROM.

Mapping the 6116 into the three areas occupied by the standard RAM, extended RAM and RAM /I/O but keeping it away from any other part of the range could present a challenge - could use a 74S571 PROM as an address decoder of course, but it would be better to shy away from devices which hardly anyone can program now. MK14man's project, although brilliantly executed, was problematic for the same reason, not many people have facilities for programming FPGAs and other programmable logic.

My original MK14 is almost too precious to run now, I keep thinking that the next power-on inrush will be the one which finally finishes off a vital component, so it would be good to have a second, expendable machine for playing with, but quite honestly the difficulty and expense which you and others have incurred while trying to acquire parts, get PROMs programmed and so on would really put me off building a precise replica.

Oh by the way, if you are feeling particularly enthusiastic, how about tracking the address, data and control buses to the fingers on the underside of the top edge connector? SOC never did bother to do that, but it would have been a very sensible thing to do especially after the introduction of the VDU, which required all those connections.

Last edited by SiriusHardware; 12th Dec 2018 at 1:30 am.
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Old 15th Dec 2018, 12:32 am   #128
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Default Re: MK14 schematic revisions

Quote:
Oh by the way, if you are feeling particularly enthusiastic, how about tracking the address, data and control buses to the fingers on the underside of the top edge connector? SOC never did bother to do that, but it would have been a very sensible thing to do especially after the introduction of the VDU, which required all those connections.
Hmm.. That probably wouldn't be to hard since quite a lot of those lines go up to the RAM/IO anyway. Do you know if there is a "definitive" mapping (i.e. order of pins) or was it just left to user whim?

I just checked my circuit with 6561 RAM chips by changing JP1 on the schematic I posted earlier and it works fine. One slight difference is all the memory locations reset to FF rather than random values on initial power up which is interesting but unimportant. The "add on" ram chips U6 & U7 work too. I wasn't able to test the RAM/IO because due to the heatsink the chip doesn't go into the socket - on the new PCB I've moved the RAM/IO chip to give more room for the heatsink. It only involved removing and relaying about 50 tracks!
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Old 15th Dec 2018, 3:11 am   #129
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Default Re: MK14 schematic revisions

If you are serious about it I would look at the connections for the VDU and try to have the various signals arrive at the edge connector in the order in which the VDU needs them. Probably the greatest difficulty presented by the VDU was the fact that there wasn't any provision to connect it to the main board. Later issue boards had contact fingers on the underside of the top edge connector but they still weren't tracked to anywhere.
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Old 15th Dec 2018, 7:27 pm   #130
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Default Re: MK14 schematic revisions

I've found the pinouts for the VDU in the VDU manual I downloaded from the internet but my question is, when viewed from the top (component) side, is pin 1 (0v or ground) on the left hand or right hand side? I've looked at google for images of the PCB but all I seem to find are replicas that seem to have 64 way DIN connectors which I doubt would have been an extravagance that SOC would have had any truck with! If you know of any photo's of the original board or perhaps a component layout diagram with the connectors marked then that would be nice.

Ian
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Old 16th Dec 2018, 1:45 am   #131
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Default Re: MK14 schematic revisions

As far as I remember the MK14 board layout diagram from the manual was actual size and showed the true locations and spacing of the keys. Online versions of the original manual rarely include that page because it, like the circuit diagram, was intended to be removed and in most cases was, but I still have mine, I should maybe scan and upload it here.

Last edited by SiriusHardware; 16th Dec 2018 at 1:52 am.
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Old 16th Dec 2018, 1:50 am   #132
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Default Re: MK14 schematic revisions

Away from base but I have an actual VDU that I can examine when I get home. The VDU was laid out for a card rack style connector and in fact mine has one fitted although SOC did not supply one.
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Old 17th Dec 2018, 2:05 am   #133
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Default Re: MK14 schematic revisions

Image #1: Rough sketch of MK14 VDU connector pin numbering seen from top (component) side. I made some quick measurements to verify that the 'a' row is definitely the row closest to the PCB edge and the 'b' row is the row further away from the edge.

Image #2: Top side showing '1' at right hand side and '32' at left hand side

Image #3 Underside of connector area

Image #4 The reason mine has a DIN connector fitted. In 2013, after the VDU card had languished in a drawer for at least 30 years, I took pity on it and connected it to a PIC micro evaluation board programmed to emulate a 512 byte block of RAM which was preloaded with a static message for the VDU to display. The aim was basically just to see / show the VDU card working without having to connect it back up to the MK14.
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Old 17th Dec 2018, 11:40 pm   #134
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Default Re: MK14 schematic revisions

Well thats fortunate because it looks like most of the signals come off the RAM/IO in the correct order. And there's not much on the bottom of the PCB in that area so its looking very possible.
Thanks for the photos, looks like I'll have something to do while the family's wathing Die Hard etc this Christmas!

Happy Christmas to all on the Forum and a Vintage new year!
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Old 18th Dec 2018, 12:06 am   #135
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Default Re: MK14 schematic revisions

Good luck and... um... maybe you can take out those 1.5K of unwanted PROM images as well? (I may be pushing my luck, but that, along with the availability of the data bus, control lines and most of the address bus on the top connector would create the possibility of making an offboard 'RAM Pack' (in true Sinclair tradition) to go either inline between the VDU and MK14 main board, or just by itself for an instant increase in RAM).

All the best.

Last edited by SiriusHardware; 18th Dec 2018 at 12:15 am.
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Old 18th Dec 2018, 1:17 am   #136
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Default Re: MK14 schematic revisions

I spent a lot of time thinking about that because memory size is one of the biggest limitations of the MK14, especially if you go to the trouble of adding the VDU, and to do it I'd have to either remove support for 6561 memories or add another 74 series chip (which I suppose wouldn't be too bad if I could squeeze it in unobtrusively).

Since the 2111/ AM9111 memory availability is patchy I'd rather not drop 6251 support as it gives more options. The 2111 compatible memories are one of the hardest part to get if you don't want to pay 30+ each from a broker.

Unfortunately there aren't really any other memory alternatives, other than the MK14.5 approach of using something like a 6116 and logic to recreate the memory map...
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Old 18th Dec 2018, 11:30 am   #137
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Might it be possible to have an option to use 2114s in place of 2111s, even if you 'waste' most of each chip, as they seem cheaper and more widely available? They look almost identical, so they would preserve the original appearance while making it much easier to source memory.

Of course if you could fully utilise and map the extra memory available in 4 * 2114, so much the better.
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Old 20th Dec 2018, 1:57 am   #138
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Quote:
Originally Posted by SiriusHardware View Post
Might it be possible to have an option to use 2114s in place of 2111s, even if you 'waste' most of each chip, as they seem cheaper and more widely available? They look almost identical, so they would preserve the original appearance while making it much easier to source memory.

Of course if you could fully utilise and map the extra memory available in 4 * 2114, so much the better.
While the 2114 is almost pin compatible with the 2111. the ground and write enable pins have been swapped and the 2 extra chip enables have been lost, so I'd have to add in some extra gates and it would be hard to add jumpers to cater for it due to the ground pin moving, so its one or the other rather than an option.

As for fully mapping the extra memory, it's a big change but I get your point! It means ripping out all the address decode logic and replacing it, I've worked out whats required with my logic simulator but it'll need reworking to simplify and use 74 gates you can actually get (I'm thinking a 74ls138 and some AND gates), but you end up with 1.5k RAM with "holes" in for the display and ram/io at $D00-$DFF and $800-$8FF respectively. I think I would do this as a "Rev 3" board a little later, since I do have 3 sets of 4 2111 compatible chips at the moment.

If I'm putting the address and data on the edge connector of course you could just leave out the 2111's and put in memory of your choice with logic that maps to the right places on an external card!

Anyway I'm not fully decided yet, I think I shall think about it over christmas!
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Old 20th Dec 2018, 3:30 am   #139
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Originally Posted by Slothie View Post
While the 2114 is almost pin compatible with the 2111. the ground and write enable pins have been swapped and the 2 extra chip enables have been lost, so I'd have to add in some extra gates and it would be hard to add jumpers to cater for it due to the ground pin moving, so its one or the other rather than an option.
I remember that the 2114 was different in terms of chip select lines and so on, but I am suggesting - maybe not on your initial V1.1 revision, but on V2.0, that you rearrange it so that it -only- uses 2114s since all of the 256-byte 4-bit types are becoming economically unobtainable.

Assuming you are still going to use BPROMs to hold the monitor code then you might consider using a third BPROM as a custom address decoder to generate with one IC the awkward chip enable intervals which would otherwise have to be generated with a 74138 plus more logic. (You may have your Tesla programmer up and running by that time).

Quote:
Originally Posted by Slothie View Post
If I'm putting the address and data on the edge connector of course you could just leave out the 2111's and put in memory of your choice with logic that maps to the right places on an external card!
Yes, Martin Lukasek has done that, except that he removes the original RAMs and plugs an overhead daughterboard into the SC/MP socket. It may depend on the PROM images at 0200 - 07FF already being absent (his replica is of the issue V PCB. I still have not established whether the issue V did or did not have this mod incorporated).

http://www.8bity.cz/2018/science-of-...ram-expansion/
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Old 20th Dec 2018, 4:02 am   #140
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Originally Posted by SiriusHardware View Post
I remember that the 2114 was different in terms of chip select lines and so on, but I am suggesting - maybe not on your initial V1.1 revision, but on V2.0, that you rearrange it so that it -only- uses 2114s since all of the 256-byte 4-bit types are becoming economically unobtainable.
Thats pretty much what I was thinking... I would end up with 2 versions, one "authentic" and one thats possible to put together from more available components. If was doing that then I'd think about adding a footprint for a 2716 EPROM since its not given that the TESLA proms are going to be around forever...

Quote:
Originally Posted by SiriusHardware View Post
Assuming you are still going to use BPROMs to hold the monitor code then you might consider using a third BPROM as a custom address decoder to generate with one IC the awkward chip enable intervals which would otherwise have to be generated with a 74138 plus more logic. (You may have your Tesla programmer up and running by that time).
My concern other than availability would be that the BPROMs may not be fast enough when its access time is added to that of the RAM and Monitor ROM, but that would be something that would be easy enough to try out, and would make fully expanding the MK14 to 512b prom + 3k RAM possible, and would make many more complex VDU projects feasible (Like MK14 space invaders ??!!)

Anyway, since I now seem to have a decent collection of SC/MPs I don't see why I shouldn't build a number of variants! I'd like to see a small computer with NIBL on it and a very simple display. I do have some 9 digit 14 segment alphanumeric displays somewhere...
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