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Old 20th Jun 2020, 8:15 am   #581
SiriusHardware
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Default Re: MK14 schematic revisions

When I was a young electronics hobbyist in the late seventies my dad, like most dads at the time, used to go out to the pub every night and as a little part of that ritual, he would bring back three packets of Tudor crisps, one for my mother, one for me and one for the dog. Like most people reading the electronics hobby press I had seen those beautifully illustrated 'woodcut' style adverts for the MK14 and decided I had to have one, but £39.95 was quite a lot of money for someone still at school so I negotiated a deal: Instead of the crisps, my dad would give me the price of the crisps which I would put in a jar.

I kept this up for weeks on end until I had more than half of the amount saved up and then my parents relented and gave me the rest, although they still couldn't see why I would want a 'computer' or what I would do with it. I assuaged my parents' doubts by getting the machine to do all sorts of impressive things, although some - like making it play 'God Save The Queen' - came straight from the manual. I later edited the 'song data' to play several other tunes as well. That's how most of us learned, by entering and improving / modifying software from manuals and other sources.

This probably explains why I still have it although I have acquired a few machines since - it took an effort to acquire it, which made it harder to consider discarding it. I've also only changed address once since I acquired it, and that seems to have been a pivotal point at which many former MK14 owners lost theirs, discarded in a house move and later regretted, so I'm impressed that Mark chose to include his MK14 - not even his originally - in the relatively small number of items he must have been able to take to Canada with him.

My original machine's 8154 also came from Tandy (Shields Road, Byker) in the 'Archer' blister pack with the folded data sheet, which I was looking at just last night.

My MK14 is issue II so it really is a museum piece as I can't consider doing any of the mods which would bring it up to to spec, so no extra RAM beyond the 640 bytes it already has, no connections for the VDU, the Single Step hardware patch was removed during a renovation phase in the late noughties as it made the PCB look untidy. That's why I'm grateful to those individuals who have produced replica PCBs, especially to Slothie whose 'issue VI' is the most practical and functional of the lot. If I break a replica it's not the end of the world although I am already too fond of my issue VI to be able to consider doing anything drastic to it.

Although we didn't really mean it to turn out this way this 'Vintage Computing' section of the UKVRR site has become a kind of unofficial repository for all kinds of things MK14, there are quite a few MK14 threads, many now closed, which are stuffed with all sorts of interesting links, information, files and so on, in no small part thanks to Tim who is an obsessive researcher sans pareil when it comes to anything he takes an interest in.
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Old 24th Jun 2020, 9:40 pm   #582
philoupat83
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Default Re: MK14 schematic revisions

Hello
thank you my friends received .
soon I will be able to assemble this famous mk14 v6 of slothie and add my vdu to it
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Old 27th Jun 2020, 1:43 pm   #583
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I am glad you received it OK Phil - the (labelled) additional connections on the rear edge connector of the V6 make it much easier to connect a VDU.
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Old 28th Jun 2020, 11:08 am   #584
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Default Re: MK14 schematic revisions

Yes good news Phil that it arrived safe - look forward to seeing you on the VDU thread.

I have to say Sirius a great story on your MK14 origin. I also have fond memories of Tandy - I recently found a 1980s catalog which contained the speakers I bought from there for my first stereo I built.

It is this human side of the forum around these machines that makes it so important that we preserve and index this information. Appreciate the praise on my obsessive research - very similar to the headmaster at my old school and my dad who pretty much added the caveat "If only he was interested in the things that he needed to be..." and of course I have often thought it should have "..or will make him rich."
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Old 7th Jul 2020, 6:51 pm   #585
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Default Re: MK14 schematic revisions

Quote:
Originally Posted by Slothie View Post
Well.when I get my stuff back I'll have a number of spare 1.2 boards if someone wants one to hack.
Hi
I saw the MK14 board on the ebay and that brought me back again to this thread. If you have one of the 1.2 boards I'd love to have one to progress on from my current replica from Martin. The extending the signals to the end of the board looks great.
Thanks
David
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Old 7th Jul 2020, 7:06 pm   #586
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Default Re: MK14 schematic revisions

Quote:
Originally Posted by DeltaAlpha52 View Post
Quote:
Originally Posted by Slothie View Post
Well.when I get my stuff back I'll have a number of spare 1.2 boards if someone wants one to hack.
Hi
I saw the MK14 board on the ebay and that brought me back again to this thread. If you have one of the 1.2 boards I'd love to have one to progress on from my current replica from Martin. The extending the signals to the end of the board looks great.
Thanks
David
Well all my stuff is still in storage but I may be able to poke around in a few weeks time once I can get to the storage if I am well enough for the walk! I'll message you of one comes up.
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Old 7th Jul 2020, 9:20 pm   #587
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Default Re: MK14 schematic revisions

Thanks for responding - sorry to hear you are unwell - I've not been following this thread for a while ( been getting other ancient hardware working ). But getting the old stuff out of the loft has been a godsend during this lock down. We have been shielding so not really left the house since March. If you do manage to find one that would be wonderful but I fully understand if not.

Hope you get better soon and are able to "work" on these ancient systems

Take care
David
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Old 21st Aug 2020, 4:40 pm   #588
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Thought I would update this thread to say yet another feature of the VI board is confirmed as working. I bought some 65x61 which are in nice ceramic packages from the bay and having cut the link and jumpered for them underneath it seem to run all that I have thrown at it so far... so another bit of evidence of the godlike status of Slothie and good news for users of the design...
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Old 21st Aug 2020, 6:16 pm   #589
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Default Re: MK14 schematic revisions

There is one slight hiccup - when the VDU is connected it seems that only the first byte of data is ever picked up on each line and that is replicated in every position on the screen - it is not a fault on the VDU as I have put the AM9111 chips back in and it works again. So something strange about the address incrementing being driven from the VDU.
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Old 21st Aug 2020, 6:35 pm   #590
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The VDU asserts the 'read' signal at the beginning of the line and continues to hold it as it works though the line, advancing the address and reading the data, advancing the address and reading the data. Only when it gets to the end of the line does it release the read signal.

I found this out when I made my PIC project which was designed to show the VDU working without having to connect it back up to the MK14. The project first watches for the _RD signal to fall low and supplies the VDU with the first byte of data. After that it watches for the change of state on the VDU A0 line. Every time that changes state the PIC supplies a new byte of data to the VDU, all the way up to the end of the line.

It sounds as though the IM... memory only outputs new data on the falling edge of the _RD signal. That would be unusual.
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Old 21st Aug 2020, 7:17 pm   #591
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Default Re: MK14 schematic revisions

A nice datasheet for the IMX65X61

https://www.tvsat.com.pl/pdf/I/IM65X61.pdf

Attached, a cutout of the section regarding read mode, which does seem to suggest that some of the signals are edge triggered rather than level-sensitive.
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Old 21st Aug 2020, 7:30 pm   #592
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Default Re: MK14 schematic revisions

Quote:
Originally Posted by SiriusHardware View Post
It sounds as though the IM... memory only outputs new data on the falling edge of the _RD signal. That would be unusual.
I've looked into this and the 65x61 memories latch the address on the falling edge of the enable input (in this case wired to NRDS). I assume the 2111 style memories don't latch the address.

Not sure what can be done about it at this point but I'll add a conspicuous note to the documentation for my boards that I'm writing....
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Old 21st Aug 2020, 7:39 pm   #593
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It suggests that it might be an idea for your forthcoming SOC VDU issue 3 to properly assert and release _NRDS as it reads each byte from RAM, unlike the original SOC VDU which holds NRDS low all the way through the line. Same for Karen's one-chip PIC VDU if she can squeeze it in.

This would have been a very obscure problem to see coming. The MK14 itself seems to work fine with the IM65X61 RAMs because it drives the NRDS line properly.
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Old 21st Aug 2020, 7:54 pm   #594
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Default Re: MK14 schematic revisions

It looks like this would make it easier to interface the RAM to an MCS85 or MCS48 type processor, where the address and data are multiplexed on the bus.
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Old 1st Dec 2020, 7:53 am   #595
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I was checking the connections at the edge connector on Slothie’s VI, just to see if instead of plugging into the sc/mpII socket an expansion board to replace the sc/mpII could be connected to the edge connector. It seems to have everything that would be needed except for reset to the INS8154.

I saw a mention earlier in this thread of possibly adding NENOUT to the edge connector if there was a future revision.

Any thoughts on which of the six unused connections might be reserved for future use?
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Old 1st Dec 2020, 10:13 am   #596
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Default Re: MK14 schematic revisions

Logically, it should go into the group of similar control signals, ie, between NENIN and NWDS, but there may be physical (layout) reasons why it would be easier to put it somewhere else. Slothie?

There is a sketch of the edge connector layout on the current (rev 1.2) issue VI PCB here:-

https://www.vintage-radio.net/forum/...&postcount=505
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Old 1st Dec 2020, 10:41 am   #597
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Quote:
Originally Posted by Mark1960 View Post
I was checking the connections at the edge connector on Slothie’s VI, just to see if instead of plugging into the sc/mpII socket an expansion board to replace the sc/mpII could be connected to the edge connector. It seems to have everything that would be needed except for reset to the INS8154.

I saw a mention earlier in this thread of possibly adding NENOUT to the edge connector if there was a future revision.

Any thoughts on which of the six unused connections might be reserved for future use?
Well I was going to need to put NENOUT (between NENIN and NWDS) for an expansion board I was considering.

It would be possible to put NRESET on the connector, probably on the spare pin next to XOUT, It would of course be an output only, if we wanted to be able to reset the MK14 you'd need to use the reset input on the keyboard connector.

As for the remaining 4 connector locations, there are only 3 SC/MP signals that are not brought out, namely NBREQ, NHOLD and CONT. NHOLD and CONT share a pullup, so an extra resistor would be required, but they could be brought out for completeness, although the routing for those signals in that area would be very tight so I'd have to be really sure it was worth it - I'd probably just leave them for people to wire as they require.

I had originally left the 4 pins between data and address clear for the upper 4 address bits if there was ever an address latch added, but since then I decided that the rework necessary would make the board cease to be an MK14, and the effort involved would be better put into a proper SC/MP card for a standard machine like the Acorn 1 rack system, or one of the other standard rack systems of the time.

The connector fingers were deliberately made long to allow mod wire to be soldered to the top of them and still go into an edge connector. I'd suggest temporarily sticking a bit of kapton tape to the edge connector to stop solder "wicking" down the connector fingers leaving a few mm spare to solder to.
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Old 1st Dec 2020, 12:14 pm   #598
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Default Re: MK14 schematic revisions

I think it's a good idea to put reset-out on the rear edge on future revisions, with the bus now properly available on the edge connector who knows what interesting or classic peripheral ICs people may choose to try to connect to the system?

I have an unused SPO256-AL2 (Allophone speech synth IC) somewhere. "As you can hear, Science of Cambridge is a jolly fine buy"? Although, a very British phrase like that might sound odd in a synthetic American accent..
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Old 1st Dec 2020, 4:10 pm   #599
Mark1960
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Default Re: MK14 schematic revisions

Quote:
Originally Posted by Slothie View Post
It would be possible to put NRESET on the connector, probably on the spare pin next to XOUT, It would of course be an output only, if we wanted to be able to reset the MK14 you'd need to use the reset input on the keyboard connector.
I might have got confused here, I was thinking I’d need to generate a reset for the off board processor, then thought I’d also want to reset the 8154. As you state It makes more sense for the MK14 to generate the reset as an output.

Quote:
Originally Posted by Slothie View Post
As for the remaining 4 connector locations, there are only 3 SC/MP signals that are not brought out, namely NBREQ, NHOLD and CONT. NHOLD and CONT share a pullup, so an extra resistor would be required, but they could be brought out for completeness, although the routing for those signals in that area would be very tight so I'd have to be really sure it was worth it - I'd probably just leave them for people to wire as they require.
I wonder if NHOLD and CONT could remain a single connection as they are both active at different times. This would save a connection and avoid a modification. If generated during a bus cycle it would extend the cycle and if generated at a different time would stop the processor starting a new instruction.

I’d expected the additional connections to be wire mods on the MK14, but just thought it would be nice to have a standard wiring.

Quote:
Originally Posted by Slothie View Post
I had originally left the 4 pins between data and address clear for the upper 4 address bits if there was ever an address latch added, but since then I decided that the rework necessary would make the board cease to be an MK14, and the effort involved would be better put into a proper SC/MP card for a standard machine like the Acorn 1 rack system, or one of the other standard rack systems of the time.
It did look like an obvious hole for the extra address lines. Maybe a rack based system could still follow the same pinouts with the form factor of the cards following the mk14 vdu. That does only leave two pins available for additional control signals to squeeze in NENOUT, NBUSRQ and CONT/NHOLD.

Maybe NENIN and NENOUT should be a daisy chain of connections separate to the bus, with the existing NENIN from the VDU redefined to kick everything off the bus.

Reset to other cards could possibly use one of the flag outputs, which would give the processor control over a bus reset.
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Old 1st Dec 2020, 10:41 pm   #600
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Default Re: MK14 schematic revisions

Every time I look at the MK14 schematic I wonder why the address decoding is using three chips, LS00, LS08 and LS04 instead of just using an LS41 or similar to decode 512 byte blocks. Was it any cheaper at that time to use the three simple gates instead of a single decoder?

The cynic in me thinks it was probably just that Nat Semi wanted to unload a bunch of stock on Science of Cambridge, the extra chips were a cost to SoC and not to Nat Semi, but Nat Semi defined the chip set. This is also reinforced by the strange use of LS157s to latch the LED data.

Is there any references to the original design by Ian Williamson floating around on the internet somewhere? I wonder how that compares.
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