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-   -   MK14 schematic revisions (https://www.vintage-radio.net/forum/showthread.php?t=145663)

SiriusHardware 20th Jun 2020 8:15 am

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.

philoupat83 24th Jun 2020 9:40 pm

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 :thumbsup::wave:

SiriusHardware 27th Jun 2020 1:43 pm

Re: MK14 schematic revisions
 
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.

Timbucus 28th Jun 2020 11:08 am

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."

DeltaAlpha52 7th Jul 2020 6:51 pm

Re: MK14 schematic revisions
 
Quote:

Originally Posted by Slothie (Post 1259276)
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

Slothie 7th Jul 2020 7:06 pm

Re: MK14 schematic revisions
 
Quote:

Originally Posted by DeltaAlpha52 (Post 1267550)
Quote:

Originally Posted by Slothie (Post 1259276)
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.

DeltaAlpha52 7th Jul 2020 9:20 pm

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

Timbucus 21st Aug 2020 4:40 pm

Re: MK14 schematic revisions
 
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...

Timbucus 21st Aug 2020 6:16 pm

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.

SiriusHardware 21st Aug 2020 6:35 pm

Re: MK14 schematic revisions
 
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.

SiriusHardware 21st Aug 2020 7:17 pm

Re: MK14 schematic revisions
 
1 Attachment(s)
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.

Slothie 21st Aug 2020 7:30 pm

Re: MK14 schematic revisions
 
1 Attachment(s)
Quote:

Originally Posted by SiriusHardware (Post 1282014)
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....

SiriusHardware 21st Aug 2020 7:39 pm

Re: MK14 schematic revisions
 
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.

Mark1960 21st Aug 2020 7:54 pm

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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