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SiriusHardware 6th Jun 2020 3:48 pm

Re: MK14 schematic revisions
This is starting to get a bit (feature) creepy...

Timbucus 6th Jun 2020 4:17 pm

Re: MK14 schematic revisions
1 Attachment(s)
Yup but, it is my ultimate goal to get to what 'could have been' in the SoC lab before they abandoned it and ran off to develop the Acorn 1 because allegedly Clive was not really interested in developing a BASIC computer.

It is all modular so people can use what they want - we will still want the simple 1.5K expansion that either fits between the board and the VDU or can sit on the long thin expansion / U bend backplane.

The NIBL board on top of the CPU will just mean you do not need the 1.5K expansion.

If you really want to get creepy the NIBL board should have space for stacking upto four SC/MP in a multiprocessor arrangement... :) like this one from Mark Pepper:

Attachment 207825

TonyDuell 6th Jun 2020 5:10 pm

Re: MK14 schematic revisions
OK, some comments on the original keyboard parts.

The metal sheet is coated (mild) steel. It's magnetic.

The dimensions are as follows (Letters in brackets refer to the sketch in an earlier post):

The hole for each key : Width (C) = 12.6mm, Height (D) = 10mm. The corners are radiused at about 2.4mm

Horizontal measurement for the whole plate :

Overall width : 89.2mm

Fixing hole spacing (A) = 79.4mm

The 'dividers' between the keys (both from edge to first hole and between the holes) (E) = 7.7mm

As any metalworker knows, adding up dimensions generates small errors. So I measure the distance from the left hand edge of the plate to the left edge of each hole and got
7.7mm, 28mm, 48.4mm, 79.4m


Vertical measurements for the whole plate :
Overall height = 92.6mm
Distance from edge to nearest edge of a key hole ('edge divider') = 10.1mm

Fixing hole spacing (B) = 81mm

Internal dividers (F) = 5.2mm

As before I measured the 5 distances of the bottom edges of the holes from the bottom edge of the plate to avoid accumulating errors : 10.2mm, 25.4mm, 40.7mm, 56mm, 71.3mm

The fixing holes are about 3.7mm diameter.

As for the overall construction of the keyboard :

The first version used conductive rubber. There were 4 parts to the keyboard :

A plastic sheet with a circular hole for each key that was stuck to the PCB
A rectangle of conductive rubber
A legend sheet (thin plastic)
The metal frame.

These were supposed to be held to the PCB by plastic clip pegs through the corner holes. Almost everybody (including me) used nuts and bolts... Note there were no buttons, you pressed on the legend sheet to push the conductive rubber against the PCB.

The second version was the metal dome switches with plastic buttons.

There was the same sheet-with-holes stuck to the PCB.
A metal contact dome went into each hole
Then a double-sided self-adhesive sheet on top of the dome.
Then the (same) legend sheet
Then the buttons through the holes in the (same) metal plate.
Again fixed together by plastic clips or nuts and bolts.

I have made high(ish) resolution scan of the metal plate and legend sheet. Too large (12MBytes or so) to post here, but I can send it to anyone who needs it, e.g.if you want to copy the original font from the legends.

SiriusHardware 7th Jun 2020 10:06 am

Re: MK14 schematic revisions
Thanks for the very comprehensive measurements and information Tony. I would certainly appreciate it if if you would email me the scan of the key legend sheet, because at some point I will need to legend the key switches of the 'new' PCB. I'm not sure if my email provider can cope with such large files, just have to try it and see?

Tim, if you find the key caps in white rather than green, please let me know as I have not (yet) ordered any of the green ones.

Slothie 7th Jun 2020 10:32 am

Re: MK14 schematic revisions
I've looked about everywhere I can think of and those green switches are the only type I can find for sale at the moment with the clear covers. Perhaps a can of plastic primer? :)

Slothie 7th Jun 2020 11:51 am

Re: MK14 schematic revisions
1 Attachment(s)
Well I suppose I could have predicted as soon as I posted the above I would find a source of switches. I searched for "A14 clear keycaps" on a famous auction site and found these:
Attachment 207907
Presumably those search terms or similar should work on other sites too. It would be nice to find an actual distributor like Farnell or Rapid that stocked them.

SiriusHardware 7th Jun 2020 12:24 pm

Re: MK14 schematic revisions
The first site which came up sells a fantastic range of switches and caps but unfortunately the switches are quite expensive and there doesn't appear to be any discount for 'bulk' so a set of switches with black square tops, not including clear covers, would cost ~£50. Probably still worth buying the green ones from the source you originally linked and trying to find the A14 caps separately.

They seem to have a nice low profile, those A14 switches, what do they actually feel like?

Remember the 'Schadow Digitast' keyswitches so beloved by Elektor that they used them on practically everything? The ones with the keytop hinged at one end? I discovered the other day that C&K still make them, or something very like them, also called 'Digitast'. Of course they don't fit in the footprint of these PCBs.

SiriusHardware 7th Jun 2020 1:07 pm

Re: MK14 schematic revisions
I've just bitten the bullet and ordered a set of the ones Slothie linked to in #327 of this thread. I agree the green doesn't sit too well with the Gothic black of the PCB, it makes it look more Ghost Train than Gormenghast.

You may have noticed I even went for a black reset switch, even though I had an authentic red one available - but needs must. I can always learn how to wield a paintbrush again, if necessary.

Slothie 7th Jun 2020 3:09 pm

Re: MK14 schematic revisions
1 Attachment(s)
Here's a diagram showing the key coordinates in mm for the original keyboard. Measurements are as per Tony's post #533 - one measurement for the rightmost horizontal button was wrong but I've made a calculated position for it.

TonyDuell 7th Jun 2020 4:51 pm

Re: MK14 schematic revisions
Yes, sorry... The postition of the left edge of the 4th column was measured as 68.7mm from the left edge of the plate (as you calculated it).

Timbucus 13th Jun 2020 2:16 pm

Re: MK14 schematic revisions
It seems you can now buy one of the V1.2 boards on ebay...

Slothie 13th Jun 2020 3:40 pm

Re: MK14 schematic revisions least they didn't take my name and logo off it.

SiriusHardware 13th Jun 2020 9:13 pm

Re: MK14 schematic revisions
I was rather afraid that might happen. Although it was good of Ian to make the rev 1.2 available to be beta tested, I suggest any further revisions remain his to get made and to sell, and the same for any other boards he produces such as memory expansions, etc.

Timbucus 13th Jun 2020 10:14 pm

Re: MK14 schematic revisions
Looking at the listing it is someone who has had a small batch made and is selling the extras I think. Maybe they were more expensive to get made than ours - let's hope it adds some extra MK14 enthusiasts who find their way here but, it is a shame Ian does not see any reward for his hard work.

Slothie 13th Jun 2020 11:16 pm

Re: MK14 schematic revisions
I have the reward, I have a working MK14 and the knowledge I picked up on the way, and also the knowledge that I have made something other people appreciate.
As for anything further? Well maybe under my current circumstances I might keep designs private until I've made a little from them. I'm mainly in it for the fun and nostalgia, and 'meeting' people like you guys.

SiriusHardware 13th Jun 2020 11:37 pm

Re: MK14 schematic revisions
Without the new PCB I would never have considered connecting the SOC VDU up to an MK14 again, the routing of the address, data and control signals to the rear edge connector, along with the all important memory hole at 0200-07FF (another thing my original machine does not have) are what made it for me. I love my original old issue II machine but its early PCB severely limits it in some ways and I could not consider modifying it in any way for any reason now.

The problem I have now is that the issue VI replica is just too nice to modify or hack about for any reason. I know Tim wants to boldly go where no MK14 has gone before with his, but I'm content for mine just to be the MK14-GTi I always wished the original machine had been.

I assume Phil in France has one either already or on the way to him, although he seems to have a lot of plates spinning so it may be a while before he is able to populate it.

Slothie 13th Jun 2020 11:44 pm

Re: MK14 schematic revisions
Oh well Sirius there's always 1.3 to look forward to.. then you can build one to keep and hack the 1.2! I

SiriusHardware 14th Jun 2020 9:53 am

Re: MK14 schematic revisions
If there had been just one or two mistakes on the PCB which made it necessary to cut tracks and add links to get it working then I wouldn't have minded doing further messy mods to it whenever the mood took me but as it is, it looks so smart / neat that I wouldn't want to do anything to spoil that.

I have tact switches on the way now so I will be printing and cutting out a set of key legends from the scans posted here recently and hopefully it will look even nicer in the not too distant future.

Timbucus 14th Jun 2020 10:37 am

Re: MK14 schematic revisions
Same here - I have even swapped the green key tops for black and my red reset switch for black as well - I will keep a look out for a black axial cap to swap for my Yellow one. I think my legends will be printed inverse white on black to complete that 'Black Edition' look.

I know you think I want to mod it heavily but, actually I think a plug in "black" NIBL board on the CPU will be non-destructive and just need the omission of some of the chips which is the route I wish to go down. The memory and VDU will be on DIN connected expansions from the back and will continue to be best used from this Ian Issue VI. That plug in board will even likely work with my JMP or the Martin V one if it is designed correctly.

I am now thinking I may have a craft project one day to make a copy of the SoC Kit Box to keep my Martin Issue V in as a smart display and useful way to store a set of spare parts for this and my JMP - which I would like to get the V1 SCIOS burned for at, least to experience it as I cannot see it being used that often now.

SiriusHardware 14th Jun 2020 11:24 am

Re: MK14 schematic revisions
You could always sell the JMP replica, I have seen even replicas go for astoundingly high prices, even taking into account the increasingly high cost of populating one of these PCBs now. (The SC/MP, 4 * RAMs, a pair of programmed PROMs and a suitable display and a set of key switches can easily set you back well over £150, especially if you prefer to avoid the minefield of buying from China. And that's without also adding the optional INS8154 I/O IC).

I think there is no sadder sight than an unbuilt kit (of any sort, be it electronics or 'Airfix') so I think it would be a bit mad to never build the 'Martin' kit and put it in an authentic box if that is what you are contemplating, but I would certainly urge you to make high quality scans of both sides of that PCB because who knows when Martin may cease production of them? They are 98% SOC's issue V, the layout is only different in the specific area of the dome switches as far as I know so I think there is a valid case there for archiving the track layout just in case.

As far as the original SCIOS is concerned, there is a reason that got replaced - it was very clunky compared with the 'new' OS, even my original issue II has had the 'new' OS since as soon as that was made available as an after fit option, some time in 1979 probably. I still have the original PROMs - no idea where - but they are never going back in.

I think the only good reason to fit the 'old' OS is if you also have an original V1 paper manual because then you might want the machine to work as described in the manual if you plan to sell the machine and the manual as a 'set'.

Slothie 14th Jun 2020 11:26 am

Re: MK14 schematic revisions
Well.when I get my stuff back I'll have a number of spare 1.2 boards if someone wants one to hack.

SiriusHardware 14th Jun 2020 11:54 am

Re: MK14 schematic revisions
I'm holding one of the black PCBs for you and I had intended to offer up the remaining black rev 1.2 PCB for exactly what it cost to have made and posted to me: £5.

I won't be doing that now though. I'll probably keep that one as my 'hacker'.

Slothie 14th Jun 2020 12:36 pm

Re: MK14 schematic revisions
Good. One of the great things about computers from this era was you were supposed to modify and extend them. I can completely understand not wanting to mess with your original considering the rarity, however.

Timbucus 14th Jun 2020 1:10 pm

Re: MK14 schematic revisions
2 Attachment(s)

Originally Posted by Slothie (Post 1259309)
Good. One of the great things about computers from this era was you were supposed to modify and extend them. I can completely understand not wanting to mess with your original considering the rarity, however.

Indeed hence my 'Inversion' hack for the VDU - in fact it is the very nature of these, as ancient arduino's, that has attracted me away from my other retro interests to allow the reconnection with code and electronics as a pair.

I may take you up on the green 1.2 option as a hacking board!

But, I am unlikely to sell the JMP just because of the eye watering cost of some of the chips which are bound to fail (as I have found to my cost) - as I experiment...

I will take building the Martin Kit under advisement (but, will scan the board) I have a few other projects that are nearly parts complete first - as I robbed a few for the Issue VI...

Anyway here is the pair now (I decided to complete the Issue VI inverse keyboard printing) in the second photo I have also made up one of the new displays to prove they work - they do but, you need to clean the pads to get any solder to adhere and it still looks like I can do solder blobbing only...

Attachment 208444Attachment 208445

SiriusHardware 14th Jun 2020 1:44 pm

Re: MK14 schematic revisions
Aww, those keys look so nice. I might do mine in conventional black text on white for purely practical reasons, I find it hard enough to read the dim white-on-black legends on this computer keyboard in anything less than perfect light nowadays.

But there's the beauty of the clear keytops system, you can do them one way and then change your mind 5 minutes later. It might be worth buying a full extra set of the clear covers and keeping them in a light proof container because I have similar (but much bigger) keyswitches here from the keypad of a 1970s EPROM programmer and the clear covers have gone tinted yellow. Mind you, the age we all are now, that probably won't be our problem to care about by the time it happens to these key caps. It probably is worth having a few spare key covers, key tops and key switches though, as those are the sorts of things which could fail or get damaged or knocked off over time and as we all know a missing knob or switch cap can have a disproportionate effect on the aesthetic appeal of any item of equipment. I even have four spare switches for the homebrew keypad on my original MK14.

The JMP PCB with its beautifully designed companion keypad looks really classy, it's only due to the fact that it lacks some of the final refinements (PROM images are not removed, no edge connector contacts on the underside) that I haven't got one myself - I already own an original MK14 with exactly those limitations and a fairly decent keypad so the JMP version PCB would not give me anything I don't already have.

Slothie 14th Jun 2020 3:43 pm

Re: MK14 schematic revisions
Those JM keys look nice, and the others look good with the black caps look good too. When I get a 3D printer I hope to make a frame to align the switches because the caps do tend to swivel slightly out of line. If the frame was black and the keycaps white it would also make the board look closer to original.
Now, if I could find a source of INS8154D chips for a non ludicrous price I would be happy! I've got 2 but I have no idea if they work, and I'd like the option of getting more as they'll be useful for future projects... Just a case of keeping an eye on t'internet I suppose. I would never have guessed that it would be the one chip that's hard to find considering how widely it was used at the time.

SiriusHardware 14th Jun 2020 4:07 pm

Re: MK14 schematic revisions
Failing all else you could always send the ones you have to me or Tim... if he promises not to connect them up to anything... for testing, but I doubt whether you have them in your back pocket just now.

Mark1960 20th Jun 2020 12:31 am

Re: MK14 schematic revisions
As suggested by Sirius, replying in this thread for how I ended up with an MK14.

I bought it second user, already built and in a simple wooden tray in about 81 or 82, too long ago to remember accurately. It was in a classified advert in either Northern echo, south durham times or possibly even the Newtonian, which I guess some will identify as putting me not far away from Sirius.

The original owner made quite a good job of building it with some very nice red RS component IC sockets for all except the processor which is a boring low profile socket. I added the 8154 socket and the 8154 itself after buying it in a blister pack from Tandy in Darlington soon after aquiring the MK14. I still have the printed spec sheet that was stapled to the back of pack somewhere, together with the original MK14 manual.

I 've recently been able to get it running again after sourcing new RAM chips. I think the original owner took it a bit too literal when directed by the manual to add solder sparingly and a couple of the joints had broken due to the pins being bent backwards and forwards a few times. Suspect this caused some bus contention. I removed the 8154 fairly early in the attempt to get it running again so hopefully it has survived. I've also removed an external ram board that I had connected.

I've been trying to find some matching sockets to add the extra ram, it just doesn't seem right to use modern sockets on what is possibly a museum piece.

It's a rev IV, with the individual switches, not the metal dome keypad, and it can still enter some of the programs from an online scan of the manual. It does take some practice to avoid double key entry by picking the best position to press on each button. This is a bit of a surprise as it has spent much of the last 20 years stored in a bubble pack envelope in a damp garage.

I'm interested in experimenting with the sc/mp again but don't want to cause any more damage to the old MK14, so thinking about building something along the lines of the RC2014 but using an sc/mp. Still trying to decide if I can trust the parts for sale from china. It would also be interesting to try the INS8070, but they seem even more difficult to find.

I don't seem to be able to add pictures yet, but maybe this is another part of being on probation on this site.

Timbucus 20th Jun 2020 2:00 am

Re: MK14 schematic revisions
Mark - I don't remember a limit for posting pictures but, I have seen and bought some of those red Camion sockets on e-bay recently - in fact my Black MK14 VI had one of them to start with but, it was stripped off when I damaged it which is a shame as they look really cool.

Maybe you would like to join our effort to rebuild the SCRUMPI - other threads are available.... that is a real experiment in SC/MP...

I did comment in the VDU thread before I noticed you had changed that your observation on the Virtual Memory support was interesting as page exceptions could work in that way, an interesting project. I propose though that the reason was the Multiprocessor support which was baked in. You would need to deal with data changes part way through cycles for that to be deterministic.

Mark1960 20th Jun 2020 3:24 am

Re: MK14 schematic revisions
Hi Tim, I did see the red cambion sockets but they look to be low profile types, though I do think they would look cool using a full set on a retro build. The RS sockets are a slightly darker red and have what would have been called standard profile back in the seventies, they also have the RS logo printed in white on the side. The RAM has been mounted using two nine pin SIL strips, no logo on the side but very close match for colour and height. I think the blue cambion sockets are closer style to the RS standard profile sockets, it also looks like they have similar pin numbers moulded in the plastic, so maybe they shared some tooling.

For multiprocessing it seems possible to piggyback an sc/mp on top of another, with maybe three or four pins cross wired. Itís probably a bit destructive given the rarity of these parts now. Address decoding could override the nenin and the other processor could copy memory contents or update a memory mapper then reenable the nenin.

Anyway first step for me is to try and get NIBL or NIBL-E running.

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
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

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.

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.

Slothie 7th Jul 2020 7:06 pm

Re: MK14 schematic revisions

Originally Posted by DeltaAlpha52 (Post 1267550)

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.

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.

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

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

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)

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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