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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 1st May 2019, 10:32 pm   #181
SiriusHardware
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Default Re: MK14 schematic revisions

I've been mucking about with the VDU again tonight, and here are a couple of snippets arising from that.

Earlier, I said that the relationship of screen RAM locations to text character positions was the way you'd logically expect, but I also said I didn't think that applied to bitmap / graphics mode. I don't know why I thought that, because the allocation of memory locations to pixels does follow the most obvious scheme you could imagine.

The first byte of screen RAM is represented by the first eight pixels at the upper left of the screen, with the LSB state indicated by the rightmost pixel of eight and the MSB state indicated by the leftmost pixel of eight.

The second byte of screen RAM is represented by the next eight pixels to the right, the third byte of screen RAM by the next eight pixels to the right of those and so on, so that the first eight bytes of screen RAM are mirrored by the uppermost 64 pixels. The next eight bytes are represented by the second line of 64 pixels, the eight bytes after that are represented by the third line of pixels and so on. All just as you would hope / expect.

The second snippet of information is the VDU character set, which is as follows: (Each character, followed by its MK14 VDU hex code).

Code:
@ 00   A 01   B 02   C 03   D 04   E 05   F 06   G 07   H 08   I 09   J 0A   K 0B   L 0C   M 0D   N 0E   O 0F
P 10   Q 11   R 12   S 13   T 14   U 15   V 16   W 17   X 18   Y 19   Z 1A   [ 1B   \ 1C   ] 1D   ^ 1E   _ 1F
  20   ! 21   " 22   # 23   $ 24   % 25   & 26   ' 27   ( 28   ) 29   * 2A   + 2B   , 2C   - 2D   . 2E   / 2F
0 30   1 31   2 32   3 33   4 34   5 35   6 36   7 37   8 38   9 39   : 3A   ; 3B   < 3C   = 3D   > 3E   ? 3F
Please note these characters and codes are correct for the character generator IC originally supplied with the MK14. I don't know if this is true for the alternative character generators which are being supplied with Martin L's VDU clones.

I didn't forget the character whose code is 0x20 - that would be 'space' which is why you don't see anything there in the table. At first glance this is awkwardly different to the standard ASCII character set, but in fact some portions are the same and other values are easily derived from the original ASCII values.

Assuming you have some genuine ASCII text which you want to display on the MK14, then:

Send characters with codes in the range 0x20 to 0X3F to the VDU unchanged, as they are the same as in standard ASCII. This range includes the numerals 0 to 9.

For characters with codes in the range 0x40 to 0x5F, - mainly upper case letters, but also including the '@' character whose code is 0x40 - subtract 0x40 from the ASCII code to convert it to the VDU code.

For characters with codes in the range 0x61 to 0x7A - lower case letters - subtract 0x60 from the ASCII code to convert it to the VDU code

If the original ASCII character does not fall within any of these ranges I convert it to 0x3F, which is the code for a question mark '?'.
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Old 14th Jun 2019, 1:23 pm   #182
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Default Re: MK14 schematic revisions

Well, since my eye is improving I thought I'd get back to sorting out the new PCB layout. I've managed to wire up all the connections on the edge connector to better match the VDU card (XOUT is on the bottom rather than the top, as discussed previously). I'm now trying to squeeze in an extra gate to fully decode the prom as this is going to make it possible to add extra RAM for VDU users and for some ideas I have. Here's a few screendumps of my progress so far.....

If there are any glaring omissions please feel free to point them out!
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Old 14th Jun 2019, 4:22 pm   #183
SiriusHardware
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Default Re: MK14 schematic revisions

One very minor non-fatal error so far - on the circuit diagram, on the lower edge of the display, the terminal marked 'e' is probably meant to be marked 'col 3'.
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Old 14th Jun 2019, 6:08 pm   #184
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Default Re: MK14 schematic revisions

Well spotted! I did actually notice this sometime before Christmas and then instantly forgot all about it.... I guess I need to edit the footprint sometime.
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Old 14th Jun 2019, 6:28 pm   #185
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Default Re: MK14 schematic revisions

I see you did get the reset line out to the keypad edge connector.

Regarding the 'P4' connector row which I assume is the new lower side of the top / rear edge connector, at which physical end of the connector will pin 1 be? The end nearest the regulator? Or the end nearest the four vertical resistors? I'm guessing you have numbered them in the same order as the corresponding VDU row pins rather than in the same order as the MK14 top side pins?

With respect to the (obviously unfinished) PCB layout, I don't see many mounting positions for supply decoupling capacitors. While this is authentic with respect to the original MK14 which has very few supply decoupling capacitors and somehow gets away with it, it would probably be better practice to provide a mounting position for a (ceramic) decoupling capacitor for pretty much every IC, if possible.

Last edited by SiriusHardware; 14th Jun 2019 at 6:54 pm.
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Old 14th Jun 2019, 8:14 pm   #186
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Default Re: MK14 schematic revisions

Due to technical issues (OK, mistakes!) the bottom connector P4 is numbered the other way round - so the address lines are on the right (near the vertical resistors) and the VCC/XOUT/NRDS/NWDS signals are to the left, underl the power/ground connections. I'm not sure if the VCC is going to be all that useful given that the regulator glows in the dark as it is, but it might be useful for something.

As for decoupling capacitors - I have been thinking seriously about this and I am glad that you also think it would be a good idea to make room for at least a few more where they can be fitted. The design as originally given works but I think that it can't do any harm to place holes for them even if I don't choose to populate (all) I'm not convinced my layout of the power rails is particularly optimal and I'm using a modern switching power wall wart which typically generate a lot of high frequency noise of its own. With the additional chip and a few other changes I have made its not like its going to be mistaken for an original unit anyway! Looking at the power and signals on an oscilloscope reveals a lot of noise, which might explain the occasional reset problems I had noted.
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Old 14th Jun 2019, 8:59 pm   #187
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Default Re: MK14 schematic revisions

I prefer to think of this as an 'ultimate' MK14, with all the refinements that the issue VI would have had, had there ever been one.

You've obviously gone away from the possibility of using PLDs or GALs or PROMs as custom address decoders, a decision which I can safely applaud since I'm not the one who actually has to get all the necessary standard logic onto a PCB.

Ultimately, that decision will make it easier for anyone else to rock up and populate one of your replicas, with only the PROMs then still being really difficult and expensive to obtain - hopefully the 6xxx RAM alternatives are still available at a reasonable price.
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Old 14th Jun 2019, 9:53 pm   #188
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Default Re: MK14 schematic revisions

I am rushing away for the weekend but, this looks great - I will check it out in detail when I get back to a machine on Monday night. Just a note that utsource currently list stock of over 147000 AM9111 RAM chips and a similar number of the PROM's so I do not think the world is going to run out very soon and the Tesla PROM's seem pretty plentiful on auction sites still.
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Old 14th Jun 2019, 10:23 pm   #189
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Default Re: MK14 schematic revisions

Ah, well, the Tesla PROMs are Slothie's other problem.

I could send you what info I have about those - over and above what was discussed earlier in the forum, and maybe the two of you can forge ahead on that problem either together or independently.

With regard to Utsource, I know you (Tim) had a good experience with them recently but someone else sent me a pair of PROMs obtained from there which, although they were genuine DM74S571s, turned out to be already programmed. Granted they were suspiciously cheap, and if you paid more you'd be more likely to get genuine unprogrammed blanks even from the same source.

Everything is still available - if you can afford it.
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Old 14th Jun 2019, 11:30 pm   #190
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Default Re: MK14 schematic revisions

Quote:
Originally Posted by SiriusHardware View Post
You've obviously gone away from the possibility of using PLDs or GALs or PROMs as custom address decoders, a decision which I can safely applaud since I'm not the one who actually has to get all the necessary standard logic onto a PCB.
Yes I haven't gone in that direction partly because I want the result to look like it could have been from the 70's but mostly because I've used CPLDs in the past and the software you need to make the programming files is a pain to use (Quartus I'm looking at you!) GALS are getting rarer to find too even if you can find the software for them.

As for PROMs I've blown up a few MOSFETs on breadboards trying to get the programming wave-forms right but then all this excitement with my eyes started and I've yet to get back to it!
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Old 22nd Jun 2019, 8:50 pm   #191
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Default Re: MK14 schematic revisions

Well I've squeezed in some decoupling capacitors which was the last thing on my list, so I'm going to give it another few days and then check everything over again and I think its time to send off to have the boards made...
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Old 23rd Jun 2019, 12:54 am   #192
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Default Re: MK14 schematic revisions

Maybe give Timbucus a chance to look it over before committing to hardware, he seems to have the same gift for effortless understanding of traditional logic circuits which you have.
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Old 23rd Jun 2019, 1:03 am   #193
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Default Re: MK14 schematic revisions

Quote:
Originally Posted by SiriusHardware View Post
Maybe give Timbucus a chance to look it over before committing to hardware, he seems to have the same gift for effortless understanding of traditional logic circuits which you have.
Lol a lot of that "effortless understanding" is the python parallel logic simulator I wrote
But I accept your compliment and it'll be towards the end of next week before I send off anything so Timbucus should get a chance to comment!
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Old 23rd Jun 2019, 8:49 am   #194
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Default Re: MK14 schematic revisions

I take it we can assume that the actual circuit diagram remains unchanged from the one you attached in #182, you've just added physical places on the layout where a few supply decoupling capacitors can go?

With regard to the python-language logic simulator, that sounds like a nice handy tool. I seem to remember there being something a bit like that available for the Spectrum in one of the official Sinclair branded software cassettes - don't remember what it was called now. Maybe just 'Logic'.?
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Old 23rd Jun 2019, 9:27 am   #195
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Default Re: MK14 schematic revisions

Yes, aside from the decoupling capacitors everything is as it was.
The logic simulator is very basic and just produces truth tables - I've attached it zipped with an example input file if you want to play with it!
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Old 23rd Jun 2019, 10:55 pm   #196
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Default Re: MK14 schematic revisions

I've just arrived back from the Hebrides so, although I've grabbed the package in the previous post I have not seen it yet. I'll have a look at it tomorrow (Monday) evening. Thanks for putting it up.
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Old 24th Jun 2019, 1:21 am   #197
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Default Re: MK14 schematic revisions

Aha! The Spectrum software was called Make-A-Chip. I'm pretty sure I still have a boxed original tape copy somewhere. Limited to gates with two inputs and twelve gates in total, which is a shame.

https://www.petervis.com/Sinclair/Ma...ke-a-Chip.html

Sorry, going off on one there. Back to the MK14.
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Old 25th Jun 2019, 9:08 pm   #198
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Default Re: MK14 schematic revisions

Quote:
Originally Posted by SiriusHardware View Post
Maybe give Timbucus a chance to look it over before committing to hardware, he seems to have the same gift for effortless understanding of traditional logic circuits which you have.
Ha ha not sure where you get that impression! Slothie deserves it for the excellent work on the PCB which looks really good, although I have not had time to go through it with a tooth-comb but, I think his goal of a believable V5.5 has been achieved - I will stick my hand up to try one when they are available as it will make using my VDU easier!

He also deserves praise of course for his simulator... "Make a Chip" is considered the booby prize in any collection of spectrum software as everybody seems to have a copy (except me strangely).

https://youtu.be/JMLvJHdCmPo?t=256

Staying off main topic for a while on that one we used to publish software (from a friend of mine Peter Armitage) for the BBC BITD!

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Also I thought you would be interested that Geoff Phillips uploaded his surviving "Complement and Add" user group magazines to archive.org - I have not read them all yet but, he also added a nice preface.

https://archive.org/search.php?query...lement+and+add
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Old 25th Jun 2019, 10:04 pm   #199
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Default Re: MK14 schematic revisions

Another mega information find!

It was remarkably kind of Mr Philips to put all that up. If you have a contact address or email for him please offer him our sincere thanks.

It beggars belief that the chain-letter format worked as well as it sometimes did (and sometimes didn't). All pre-internet and pre dial-up BBS of course, never mind the fact that for most of the club members the MK14 was the only sort of 'computer' they had.

Imagine the stuff which could be contributed to that newsletter now, if it was still going. Just the material contained within several threads (some now closed) on this forum - plus numerous off site sources pointed to by links in the same - would be enough to fill 100 such newsletters.

I have (ahem) 'reserved' one of Slothie's issue 5.1 / 5.5 PCBs as well, Tim, the notion of owning an MK14 with all the bus signals neatly available at the rear edge would be too interesting to resist.
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Old 25th Jun 2019, 10:11 pm   #200
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Default Re: MK14 schematic revisions

Since a queue is forming I'd better check that its going to work carefully!
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