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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 24th May 2020, 11:44 am   #21
SiriusHardware
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Default Re: Mk14 vdu

Certainly useful to have that information, although I find it hard to imagine anyone putting all of those components onto a daughter board which then plugs into the CG IC socket. Regarding the use of an EPROM to substitute for the original PROM, this is one of the rare cases where that might not work because of the high access speeds involved. Even by modern standards, Bipolar PROMs are still very fast compared to EPROMs.

Karen could probably replicate the whole SOC VDU with a single PIC chip if asked nicely. Direct video output from PICs is something of a speciality for her.

I've often wondered whether, when you're starting with no VDU, it might be better to recreate the PE (Practical Electronics) VDU for the MK14. I never studied it in detail because I already had the official SOC item but by all accounts the PE one was better thought out. I think, though I'm not sure, that it had its own onboard RAM.

Tim posted link after link to MK14 related items in magazines in old British electronics magazines hosted on the americanradiohistory website, I'm sure one of those will be pointing straight at the PE VDU article(s).
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Old 24th May 2020, 12:03 pm   #22
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Default Re: Mk14 vdu

Yes, I knew Tim wouldn't have let me down.

PE October, November and December of 1978. The RAM module, constructed by stacking 2112 RAM ICs between rectangles of veroboard, would have to go and be replaced with a one-chip SRAM. It also uses a specialised CRTC IC, but not a common one like the 6845, so that could be a technological dead end.
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Old 24th May 2020, 12:42 pm   #23
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Talking Re: Mk14 vdu

Of not an original VDU I was going to try Grant Searles AVR based design which has it's own memory and is only a couple of chips. It can be building with 4 or 8 bit parallel interfaces and its also pretty high spec with different character sizes and graphics. It also has a PS/2 keyboard and serial interface optionally.
http://searle.x10host.com/MonitorKeyboard/index.html

I would have tried the PE design as it's more "period" but the CRTC chip seems to be unobtainable. Its possible theirs an equivalent device on a different number but I wasn't able to find one. The other option would be to.make a 6845 design adapted to the MK14 since I have several!
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Old 24th May 2020, 1:05 pm   #24
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Default Re: Mk14 vdu

If you are really game for a challenge we could recreate the Display Circuit from the SCRUMPI 3 - two challenges met with one task... Just saying like. I paid for access to the Journal article where he describes how it works but, they did not supply the circuit diagram he mentions - I need to chase that up.

For those who remember the Pong video I had a reply from him and he confirmed he was using a RAM expansion - this thread has some nice pictures of the backplane he made to have an external RAM board and the VDU...

https://translate.google.co.uk/trans...78640%23p78640

Scroll up a bit to see the other photo as the link is direct to one of them half way through the thread.
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Old 24th May 2020, 1:53 pm   #25
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Default Re: Mk14 vdu

Actually that would be a great idea, kill 2 birds with one stone, and the SCRUMPI 3 VDU would be age-appropriate on an MK14. I hope you can get the schematic because info on the SCRUMPI 3 is very scarce.
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Old 1st Jun 2020, 12:10 am   #26
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Default Re: Mk14 vdu

Well, to bring things up to date: Thanks to Slothie the world now has an MK14 replica PCB with all of the lines needed for the VDU already tracked to the rear edge connector, the first time this has ever been done.

Moving on, Tim had to muck about with the components on his replica VDU's composite output, but I didn't quite follow exactly what he did, so Tim, can you attach a sketch of what the composite output on your VDU now looks like - and is there still an issue with inverted (black text on white background) not looking right?
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Old 1st Jun 2020, 8:08 am   #27
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Default Re: Mk14 vdu

hello
for video output there are more schemat for zx80 or zx81 see
http://www.user.dccnet.com/wrigter/i...zxinverter.htm
Do you have the definitive version of the slothie pcb?
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Old 1st Jun 2020, 8:19 am   #28
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Default Re: Mk14 vdu

To make his TV / display work with the composite output from his 'Martin' replica VDU, Tim had to change some components around, so I was asking if he will post the changes he had to make, even if it is just a sketch with a pencil.

Tim and I have just built up two of Slothie's latest version of his PCB, see recent posts on this page of this thread:-

https://www.vintage-radio.net/forum/...145663&page=23

It appears to work very well.
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Old 1st Jun 2020, 7:26 pm   #29
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Default Re: Mk14 vdu

Well as requested - the short story is here are the mods I did to get a nice clean composite on my Phillips CRT. Basically the manual BOM has to have R9 and R7 changed and the omitted R6 needs to be installed - C10 is still omitted.

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Note that I was unable to tune the modulator - I have since swapped it for another and still cannot on either my Phillips or my period Sony - the latter does give a poor picture.

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It is possible I need to revert the component values to allow the use of the modulator - I was unable to find its DataSheet UM1233 but, maybe it needs the near 5v p2p that the system was putting out that was blanking my usual desktop CRT. Martin has said that his LCD works with the original values on composite - it does not seem to mind the higher voltages.

The composite image on my Phillips is great though!

For completeness of mods done: I have bent up b27 (as that would short the clock to a GND on the b side so a wire patch underneath (tiny one, one pad down from the resistor) takes it to XOUT on the 'a' side where it comes from the MK14.

The resistor is for the pullup for NWDS which is mentioned in the VDU Manual as needing to be done on the MK14 - note that changes to NENIN have already been done on the Slothie V6 board - you would need to do those on a real one or the JMP/Martin clones along with the pullup. I did it on the clone VDU as it is removed when I unplug it.

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Finally the two white wires (and the associated bent up pins, move b13 the VDU ON/OFF from connecting to PA0 on the INS8154 I/O chip to FLAG1 (on pin b1) as recommended in the manual - I prefer this as you can turn it on and off (which you need to do to use cassette or a KeyProgrammer) still without an IO chip (I have used patch wires in its socket to pull the relevant pins up and down) - and it is canon from the manual - I may well hook my Graphics/Chars (b14) over to Flag2 for a similar reason.

The other wire is the Reverse Pages pin which I move from INTR (an input to the IO chip) onto PA3 which can be an output.

My next step will be to make a neat 8154 plug in module with some switches for the other lines and try my actual 8154 to change things within a program.
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Old 1st Jun 2020, 7:29 pm   #30
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Default Re: Mk14 vdu

I should have mentioned that inverting the signal to black on white creates a useless image - I assume it has the same problem as the early ZX81 that there is no Back Porch on the signal to set the White level. I may investigate adapting one of the (555 based) ZX81 fixes.

http://zx.zigg.net/misc-projects/ZX8...nditioning.pdf
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Old 1st Jun 2020, 7:38 pm   #31
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Default Re: Mk14 vdu

Thanks for the info - I believe it is a fact that the composite output circuit ideally needs to be distinctly different depending on whether you are driving a UM1233 modulator (as per original) or composite video directly, for one thing a composite video input on a TV or display typically has a rather low termination resistance of 75R.

If you followed any of John Erland's adventures in his ZX81 thread you may have noticed that his replacement 'ULA IC' even had selectable output modes, depending on whether it was going to output to a modulator or output direct composite video.

Both of those conditioning circuits look as though they could be built inside the empty shell of a UM1233 modulator, with the 'UHF out' connector then becoming composite-out.
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Old 1st Jun 2020, 9:38 pm   #32
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Default Re: Mk14 vdu

Well I had to know so I did a temp patch back to the VDU manual values... (yes its messy):

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anyway now I still get a composite image on the Phillips (as the Modulator is also in circuit I assume it drops the levels a bit) although it did blank on me once or twice. Inverse is still junk.

UHF is awful though

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But, the Sony can now get a stable picture in both Graphics and Text - although you have to twiddle the horiz. hold every time you turn the MK14 on or off.

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So for those who were observant on the rubbish picture above no surprises here:

https://www.youtube.com/watch?v=vtL2yKEOh74
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Old 1st Jun 2020, 10:21 pm   #33
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Default Re: Mk14 vdu

Brilliant!
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Old 2nd Jun 2020, 8:03 am   #34
SiriusHardware
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Default Re: Mk14 vdu

Wow, the old 'Falling Man' demo... I have not seen that running since about 1979. That's also the first time I have actually seen the uploader being used anywhere else so that was interesting for me too.

If you suspect a possible problem with the modulator (where did it come from?) forum user 'electroanorak' mentioned in another recent thread that he has a few NOS ones.
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Old 3rd Jun 2020, 9:47 pm   #35
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Default Re: Mk14 vdu

I do not think the modulator is (or was) faulty (I had already swapped it) it is simply the lack of Back Porch that makes Black on white unusable (and of course the perfect values for a NO modulator fitted board that stopped it working probably). So the original was probably OK but, I switched it for one on a Spectrum Issue 2 I know is good, that has had a wire Composite bypass (which I reversed) so does not need a working one if the original indeed proves faulty.

Anyway tonight I miniaturised my INS8154 eliminator allowing a switch for Graphics/Text and for Invert video anyway to experiment - I have one spare switch I can use to vary something else (the green one)...

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For those who wish to repeat the demo from the video here is the FallingMan as far as I have got - I am intending to replace the jumbled man bits above the horizontal line with a picture or the text Falling Man - 1979 if I can work out the bit pattern....

(just delete the .TXT and restore the last three chars for the file extension)

FALLMANASM.TXT

FALLMANHEX.TXT
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Old 3rd Jun 2020, 10:02 pm   #36
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Default Re: Mk14 vdu

Thanks, I was hoping you'd have that handy. It will be a few more days before I get mine connected, but it will be good to have something to fire into it straight away when I'm ready. When I last ran that about... 41 years ago, I had just typed it all in by hand.

You seem to be having fun with that. I still plan to use the actual 8154 to control the VDU initially, just because it's already there.
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Old 3rd Jun 2020, 10:06 pm   #37
Timbucus
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Default Re: Mk14 vdu

Great - when you have worked out the initial settings to get it to display something I can steal them and put mine in! I just wanted to get rid of the breadboard as it was awkward and a ten minute job turned into a major event...
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Old 3rd Jun 2020, 11:20 pm   #38
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Default Re: Mk14 vdu

Just a word of caution - try testing the voltages after the VDU is hooked up on the expansion bus on PA0-PA7 BEFORE you plug in the 8154 - I was getting readings around 6v and could not control text/graphics mode using PA6 by writing bits to it after setting it to an output port. Maybe we have to sink too much using the IO/chip alone if they are normally high - some study needed of the circuit I think.

It might be worth on your little conversion device having a set of links that can be made or broken for each PA0 connection - I need to look into what is happening - hoping I have not done any damage to my only 8154! The eliminator will be back in for the moment.

I have also found a flaw in our 1:1 mapping - hooking the "top page" line b17 to PB0 as an input is great to sense in software (in theory) when you are there but, you also need to link it over to b11 on PA1 so that the VDU board auto swaps the memory page to display both pages - that cannot be done fast enough in software.

I started down this road typing the Charset demo from the manual which 'only' needs the I/O chip to execute from so though what the hell and plugged it in and started using your trick of just writing to the 0800 areas after setting 0822 to 1...

I did this as Sirius asked if the char set was the same. Anyway more updates later in the week.

Actually I am getting high readings >7v on SIN and SENSE_A and other lines when my VDU is hooked up with no INS8154 - I wonder if something is coming back in that should not... I will have to take a serious look at the connections - especially if it is coming through the SC/MP!
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Old 4th Jun 2020, 8:12 am   #39
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Default Re: Mk14 vdu

I would view 7V anywhere east of the regulators with a great deal of alarm! I was actually already thinking of replacing one of the many decoupling capacitors with a 5.6V zener so that in the event of regulator failure (short from input to output) the zener would at least limit the voltage and more likely fail s/c, which would be bad news for the zener but would hopefully save everything else.

What's the actual supply voltage to the chip supply pins on the VDU? It's possible, I suppose, that somehow the unregulated input has found its way to the +5V - out terminal on the edge connector. Always remember, this was a previously untested PCB so you have to consider things you wouldn't normally consider.

I was aware of the likely problem with 'top page' usually needing to be connected to one of the address select lines in order for the VDU to display two different 256-byte blocks.
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Old 4th Jun 2020, 9:27 am   #40
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Default Re: Mk14 vdu

On a different note, I've been reading the articles in PE on their VDU project and it looks interesting. It uses a Thompson SFF96364 CRTC which when I looked at it last year seemed unobtainable, but now I see Littlediode advertising on eBay. It's an odd design but has some interesting features, including a RF modulator with inductors and transformer printed on the PCB layout. It has its own 1k of RAM that you could map to 400-7FF and a few control lines that could be connected to the 8154. The RAM is 8 2102 memory chips soldered between two pieces of veroboard to form a memory module that is connected by wires to points on the PCB. This memory chip is unusual in that it has separate inputs and outputs - modifying the circuit to use more easily obtainable ICs might be an interesting project.
Just need to get my stuff back so I can start clearing my backlog of projects!
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