10th Aug 2020, 1:59 pm | #141 |
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Re: Mk14 vdu
OK, I've got one of those. I thought I had at least one more, but maybe they have found their way into projects. I'll start rounding up the things I will obviously need, like a 32 * 2 -way edge connector. (No pressure!). Are you planning to run the uP from the host system's 4MHz clock or from a faster local clock, if so, what frequency, 20MHz?
Last edited by SiriusHardware; 10th Aug 2020 at 2:04 pm. |
10th Aug 2020, 2:09 pm | #142 |
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Re: Mk14 vdu
The PIC will clock itself.
You won't have to change the crystal on the Mk14 The PIC crystal will be 16MHz so it'll require a 20MHz part. Not that it bothers me - I regularly thrash 4MHz parts at 20MHz There is an option that might be available to us: if I were to use a 14.745MHz crystal, then the graphics mode pixels will be theoretically square (though they're not far off square as they are, and it's probably best to stay true to the original). |
10th Aug 2020, 6:04 pm | #143 |
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Re: Mk14 vdu
In the matter of the 16MHz crystal I find myself defeated, I have a bag of microprocessor crystals among which there is not a single one for that frequency. Nor have we ever used them at work. I'll order some.
Knowing your past work I assume the project will consist of the crystal, the PIC and very little else, although I would think the the video output will be 'conditioned' by one or two transistors. |
10th Aug 2020, 6:28 pm | #144 |
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Re: Mk14 vdu
You surmise correctly, SH
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10th Aug 2020, 6:49 pm | #145 | |
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Re: Mk14 vdu
Quote:
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10th Aug 2020, 9:39 pm | #146 |
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Re: Mk14 vdu
Hi all,
Doubtless the original SOC VDU generated a compromise non-interlaced signal. I can do better -for instance, I can ensure that five broad sync pulses are generated for frame sync. But there'll be no equalising pulses or interlace. I find most monitors (even modern ones) can lock to this, but there will always be the occasional modern TV that turns its nose up at my signal. There should be no problem with older (e.g. CRT) monitors. |
11th Aug 2020, 12:18 pm | #147 |
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Re: Mk14 vdu
Crystals are ordered. Your move, Karen...
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16th Aug 2020, 7:49 pm | #148 |
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Re: Mk14 vdu
Hi All,
I've been lurking on this thread, and the other MK14 stuff, because back in the day I had one. I built it from a kit, and put it in a sloping aluminium case, with the cassette interface, better keyboard and display, and a PSU. But the pièce de résistance was that I put a breadboard on the front, with all the IO brought out to sockets nearby. I experimented with all kinds of gadgetry connected to my MK14. One I remember was a 4 channel 'chopper bar' chart recorder. I turned that ito the world's slowest and worst performing colour printer. A VDU was something I wanted, but never got. Something I did get was a few chips used in arcade video games. I used these to program a rather clunky picture of train, with an old-school loco puffing clouds of square smoke over a couple of windowless carriages. Forty odd years, three house moves, and two kids later I can't find my MK14. But I did find the chips !. They are from the Signetics 2650 CPU family, but were used in Z80 based arcade games. The chips are the 2636 Programmable Video Interface, and the 2621 Video Sync Generator. ( The 'official circuit' also used an RGB video combiner, but I just used a resistor network to make a grayscale. ) The 2636 is rather clever as it has hardware collision detection, which makes missile/target detection very easy. If anybody wants to try replicating my train, or build Space Invaders out of an MK14, then I'll be happy to send the chips on to a better home. The chips might not work after 40 years in the attic, but they're old-skool white ceramic, so they will look cool on any PCB, even if they do nuffin ! ) Cheers, Buzby |
16th Aug 2020, 11:28 pm | #149 |
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Re: Mk14 vdu
Hello Buzby - thanks for joining, interesting story and probably typical of the kinds of experimentation and self-teaching these things were used for at the time.
House moves seem to be the most dangerous moments in the lifetime of any original MK14, I have heard or read that over and over again. For a while the only people who had MK14s or MK14 VDUs were people like me who had managed to hang on to them, then slowly but surely MK14 replicas started to appear, MK14man's 'modern' replica built partly with modern programmable logic, Karen O's PIC14 hardware emulator, then a couple of individual 'faithful' replicas made by people who just wanted one for themselves, then finally replica PCBs produced in some numbers by JM Precision (UK) and the Czech replicas from Martin L, who also produced exact clones of the PCBs for the Science OF Cambridge VDU, and then finally the 'issue VI' from Slothie on this forum, the only replica PCB which has the system buses routed to the rear edge connector. More recently there have been some very inventive ideas about how to make an alternative VDU for the MK14, for example TonyDuell's suggestion of using a I2C controlled teletext IC and Karen's idea of replicating the functionality of the original VDU with just a single microcontroller IC. And now your suggestion of using that chipset. I know it was a long time ago but can you remember the details of how the chipset was connected to the MK14, and can you remember which issue your MK14 was? |
17th Aug 2020, 1:19 am | #150 |
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Re: Mk14 vdu
Hi Sirius,
No, I can't remember what issue my MK14 was. I only remember it didn't work when I built it, so I sent it back to SOC. They diagnosed some problem with the 7805, fixed it, and returned it with the faulty parts in a little bag. You don't get service like that nowadays !. Now I would have no problem finding a fault like that myself, but I was a real noobie then. The 2636 chips I got when I was working for an arcade company, sometime after I got the MK14. These chips are not really VDU devices, they don't produce a page of text. The basic principle is that you define the shapes of upto 4 objects ( gun, missile, spaceship, explosion, or track, loco, smoke, carriages ), then specify the positions of these objects. The chip will tell you if any of these objects overlap, so collision detection is a doddle. There is no need to continually refresh, so I drove the chip using the 8154 IO. I spent an hour tonight looking for my MK14, but all I found was a stack of old laptops, an Apple ][, a BBC, and an OPD. ( Who remembers that ? ! ) I also found some TTL LS chips, some 8060's and 8154's, but no RAM chips, so I've not got quite enough to build a new MK14. Anyway, I shall continue lurking on this thread, it's heartening to see that there is still interest in these forerunners of our modern world. Cheers, Buzby |
17th Aug 2020, 11:02 am | #151 | |
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Re: Mk14 vdu
Quote:
You know you want to get the same fun by building one with such a good leg up. Among us here we could probably arrange something to help source the other hard to get bits - pre-programmed Tesla PROMS are available from JM Precision if we cannot get Blank 571. I was a QL user so aware of the OPD but, I have only seen one in a museum they do turn up for sale occasionally. Thanks for coming out of the shadows with such an interesting story. Tim |
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17th Aug 2020, 11:19 am | #152 |
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Re: Mk14 vdu
Another USP of Slothie's 'issue VI' PCB is that it allows for the use of an alternative type of RAM (IM65X61) by setting a link on the PCB, so you could look out for those rather than the now rather expensive MM2111 / upD2111 / AM9111 etc.
If you can find blank Nat Semi DM74S571 (rather than Tesla MH74S571) PROMs I'll be happy to programme them for you. However, buying blank PROMs is a bit of a minefield, some sources may try to sell you PROMs which were already previously programmed, therefore useless. It's probably safer to buy a programmed set from JMP. I have four blank, presumed genuine DM74S571 PROMs here. I would be happy to programme a pair and pass them on for what they cost me although I won't know they are good until I try to programme them. |
17th Aug 2020, 11:39 am | #153 |
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Re: Mk14 vdu
I have a display and maybe some of the other chips - if you are interested and let us know what is missing from the BOM for you, as with Sirius happy to pass on for what they cost me -another MK14 user would be great and seeing you hook up those video chips doubly so...
Semiconductors IC1 1SP-8A/600 (8060) SC/MP Microprocessor IC2,3 74S571 512 x 4 bit ROM IC4-7 MM 2111-1 N 256 x 4 bit RAM ((IC6,7 optional) IC8 INS8154N 128 x 8 bit RAM & I/O IC9,10 74LS157 Quad 2-1 line Selector IC11 80L95 Hex Tristate buffer IC12 74LS173 Quad TriState Latch IC13 7445 BCD->Decimal Decoder IC14-16 7408 Quad 2 Input AND IC17 7400 Quad 2 Input Nand IC18 74LS04 Hex Invertor IC19 LM340T-5.0 5v Regulator You also need a 74LS20 for the VI... IC17/16 can be LS - 14 and 15 have to be 7408 with an original display. IC11 can be 74LS365 Resistors R1,11 4k7 R2,5 2k4 R3 100k R4,6-10,12-15 1k2 Capacitors C1 27p for 33p Ceramic C2 1000uf 40V (Smoothing, optional) C3,4 0.01uf = 10nF C6 22uf 16V Other relevant parts X1 4.433619Mhz Crystal Switches - easily and cheaply available for the VI - I have a good stock so could pack up just enough and some caps for you. If you are really interested let me know and I will search through my chip stock and how much I paid for them. The 7408's may be the only fiddly ones. I know I do not any LS08's either I need to get some for spares when I have chance. Edit: I do have two LS173 - I may also try some C173 as I have a lot of those from the SCRUMPI project and the 80C95 works... Last edited by Timbucus; 17th Aug 2020 at 11:57 am. Reason: correction |
17th Aug 2020, 3:23 pm | #154 |
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Re: Mk14 vdu
Hi Tim,
Thanks very much for the kind offer of the chips and stuff, but I'm not yet ready to embark on making a new MK14. Too many other projects on the go. Anyway, I would really like to find mine. I know it's not lost, I've just put it somewhere I can't remember !. From what it looked like last time I saw it, it's going to need a huge amount of TLC to get it going again, so that's when I'll be back for assistance. Cheers, Buzby |
17th Aug 2020, 4:12 pm | #155 |
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Re: Mk14 vdu
I've been thinking about the SOC VDU and it occurs to me that the "bridge boards" that you guys have made to connect the VDU to the MK14 could have one of those 4-pin 4MHz oscillators on it feeding the XOUT signal, as there seems to be no requirement for the VDU and CPU to be syncronous. Then the CPU could run at its full blistering 4.433619 Mhz!(When it gets the chance)
It could also host a dual port RAM... Confession: I've bought a DM8678cab from eBay... No concrete plan but I have started drawing out the schematic in KiCAD! |
17th Aug 2020, 5:07 pm | #156 |
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Re: Mk14 vdu
Maybe someone has an as yet unpopulated VDU PCB they can just scan for you - would suggest though that you design out the weirdly pinned (therefore expensive) 74L86 and track for a 'normal' version while you are at it. Probably miss out the modulator and design a robust composite - out stage instead. That would make it officially issue 3.
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17th Aug 2020, 5:27 pm | #157 |
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Re: Mk14 vdu
I second designing out the 86 as they are nigh on impossible to get now.
If you go direct composite I think I may have already posted this but, just in case see attached. I did not scan my PCB before building sorry - I think Martin still has them available anyway. |
17th Aug 2020, 6:16 pm | #158 | |
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Re: Mk14 vdu
Quote:
If I'm going to make an issue 3 I'd probably want to deal with the front/back porch problem with inverted video that troubles modern TV and converter boxes so much. Plenty of food for thought! Tim: I've been having trouble locating repro PCB for the VDU when I last looked on Martins site last year it implied hed sold out and didnt know when/if he was getting more. Maybe a rev 3 for the V6 MK14 is in order...! |
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17th Aug 2020, 6:41 pm | #159 | |
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Re: Mk14 vdu
Quote:
Edit: Come to that, a third option: Provide pads for the optional fitting of a four-pin 4MHz oscillator module on the PCB. Last edited by SiriusHardware; 17th Aug 2020 at 6:54 pm. |
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17th Aug 2020, 6:52 pm | #160 |
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Re: Mk14 vdu
I have another suggestion, which is going to make you groan. Since the MK14 + VDU are only barely usable with the original maximum system RAM, consider including your now proven two-chip memory add-on on the VDU PCB. You might have to make the PCB longer or wider but who cares if it doesn't look exactly like the original VDU as long as it is electrically / electronically equivalent to the original as far as the system is concerned? Functionality over form for me, any day.
Another thing: 3mm or 4mm mounting holes on the board edges if possible - I'd like to see two mounting holes near the rear end on future revisions of the issue VI PCB as well, to make them easier to install on a base plate or in a cabinet. |