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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 20th Dec 2018, 10:13 am   #141
SiriusHardware
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Default Re: MK14 schematic revisions

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Originally Posted by Slothie View Post
My concern other than availability would be that the BPROMs may not be fast enough when its access time is added to that of the RAM and Monitor ROM, but that would be something that would be easy enough to try out, and would make fully expanding the MK14 to 512b prom + 3k RAM possible, and would make many more complex VDU projects feasable (Like MK14 space invaders ??!!)
Don't get too ambitious, the dot resolution of the VDU in graphics mode is not terribly high. 'Pong' might be a more realistic aim.

BPROMs were used on the Nascoms, or at least one of them, as custom address decoders. They are actually surprisingly fast in terms of access speed - for that reason they were often used in things like video test pattern generators where the access speed of contemporary EPROMs might have been too slow.

However, as with all things that age they are becoming more expensive and difficult to get. GALs would be another option but only a small percentage of people have programmers capable of programming them. The parts would really need to be as ubiquitous as possible. A 'modern' fast EPROM might work just as well as an address decoder but it would come in a wide bodied footprint.
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Old 20th Dec 2018, 1:00 pm   #142
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Default Re: MK14 schematic revisions

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Originally Posted by SiriusHardware View Post
However, as with all things that age they are becoming more expensive and difficult to get. GALs would be another option but only a small percentage of people have programmers capable of programming them. The parts would really need to be as ubiquitous as possible. A 'modern' fast EPROM might work just as well as an address decoder but it would come in a wide bodied footprint.
GALs might be an option; my cheap 50 TL866CS/Minipro programmer will do Lattice GAL16V* and GAL20V* and Minipro works great on Linux under Wine with a modified DLL. Although obsolete too they seem to be far more available than BPROMS. Although I've never used GALs I have used CPLDs which are much the same. I'd have to find some logic compiler to generate the bitmap that the Minipro expects but I'm sure there's something out there for free. (Sadly the TL866 doesn't do BPROMS )

That said, if someones kitted themselves out with a programmer for Tesla PROMS for the monitor then programming an address decode PROM wont be an issue! eBay seems to be flooded with Tesla 74S571s at the moment, so perhaps I should get my act together and start looking at programming them!
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Old 20th Dec 2018, 1:02 pm   #143
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Default Re: MK14 schematic revisions

.... nudge.
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Old 25th Jan 2019, 1:34 pm   #144
Slothie
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Default Re: MK14 schematic revisions

Hmm things are getting a bit tight....
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Old 25th Jan 2019, 6:48 pm   #145
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Default Re: MK14 schematic revisions

Project impossible? (Within the established footprint, at least). Maybe that's why S.O.C. never did it despite it being the obviously right thing to do.
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Old 26th Jan 2019, 6:22 am   #146
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Default Re: MK14 schematic revisions

Probably not impossible if i'm prepared to rip up and re-lay some tracks. It would have been far more difficult for SOC without modern CAD tools, the board photos I've seen show they were most likely done with tape and transfers on acetate sheets by hand, so yes that's probably why SOC didn't. I haven't quite given up yet
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Old 28th Jan 2019, 8:10 pm   #147
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Default Re: MK14 schematic revisions

I've just been looking around for maybe one or two pairs of spare DM74S571 PROMs to give me the option of occasionally using the MK14 for a dedicated purpose every once in a while - the price of Tesla PROMs has gone up to about 2+ 3 each and the seller I bought two 'real' (National Semiconductor) DM74S571 PROMs from for 6 each last May is now selling them for 16 each.

Elsewhere I found someone offering programmed pairs of PROMs for well in excess of 30 which I thought was ridiculous until I saw the price the DM devices are going for now (The devices being offered in that particular case are actually DM, not MH).

In the past, it was the price of original machines and the lack of availability of PCBs which held people back from building / owning MK14s. Before long it is going to become impossible for anyone to afford the parts to populate one. I can see a situation where eventually genuine MK14s in poor condition are being robbed for their parts to populate replicas or just to sell separately.
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