Thread: Mk14 vdu
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Old 7th Nov 2020, 7:41 pm   #829
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: Mk14 vdu

Well, you now appear to have arrived at what I call a four-biscuit solution.

This is a fast moving thread. Now I'm looking for some 47pFs, I have a 'pot' full of small value ceramics originally acquired for RF projects so I'm hopeful that I might be able to scrape that many together.

After you had that mishap with the first 8154 I decided to keep the 8154 pins and flag outputs the hell away from the VDU - they are not completely disconnected from the VDU control pins but there are series resistors in between the 8154 pins and the VDU and I never alter the 8154 pins away from their default input mode. To 'programme' the VDU configuration I just use Arduino style pluggable links.

Of course it is preferable to have an agreed connection setup so that software can be written to soft-control the VDU (which in fact you have always largely succeeded in doing) but it just seems rather high-risk and I can't afford to damage expensive, hard to get ICs.

One non-canonical way to go around this would be to map something like a 74LS573 (8 bit latch) onto the final address in the memory hole (07FF) replacing one byte of RAM, and use the outputs of that to control the VDU functions. Literally as cheap as chips, unlike the antique LSI ICs.
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