Thread: Mk14 vdu
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Old 8th Jan 2020, 8:16 pm   #3
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
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Default Re: Mk14 vdu

I suppose you could call it DMA - it uses the SC/MP's ability to go hands-off on the bus to take over the memory, scans 512 bytes of the 640 bytes total memory and renders the contents as either characters or pixels. The character mode layout is 16 across by 32 down, the character lines are really too close together for comfort with a very small vertical interval between the lines.

As to the rendering mode, I can't honestly say as I have never looked into it in that kind of detail. I'll attach the manual, which includes the circuit diagram, below.

There were various problems with it:

-When the VDU was active it stopped the SC/MP whenever the scanning dot was within the 'drawable' area of the screen - in other words, instead of halting the SC/MP for just long enough to read each byte of screen data, it would keep the CPU in the halt state whenever the 'dot' was within the drawable area, which slowed the system down a lot.

-It ate up most of the available RAM, leaving only 128 bytes in which to write an actual program.

-The MK14 standard 4.43Mhz crystal had to be changed to 4.00Mhz for the VDU, so software timing-critical programs such as 'Music Box' and 'Digital Clock' didn't run quite as well as before.

-No version of the MK14 ever had proper connections provided for connection of the VDU - issue IV and V had extra 'finger' connections on the track side of the rear edge connector but they still weren't actually wired to the buses, you had to do that yourself.

For all of the above reasons mine didn't stay connected to the MK14 for long, but around 2013 I found it languishing unloved in a drawer and took pity on it, connected it to a PIC programmed to emulate 512 bytes of RAM and powered it up, essentially just so that I could show it / see it working. (see attached image). The lines of text on that demo image are written to every other character line to leave a reasonable space between lines.

You may recognise the PIC PCB as a '44 pin demo' PCB, as supplied with some versions of the PicKit2 / PicKit3 programmers. The PIC is an 18F452. The 'metal can' oscillator is providing the 4.00Mhz clock which would normally be borrowed from the MK14.

Practical Electronics magazine were enthusiastic supporters of the MK14 and produced a superior VDU project for it - unfortunately I never saw one or came across anyone who had one.
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