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Old 25th Apr 2019, 9:33 pm   #174
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default Re: MK14 schematic revisions

That was a bit more difficult than it needed to be - my archived copy of the source code for the VDU 'Demo' wasn't the final version - I have no idea where that got to - so I had to debug the next most recent version all over again.

Ordinarily, I would connect the MK14 VDU or my ZX81s to a nice little mid-late seventies B/W portable TV that I keep for the purpose, but that is mothballed just now so I dug out my Philips CM8524 Composite / RGB video monitor and fed that with composite video taken from the input of the VDU's UHF modulator.

Attached image #1, the VDU fitted with RS sourced SN74LS365s in place of its original 80L95s and displaying an appropriately informative text.

The VDU is of course monochrome, but the Philips (colour) monitor has a 'Green' switch which puts it into a mode where monochrome is displayed as restful green on black instead of white on black. The image is absolutely rock steady - the Philips has always been a great monitor for me - I bought it to use with my Atari ST in the mid eighties and it is still regularly used with that same Atari. I had to replace the power switch once, but that's all.

The SN74LS365s were two from the same batch which all worked in place of the 80L95 on the MK14 - looks like they work OK as IC9 / IC10 in the VDU as well.

Doing this reminded me again just how primitive the VDU is - only 16 characters maximum across, I had to leave a blank line between each line of text because they are insufficiently spaced out otherwise, there are only 64 characters in the character set (no lower case, and only a limited subset of 'other' characters available).

Image #2 (displayed in conventional white) is more or less centre justified within the active screen area. It gives a better idea of where the left and right boundaries are and clearly shows that even with the correct 4MHz clock the active screen area is noticeably off-centre towards the left.
Attached Thumbnails
Click image for larger version

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Name:	SOC_VDU_message.jpg
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