UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

SiriusHardware · 27th Apr 2019, 9:22 am

Again, some nice information mining there. We're going to have to make you the official MK14 historian / repository. I remember running the 'Falling man' animation on my VDU / MK14. That comment by Paul Robson (-The- Paul Robson?) is quite new, I only just noticed it myself.

The VDU 'demo driver' PCB is a Microchip '44 pin demo PCB' of the type which was included with PicKit3 programmers - the chip on it is a PIC18F452, chosen because it has a 'PLL' clock mode which allows it to effectively run at 40Mhz with a 10Mhz clock crystal. I rigged that up a few years ago now after seeing the VDU PCB sitting unloved in a drawer for decades and finally feeling sorry for it.

In theory you can test the VDU just by programming an eprom with some test text or a test bitmap image and connecting the address, data and RD lines to the appropriate connections on the VDU. (You will need to generate a 4MHz clock for it as well).

Tie all unused eprom pins - higher address pins, OE, etc to 0V and other pins to whatever state is necessary to get the chip into read mode.

On the VDU, tie all of its control lines into the obvious required states and you should get a nice static image of whatever you have programmed the eprom with.

Character mode is the easiest to try this with since the relationship of screen ram locations to screen character locations is exactly as you would expect, screen ram location zero is the first (upper left) character on the screen, screen ram location 511 is the lower right character on the screen.

In graphics / bitmap mode the relationship between screen ram locations and pixel locations is not quite as simple as you would expect, I'll go into that in more detail some other time. There's a reason why the 'Falling Man' animation scrolls downwards rather than across... I'll also post the VDU's reduced ASCII character code set as well, as I don't know if it is documented anywhere.

27th Apr 2019, 9:22 am	#177
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,569	Re: MK14 schematic revisions Again, some nice information mining there. We're going to have to make you the official MK14 historian / repository. I remember running the 'Falling man' animation on my VDU / MK14. That comment by Paul Robson (-The- Paul Robson?) is quite new, I only just noticed it myself. The VDU 'demo driver' PCB is a Microchip '44 pin demo PCB' of the type which was included with PicKit3 programmers - the chip on it is a PIC18F452, chosen because it has a 'PLL' clock mode which allows it to effectively run at 40Mhz with a 10Mhz clock crystal. I rigged that up a few years ago now after seeing the VDU PCB sitting unloved in a drawer for decades and finally feeling sorry for it. In theory you can test the VDU just by programming an eprom with some test text or a test bitmap image and connecting the address, data and RD lines to the appropriate connections on the VDU. (You will need to generate a 4MHz clock for it as well). Tie all unused eprom pins - higher address pins, OE, etc to 0V and other pins to whatever state is necessary to get the chip into read mode. On the VDU, tie all of its control lines into the obvious required states and you should get a nice static image of whatever you have programmed the eprom with. Character mode is the easiest to try this with since the relationship of screen ram locations to screen character locations is exactly as you would expect, screen ram location zero is the first (upper left) character on the screen, screen ram location 511 is the lower right character on the screen. In graphics / bitmap mode the relationship between screen ram locations and pixel locations is not quite as simple as you would expect, I'll go into that in more detail some other time. There's a reason why the 'Falling Man' animation scrolls downwards rather than across... I'll also post the VDU's reduced ASCII character code set as well, as I don't know if it is documented anywhere.