photo to follow
This is an enhanced version of the original MK14 VDU which is intended to be used with the MK14E but can be used with an original MK14 as well.
The main driver for this version was to design out the obsolete and very hard to find DM8678CAB Character Generator at the same time a number of other changes have been made:
the again hard to find 74L86 has been replaced
the Modulator has been dropped and a video output connector added (phono)
the 'b9' to 'b17' configuration links have been made easier to use
the graphics shift register has been changed to a 74LS166
inverted video has been 'fixed'
2732 EPROM character generator
inverted characters are now possible by setting either bit 6 or 7
The change of shift register for graphics (from 74LS165 to 74LS166) was made because the 74LS166 has a synchronous load input, this enables the characters and graphics to be aligned vertically when both are on the screen, it also fixes the 'jail bars' flaw.
I have not changed the logic that controls memory/CPU access for fear of upsetting some critical timing !, the only change was to replace the two 74L86 'buffers' for CMOS 4050B buffers which have approximately the same propagation delay.
The original VDU required the 'b9' to 'b17' connections to be linked to configure the VDU, I have made this easier by adding links to the most used options and to cover all eventualities both the 8154 ports are brought out on a set of pads so they can be linked in if required.
The original VDU will only work in a 4K memory space, the MK14E VDU is able to work in the full 64K by setting the additional Page Select lines.
The MK14 VDU displays 32 lines of either 8 - 8x8 pixel graphics or 16 5x7 pixel characters (from the DM8678CAB). Having replaced the DM8678CAB with an EPROM character generator this allows for the full 8x8 pixel map to be used for characters. The 2732 allows for 4 sets of character fonts to be included, I have created an image with the following fonts:
| Font A/B Links | Font |
| 0,0 | original DM8678CAB Font |
| 0,1 | ZX80 and ZX81 Fonts |
| 1,0 | ASCII Font |
| 11 | Spare |
Only the first 512 bytes of the DM8678CAB Font page are used. The ZX80/81 fonts are each only 64 characters - bit 7 selects between the two.
I've written a Python program that generates a text file showing the fonts, here it is with the output file:
Schematic
Bill Of Materials