After various failed attempts to boot, with either floppy drives or a Gotek drive (which I’ll document later), I entered a world of despair as I discovered there was an issue with my VDU2K ‘graphics card’. Days tearing my hair out ensued, and I almost abandoned the project multiple times. Finally, I had some success though. 

The investigation is too long (and boring) to document here, but it is worth capturing the quirks in case it helps others. Incidentally, I was fortunate along the way, to obtain an old modified VDUK card, and also to borrow a known good VDU2K the same as mine (along with a spare MZB-3 with DMON Boot ROM too) – this helped fault isolation considerably!

The initial display problem was that although the boot prompt displayed correctly with my VDU2K fitted, proceeding beyond the boot prompt produced the curious result of all white space displaying as a zero character and any text characters displaying as the digit 6. With the borrowed MZB-3 card fitted, even the DMON boot screen had only 6 and 0 characters. The borrowed VDU2K card was exactly the same as my VDU2K, with 6 and 0 characters only, with either CPU card fitted.

Wondering if there could be some issue with both VDU2K cards, however unlikely that seemed, I decided to take a look at my VDUK. It actually turned out to be in good shape, requiring only the replacement of 5 missing ICs. I was concerned it had several cut wires that went nowhere, but these appear to be from some modification or other that has been reversed and they can be ignored. To my surprise, the VDUK worked immediately, showing a clear boot prompt, and also able to display beyond that when I finally got the system to boot. Its only shortcoming is its lack of RAM (1K instead of 2K) means it only has a small display area.

At some point during my fault finding, with cards repeatedly in and out of slots, and sometimes in different slots, the problem suddenly changed. The VDUK was still consistently working well, but the VDU2K cards would now display a blank white screen instead of 6 and 0 characters. I can only presume there was some issue with one of the edge connectors (or the back plane) that cleared itself.

Pulling out the working VDUK and inserting the known good VDU2K, I expected the same display. Instead, I got the same boot prompt, but when the system booted I was presented with a blank white screen.

I finally found the cause of this issue, on the DRM-64 Dynamic RAM card. The replacement RAM card I had purchased had the DIL switches set to create a ‘memory hole’ at F000, where screen memory resides. It seems a system with a VDU2K installed requires DRAM at the same location as the static RAM on the VDU card, whereas a system with a VDUK installed does not!

Along the way I observed an issue with my card, in that the known good VDU2K would display a blank screen below the boot prompt, but my card would display tabular random (but repeatable at each cold boot) characters. This is caused by my Character Set EPROM, as swapping EPROMs with the good board removed the issue. With DRAM shadowing the static RAM it does not manifest however, random characters are only seen with a DRAM ‘memory hole’.

For good measure, I also discovered that I have one 74LS74 that when inserted as U21 in a VDU2K works perfectly, but when inserted into my VDUK removes the right-hand edge of the display.

Overall, I have learnt the cards are fussy, and it is best to shadow the static RAM with dynamic RAM (indeed it is essential with a VDU2K card).

It took a long time to get there, but I do now have a working Interak system (albeit with no keyboard yet!)