Sigma Electronics Systems Display Generator 5564

The fact that 2 of the boards are wire-wrapped not PCBs suggests it was a low-production unit, but equally it is too well made to be a one-off kludge IMHO. I suspect it's similar to the little state machine in the HP2623 terminal for line drawing and these RAMs contain the parameters for the state machine. I doubt it's 4 bits per colour, I suspect lines are drawn one at a time, you specify the colour, the state machine does its thing and the colour setting simply determines which bits in the video RAM are changed. So the vector generator would not know anything about the colour I think that's a state machine type of sequencer. 6 of them. It might be 2 boards per colour, but one thing I didn't notice until later were the 74189 RAMs on the video processor board (the one with the DACs on it). My guess is that these are a colour look up table forming a mapping between bits in video RAM and the DACs (maybe 3 off 5 bit DACs). This suggests 4 bits per pixel (to address the CLUT RAMs) at the video memory side, how that fits in with the 6 Pixel Store boards I don't know. It is going to be a big job to sort it out, but one that will be worth doing. That is, unless manuals for it turn up... -tony