Hi ELF-ish Fans -
If you recall from my version of the VIS, one can either use Character Generator mode, by running with the 2 * CDM6116 2k x 8-bit SRAMs, or can use bit-map mode, by running with the single CDM6264 8k x 8-bit SRAM. But you had to make that decision up front.
Wouldn't it be nice to be able to switch between the modes using software control? Note that the VIS board is already very full, so not much could be added.
I decided to add a 4013 D flip-flop to be used as a 1-bit output port, with the Q output controlling the !CS for the CDM6116's and the !Q output controlling the !CS for the CDM6264. I used a spare 4016 transmission gate together with a decoded N=2 line from the CDP1853 that was added earlier for the keyboard, to select the FF.
Also, note that the PCB input of the CDP1870/1876 is useless in bit-map mode needs to be tied high when the CDM6264 is enabled. I used another spare 4016 transmission gate and a pull-up resister to cause this to happen automatically when the CDM6264 is selected.
Here's the relevant changes:
And here's the new 4013 (original RCA type) installed in the lower left of the VIS board.
With this change, I can have both sets of memory installed at the same time.
With a slight update to the program to select the 6116 SRAMs, the Character Generator demo works as expected.
Changing the output OUT 2 byte from 0 to 1, selects the 6264 and thus bit-map mode.
It doesn't do anything more exciting than that as I haven't written a program to do anything interesting in bit-map mode yet, but at least it's functioning.
One of the keys is that we now have two independent pages and modes. You can be in Character mode, switch to bit-map mode and do stuff, and then switch back to Character mode and the display will not have changed.