I spent most of the morning today tinkering around with the PPC-21 I have here to see if I can somehow make it emulate CGA like its bigger brother, the CorData (P)PC-400.
First try was to swap out the BIOS. The PPC-21 runs 3.10, and the PPC-400 has 4.23/4.33 (at least, those images I have here). The ROM socket on the mainboard did not like the PPC-400 16K ROM (the PPC-21 has a 8K 2764 chip). So I ordered a lo-tech ROM board to see if I could make it work that way.
Surprisingly, the answer is yes. I managed to load the 4.23 and 4.33 ROM onto the flash chip on that board and map it to F800 and remove the on-board BIOS.
It boots! The screen had some trouble syncing, but after rotating its vertical hold pot a hair, it was perfectly fine with it.
Unfortunately -- no CGA. So I'm guessing the PPC-400 has some extra circuitry on board to map 16k of memory at B800 to emulate CGA. Bootup did show that it was removing 16k from the 512k on-board to do just that, but I am guessing the 16k never made it to B800.
It was obvious that its character ROMs weren't compatible with this slightly higher resolution (640x400 instead of 640x325 on the PPC-21 BIOS) as there were gaps between the lines.
So .. ultimately I accomplished nothing other than prove out that the mainboard of the PPC-21 will not support CGA emulation. Bummer.. It does still do 640x325 high res monochrome graphics, but only in an old gwbasic "screen 105" mode.
Next experiment will be to see if I can connect the internal monitor to an EGA board I have here that can drive hercules monitors.... But that's a job for another time. Looking to see if I can find the hsync and vsync on the connector to the monitor have so far proven to be fruitless unfortunately (didn't see any recognizable signals on the scope).
Nobody happens to have schematics for the Corona PPC-21, I assume?
Still a fun morning though!
First try was to swap out the BIOS. The PPC-21 runs 3.10, and the PPC-400 has 4.23/4.33 (at least, those images I have here). The ROM socket on the mainboard did not like the PPC-400 16K ROM (the PPC-21 has a 8K 2764 chip). So I ordered a lo-tech ROM board to see if I could make it work that way.
Surprisingly, the answer is yes. I managed to load the 4.23 and 4.33 ROM onto the flash chip on that board and map it to F800 and remove the on-board BIOS.
It boots! The screen had some trouble syncing, but after rotating its vertical hold pot a hair, it was perfectly fine with it.
Unfortunately -- no CGA. So I'm guessing the PPC-400 has some extra circuitry on board to map 16k of memory at B800 to emulate CGA. Bootup did show that it was removing 16k from the 512k on-board to do just that, but I am guessing the 16k never made it to B800.
It was obvious that its character ROMs weren't compatible with this slightly higher resolution (640x400 instead of 640x325 on the PPC-21 BIOS) as there were gaps between the lines.
So .. ultimately I accomplished nothing other than prove out that the mainboard of the PPC-21 will not support CGA emulation. Bummer.. It does still do 640x325 high res monochrome graphics, but only in an old gwbasic "screen 105" mode.
Next experiment will be to see if I can connect the internal monitor to an EGA board I have here that can drive hercules monitors.... But that's a job for another time. Looking to see if I can find the hsync and vsync on the connector to the monitor have so far proven to be fruitless unfortunately (didn't see any recognizable signals on the scope).
Nobody happens to have schematics for the Corona PPC-21, I assume?
Still a fun morning though!