IanSB
commented
1 year ago @Nephrite-FM
is the existing PC-98 profile meant to be connected to the digital video ports of the older PC-98s which only supported 8 colors (3 bit RGB, 1 bit per RGB)?
Yes
Has anyone else made any headway on getting later PC-9801/-21 EGC-Mode 16-of-4096 Colors working through the 12-bit board already?
Not as far as I know
Am I right in my hypothesis that it might be just like what we would have to do to the Apple IIGS, which, in one of its lower resolutions had a similar 16/4096 color palleted mode?
It looks like that should work, as long as you can find the 12 bits of RGB and separate or composite sync to pickup.
is the existing PC-98 profile meant to be connected to the digital video ports of the older PC-98s which only supported 8 colors (3 bit RGB, 1 bit per RGB)? I haven't had a chance to try it yet, I only recently acquired 2 machines (a PC-8001 and an 8801MA system) capable of digital output, the 8801 MA should be able to use or be close enough to the 640x200 profile at 24KHz. My PC-98 machines are all the later Analog-only types.
Has there been any work done yet for the later PC-98s with the EGC Mode, where it could display 16 colors from a palette of 4096 colors?
Some background:
I have a PC-9821 Ap2 which would look nice scaled up 3x to 1920x1200 and a monitor which supports the 24kHz Refresh rate. For those looking for such a monitor for their later V2-capable PC-88s or their PC-98, the HP ZDisplay Z24i natively supports PC-8801's and PC-98's 24 kHz Modes with 56Hz HSync perfectly, plus its 1920x1200 resolution is a perfect 3x upscale of 640x400 (and a 3x Vertical 4x Horizontal upscale) of the 640x200 resolution, so artifacting is minimal although it is still softened going through an analog upscaler... Plus, the RGBtoHDMI will drive it (you need a DVI adapter as this display has no HDMI port) at 1920x1200 at 24kHz as well, making it perfect for integer scaling! In digital mode the pixels are perfect 3x3 squares.
Unfortunately I can't recommend it the HP Z24i for DOS/V or AT Machines like my PS/2 8540SX, because it can't handle 640x350 and other AT DOS Refresh rates (70 Hz) as well as it should, and when using Window Accelerators on my PC-98s in Windows, I have to connect them to a different monitor and dial down the refresh rate to 60 Hz, by default a lot of the Window Accelerator drivers in PC-98 like setting the refresh rate as high as possible, and a lot don't have EDID capability to detect that this display will only permit 60hz max.
The Ap2 (and many other 9801/9821 variants, especially A-Mates and 98MULTi Ce/Cs series that also supported 256-color games like DOOM and BlackThorne) has a Rockwell Bt121 Triple 8-bit DAC. These accepts 8 bits each of Red, Green and Blue which was needed to permit onboard or optional Windows Accelerator cards which plugged into the motherboard to feed their signals out through the same Analog RGB Output connector, enabling 16- and 24-bit color modes (i.e. my AP2 is a model variant (U2) that did not have the Matrox daughterboard added to it, although there are headers for it on the board. 3 of the 5 variants which did were meant for Windows customers, they would get a 1MB Matrox Card that could handle 16.7 Million Colors at 640x480. I also have a PC-9821 Cs2 where all variants sold had a Cirrus Logic Window Accelerator GD5428 on the motherboard, and therefore could handle 16.7 Million colors for Windows). http://classic.pasocomclub.co.jp/datas/nec_pc_9821ap2.html http://classic.pasocomclub.co.jp/datas/nec_pc_9821cs5_s2-s3.html
Both these models could take Expansion Boards - the Cs2 could use 8-Bit ISA-Like C-BUS cards. The Ap2 could take these as well, but it also had a VLB-like LB Slot as well right above the C-BUS slot that allowed 32-bit access... However both cards needed a second cable and a relay to switch from the Motherboard Video to the card upon botting Windows.
Ok, so there's the backstory.
I am only interested in getting the PC-98 DOS Mode EGC 16/4096 colors mode working off the mainboard, not the 256/16777216 palette PEGC mode (24 bits needed for that). That would be sufficient for a great deal of the PC-98 game library, including Touhou Project 1-5. and if I am not mistaken, I would need the 12-bit extender to pass the input lines to this DAC to the RGB2HDMI. So, I would have to order one, I don't have one yet...
I am also making the assumption that the 4096-color palette is an equal distribution of 4 bits per color, to make 12 bits (4096) and that the DAC, when being pumped with 4 bits of data from the EGC would get the data paired up so that the distribution is uniform, like this:
RED (4 bits EGC): BT121 DAC Input R7-R0: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 1 1 0 0 1 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 1 1 1 1 etc. etc. 1 1 1 0 1 1 1 1 1 1 0 0 1 1 1 1 1 1 1 1 1 1 1 1
I could be entirely wrong about this, it would depend how the motherboard's EGC chip is connected to the DAC.
BT121_Rockwell.pdf
Has anyone else made any headway on getting later PC-9801/-21 EGC-Mode 16-of-4096 Colors working through the 12-bit board already? Am I right in my hypothesis that it might be just like what we would have to do to the Apple IIGS, which, in one of its lower resolutions had a similar 16/4096 color palleted mode?
Are there any other thoughts on this?
I'll order the 12-bit extender board, if I am the first one to get this working I'll be happy to share the results.