Found this thread while trying to find out about my own old CGA card.
Reason for wanting to find out more is that I have just bought a faulty 8088 motherboard from an Ebay seller (UK). GIven my recent interest in this old uP I thought that it would be interesting to try to repair.
I saw a YouTube video in which the maker explained that a VGA card would not work in his homebrew 8088 PC board, since the BIOS used non 8088 opcodes. I do have a VGA card, but if I try to use the old CGA card in theory it should be compatiable. Problem is I don't have a suitable monitor 😀 .
Anyway I started to search for information about this particular card. I found a number of pictures of nearly identical cards. The layout of the ICs is identical, the positions of the jumpers are identical, but there are subtle differences in the PCB layout. I'll post these other pictures in a followup posting. I searched around for datasheets on the non TTL ICs, but hit a dead end with the 100pin QFP, but finally got a break on the UM2300.
Since then I have reversed engineered the board and I am in the process of completing the schematic in KiCad.
The main IC has no device identification, just the label VDL or more likely UDL. I did find some pages which listed UDL as a company, but I cannot find anything more out about them. There is also a VGA chip from the same company.
I did confuse them initially with UMC (Unicorn Microelectronics). This lead me to searching for a UMC datasheet. I didn't find it, but I found one thaty was remarkably similar, and I was able to find a PDF with pinout and example circuit. The UMC part is a UM487 "HCGA Controller". At first I thought this was the same chip as it reads "The UM487 single-chip HCGA controller is an advanced product designed to serve as a combination MGA and CGA through the integration of most of the circuits found on MGA and CGA cards. Built with the 2um CMOS process, the UM487 incorporates a built-in 6845 CRTC circuit. Thus, for MGA and CGA functions, only a few external components are required to complete the circuits. These include: 64K bytes DRAM; 4 TTLs (74LS374, 74LS245, 74LS04 and 74LS20); one character generator (UM2310) and a 16.257MHx crystal. The UM487 comes packaged in a compact, 100-pin plastic flat pack". Wow! That description almost fits this card exactly, but as I viewed the example circuits I noted a number of differences. I also noted that the pinout didn't look right. Nevertheless I now believe that the UDL chip is very similar to this device.
I also searched for the UM2300 IC. This is a UMC part, which is why I at first thought that the UDL part was also a UMC part. I had no success finding a datasheet, but I did find the UM2303 which is a character set ROM containing character sets for both MGA and CGA. This device is however 28 pin, whereas the UM2300 is 24 pin. Eventually I had a break and found the SIS 23C00. Bingo! This is obviously the same part as the UM2300 and is called a "CGA Character Generator". This seems to imply that this card is CGA only (and does not support MGA). I have attached the PDF summary.
Some other similar cards have a DIP switch. I eventually figured out that the bank of nine 47k resistors are used for configuration and connect to an 8 bit databus and "pin 3". Five of these resistors come from the J5 jumper block. On my card J5-1 and J5-2 are tied to VCC by a wire link, the remaining three have jumpers for selection. Likewise J4 (which my card says is to Enable/Disable the Printer) also connects to one of these resistors.
I noted that the board has an extra set of pads right next to the UM2300 to take a larger 28 pin device. I have confirmed the connections and this footprint is for the UM2303 CGA/MGA character ROM. I also then realised that "pin 3" connects to the pin labelled "C/M" on this bigger ROM, which I assume selects the Colour or Mono character set. So perhaps the UDL chip can do MGA too, but is configured on my card to be CGA only. Perhaps the 47k resistor on "pin 3" selects either CGA only or CGA/MGA mode? Obviously you would need to unsolder the UM2300 and solder a socket for a new character ROM. The UM2303 is 2764 pin compatible, so this is certainly do-able.
