First post, by kinetix
Hello Vogonians
I want to show you an expansion card for XT that I found a while ago. It is one of the two main cards of the Acer 500+. I think it is the most "multi" multifunction card I have seen, as it includes:
-CGA/MGA/Hercules video controller with TTL and Composite Video outputs
-FDD
-Serial port
-Game port
-Parallel port
-Real Time Clock
However it has been cannibalized, it is missing several discrete components. One component even fell on my hand when I picked up the card from a pile of "electronic garbage" (trash for someone, a treasure for someone else), they were unsoldering!!
I´m posting one image of a full card, no the one I found, with some labels.
With the help of the few images I have found, I think I have identified all missing parts, except for one missing chip (??? in the image). The transistor on the top right I'm not entirely sure which one it is.
Have doubts about two others chips (? in the image).
The marking on the missing chip is three lines: 87G018A, 20V8-027-1-2AS, 8841.
After much searching I thought I had found a possible, minimal reference, but I lost it and have not found it again. I seem to remember that it said it was a multiplexer of some kind, but, again, the reference was not certain.
Following the traces it appears to be connected to the bus address lines (A21-A25) and has some indirect relationships with chips from several of the subsystems. It seems to me that it is the chip that controls which subsystem is served by the central unit at a given time, multiplexing each one.
Now, I was thinking about rebuilding the card later, since it is the only one I have of any type of video for 8-bit ISA, the oldest ones I have are VL-bus, and none ISA. I am trying to rebuild and resurrect two XT systems with some historical relevance in my country, but they are very incomplete, and I need a video output. Here it is very, very difficult to get the necessary parts (I am visiting a recycling center once a month, and a friend there saves things for me, but it is very random what appears before 2000 and little before 1990)
As I think it would be almost impossible to find this chip in its physical form, or its equivalents, it would perhaps be better to find its schematic and pinout and perhaps make and adapt an substitute circuit. I want (hope) at least the video controller to work, maybe the FDD too. Meanwhile I´ll keep looking for some other compatible card.
So, if any kind soul has information about it, it would be appreciated.
