The unknown board is a good exercise for reverse engineering an ISA card. No custom PALs on them, most traces visible, not too complicated. I will still spoil the result: It seems to be an 8-bit digital I/O card with opto-isolated floating inputs and open-collector outputs. It mostly works like this:
The two 74LS85 chips compare 4 bits of the 10-bit port address to the 8-bit base address configured by the jumpers. If both of them indicate a match, they enable the 74LS138 decoder, which can decode up to 3 bits, but will decode only 2 bits (8 are already decoded byte the 74LS85 chips) on this card. Maybe it also decodes IOR/IOW.
If the output port is selected, the 8-bit data value from the ISA bus is latched into the 74LS273, which drives the TD62083 octal darlingtron transistor driver. This chip can pull output lines (likely pins 1-8 on the D-type connector) to ground, and quite strongly so, but it can not provide an output voltage. External devices connected to the card need to provide their own power that to pull the outputs high if they are not pulled low by the card.
The 8 digital opto-isolated inputs have a low and high signal each. If the voltage of the high signal is high enough compared to the corresponding low signal (irrespective of the PC ground potential), an internal IR LED in the DA0-DA7 chips (not D/A converters) lights up and make a photodiode (or a photo-transistor) inside the same chip conductive. The output of the photo transistor is then compared to a threshold voltag using the two quad comparators (clones of the LM339, most chips being labeled some random letters followed by 339 are LM339 clones), to get a clear 0 or 1 out of it. If the input port is selected, the 8 output bits of the two LM339 clones (4 bits per clone) are forwarded to the ISA bus through the 72LS244 octal bus driver.
For the beginners in electronics: If a chip is called a "quad something", it means that the chip contains 4 times the same function performing "something", so a "quad comparator" contains four separate comparators, each of them comparing two different input voltages against each other, and each of them having their own output pin. The same is true of "octal something" chips, that contain 8 times the stuff to do "something".
