8bit ISA-bus expansion card (8255, 8253)
Three buffers protect computer bus from direct damages: U1 in bi-directional
mode set by BIOR (Buffered Input Output Read) for data bus and U2 and U3
in unidirectional mode for address and miscellaneous signals. The board can
without these buffers if ISA bus is not overloaded ( only graphic card and IO card are also connected on 286,386 or 486 main boards) but for a prototype card is better for user to buffer the bus.To select addresses for 8255 and 8253, A0 to A9 address lines are used and of course IOR, IOW( Input Output Write), AEN(Adress ENable) and RES(RESet) signals. All address selection circuits (74F400, 74F32, 74F30, 74F04) can be replaced with one GAL16V8 but because this board is the key of my GAL programmed (Alta Engineering modified version) have not a GAL device inside. 8255 PA, PB and PC and 8253 CLK0, CLK1 and CLK 2 ports are connected with a IDE style connector to user section of the board ( all board is a prototype PCB with 8 bit ISA bus connector). This board was tested on 286, 386SX and 486 computers. I never try on Pentium for two reasons: ONE - I haven't any Pentium motherboard, TWO - I don't want to smell a smoked Pentium board ...
All chips have a 100nF capacitor soldered between VCC and GND pins. Also two 10uF tantalum capacitors are soldered on board. To understand how is generated the address here is a little table:
Notice that (A9,A8, -A7, -A6, -A5) become -CSP signal ( Principal Chip Select ), (-CSP, A4) formed 8255 CS and (-CSP ,A2 ,-A3) is 8253 CS. The conections between integrated circuits were done with wire wrapping soldered.
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