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
work properly
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:
A9 | A8 | -A7 | -A6 | -A5 | A4 | -A3 | A2 | A1 | A0 | ||
8255 | 1 | 1 | 0 | 0 | 0 | 0 | X | X | 1 | 1 | CONTROL |
8255 | 1 | 1 | 0 | 0 | 0 | 0 | X | X | 0 | 0 | PA |
8255 | 1 | 1 | 0 | 0 | 0 | 0 | X | X | 0 | 1 | PB |
8255 | 1 | 1 | 0 | 0 | 0 | 0 | X | X | 1 | 0 | PC |
8253 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | CONTROL |
8253 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | CLK0 |
8253 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | CLK1 |
8253 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 0 | CLK2 |
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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