FPGA to ISA-bus

I bought a while ago a few of the cheap FPGA boards from ebay for 10 € a pcs and decided to test whether one could interface the old ISA bus with it. One issue is that this FPGA board is not 5 V tolerant so I decided to try with 10 kohm resistors in series with the pins and this seemed to be all that was needed or at least so far everything seems ok for standard port writes even if I'm not entirely sure if this is safe in the long run.

I ordered some rather blank ISA boards from JLCPCB for 2 € for 5 boards and soldered some tests leads to 8 data pins and 10 address pins.

The 8 data pins are located at A9-A2 and the 20 address pins are located at A31-A12. Additionally we need nIOW for I/O write which is located at B13 and GND that is located at B1.

This is how the thing looked like. I suppose one could have powered the FPGA board directly from the ISA slot, assuming it can supply enough current. That way one could make a rather standalone board, but so far I didn't try this. Actually, in this picture I have only 9 address pins connected, but it would be more standard to use 10 address pins.

I wrote a quick VERILOG to control the FPGA board LEDs. Basically all this does is set the LEDs to match the data bits when I/O write occurs and the address is 511.

module C2(LED, ADDR, DATA, nIOW);

output  [2:0] LED;
input   [8:0] ADRR;
input   [7:0] DATA;
input         nIOW;

reg     [2:0] leds;
assign LED = leds;

always @(negedge nIOW)
begin
if(ADDR == 511)
leds <= DATA;
end

endmodule

I also wrote a quick PASCAL program to test with a binary counter.

var
  a, b : byte;
  c : word;
begin
  for b := 0 to 4 do
    for a := 0 to 7 do
      for c := 0 to 32768 do ;
      for c := 0 to 32768 do port[511] := a;
    end;
  end;
end.

All seemed to work well on the first try so it appears making a rudimentary ISA device is very easy.

[http://www.hardwarebook.info/ISA]