RE: Article PC-card
Date: 2001-03-02 11:41:45

> > to use it. I will study the datasheets again to see why.
> Please do. :) I"m curious about your concerns.

First about my own solution. I thought I used 3-input OR-gates plus an 
OC-invertor to generate an IRQ for the C64 out of the IRQs of the PC-card. 
But 3-input OR-gates don't exist. Looking at the very first proto-type of 
my interface, which is dismantled partly, I think I used a 74LS14, 6 * 
invertor, a 74LS30 8-input NAND and a 7406 OC-invertor to generate the IRQ. 
A 74LS541 (or equivalent) was used to read the status of the IRQx-lines.

Ok, here are my findings. 
1) the 8259 is very complex to program
2) if you program it to give IRQs, you cannot poll the registers to read 
which line caused the IRQ. (But I'm not sure about it)
3) you can program a poll mode but then it does not generate IRQs
4) if you program it to give IRQs, you _have to_ tell the 8259 that you 
have noticed the IRQ by negating the INTA (INTerrupt Acknowledge) line 
twice. This means extra hardware. Advantage: Negating INTA the first time 
causes the 8259 to put a vector on the databus. This could be used instead 
of the polling.

But my main reason was: my gates-solution can be used on any C64/128. You 
only have to supply a PRG for the used card. Using the 8259 means you have 
to run software even if you haven't attached a PC-card.

Groetjes, Ruud

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