Having read some of the suggestions here--and having found that though the previous approach seemed to work it actually continued to be unreliable--I have a new ISP circuit that I'd like to get everyone's opinion on.

D6 is used to detect when the circuit is connected to the parallel port since the software makes sure that D6 is high when doing the programming.



Now the above circuit seems to work 100% flawlessly. The 220 ohm resistor on MISO seems to be necessary. If I remove it it fails 100% of the time. Is that to be expected? Does the above circuit look reasonable or do any of you see any blatant "gotchas" considering the device would be connected to a PC parallel port?

Also, I was thinking I'd like to use a 74HCT126 instead of the '541 which would give me the exact number of lines I need (4), do it in a smaller package (14-pin rather than 20-pin) and could do away with the inverter that is connected to /G2. But I was unable to find 74CT126's locally (in Mexico) and Mouser doesn't carry them in-stock though Digikey does. Would a 74HCT126 be considered a "hard to find part" and should I use a '541 instead just based on availability? Is there some version similar to the '541 but which is high-Z on LOW G1 or G2 rather than on high so I could avoid the inverter?

I apologize for what may be considered obvious questions. I'm usually on the software side of development though I'm quickly finding the hardware side to be quite enjoyable (despite the lack of parts in Mexico).

Thanks,
Craig Steiner