bootloader and the parallel port programmer

I built one of those parallel port programmers following the instructions on and, not looking carefully, I used 470K and 220K ohm resistors instead of the ones specified. It didn't work, of course, and then I realized I only had 560 ohm resistors, not the 470 specified. Interestingly, this made the programmer work, but unreliably. I'd get various numbers of flash errors - sometimes as few as eight and sometimes as many as 20 depending on how close I was to 470. Eventually I found some 470s and it works most of the time but is still unreliable. I still get flash errors - always at least two. If I try to flash several times and end on low number of errors the bootloader usually works and I can use it in the Arduino world.

Does anyone have any ideas for how to lower the error rate? Should I be shielding the cables or something? Should I be using ferric cores? I have no idea what speed the data is traveling at inside the programmer, though it must be some sort of serial system.

If you're on Windows, you might need to change a registry setting to stop Windows from polling your parallel port. See for more details and some other things to try.

I don't know what was going on with my parallel programmer - I resoldered it and it seems to be perfect now. Shielding and such were unnecessary. I guess I'd done a crappy solder job before. All the those resistors are buried in epoxy now and it is perfectly reliable.