A lot of people has trouble burning the bootloader using the parallel programmer (http://www.arduino.cc/en/Hacking/ParallelProgrammer). Some suggest that removing the resistors improves the results.
Why are those resistors there in the first place? If the parallel port inputs and outputs are TTL compatible, shouldn't it be connected directly to the ISP inputs and outputs?
its kind of like the current limiting resistors in an led circuit but instead of protecting the circuit they are there to protect the parallel port. Basically parallel ports are only designed to source a small amount of current but if you exceed that you risk burning out parts on your motherboard. resistors limit the amount of current you will sink to acceptable levels.
-the worst case scenario of not having them is that you burn out the parallel port on your board but depending on how it is interfaced to your motherboard it could fry your whole motherboard.
-p.s. even this circuit is only safe to use just for boot loading/ programming a bare chip. If you have lots of other things hooked to the circuit like motors or servos that are powered you could fry your parallel port. Some people buy a PCI bus parallel card to be safe or there is a better programmer design that uses some protection diodes.
Thanks. I stayed on the safe side and built my parallel programer using the resistors. It works. That is good enough for me.
Your answer makes sense, but some of the output lines of the parallel port on that programmer don't use resistors. Besides, there is a resistor connected to one input line of the parallel port. Go figure!