Hi James,

My own experience with PC parallel ports has been that they are relatively
tough, and you can communicate to and from a PIC pretty much one-to-one.

The real damage comes from ground voltage differences. If your PC is not
grounded (like if the third prong is missing from the equation somewhere)
then the PC will be 60 volts AC away from ground, and that is where there
can be some issues. People hooking their PC into their TV entertainment
system do sometimes blow up their sound cards.

However, since people get away with hooking printers and PIC programmers
up to their PCs, day in and day out, with hardly ever a problem, I'd
suggest doing the least possible. Like just hooking them together. Or
using a series resistor, and optional zener + diode.

How much this adds to the cost of your device should be a factor.

Of course, there is EMI compliance. Do you need to look at that? Because
filters and protection can be combined.

There are various standard-setting agencies each with multiple levels of
test limits and you could build to suit the one you choose.

Asking if something is protected enough is hard, because, like doctors and
lawyers, the "safe" answer is "not 100 percent safe".

Cheerful regards,

Bob

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