How exactly do PAL chips get programmed?

[Tony Duell]

> In most cased you could replace a PAL with a GAL, as long as the speed
> of the part was correct.

The main exception being the PALs with XOR terms (16X4, etc IIRC).

> 
> What's the part?  if it's something like a 16L8 or a 22V10 then you
> should have no problem.
> 
> It is possible to 'program' a pal such that it can't be read.  If your

For the record, this applies to GALs too. I _think_ you can still 'bulk 
erase' and re-use GAL that's been protected, but you can't read out the 
programming pattern.

-tony