How exactly do PAL chips get programmed?

[Tony Duell]

> 
> As some of you might recall I was looking for drivers for my Tokamac II FX
> PDS card on my mac IIfx. Well I found the drivers from the original company
> founder. I read in Macworld (and had it confirmed from the company contact)
> that you need a PAL chip on the IIfx reprogrammed (or replaced?) because
> Apple screwed up BUS mastering on the IIfx and nobody caught on at the time
> because those types of cards were rare.
> 
> Well I have found another person who has the same card AND the PAL chip
> needed on his unit but is having other problems getting his setup to work.
> 
> What I want to know is are PAL chips somewhat like eprons in getting
> programmed or is there some other process involved? Does anybody know of a

Well, physically, a true PAL (as opposed to a GAL or similar) is 
programmed like a fusible link PROM. There are little 'fuses' on the chip 
that you burn out when you program it.

However, there is one important difference between a programmable logic 
device (PAL, GAL, CPLD, etc) and a memory (PROM. EPROM, Flash, etc). And 
that is that when you read a memory, you are effectively reading out the 
programmed pattern directly. When you use a logic device you are not

Therefore, PALs (etc) have a 'programming mode'. You get to it by, 
normally, 'overvoltaging' one of the pins. In this mode, the other pins 
on the chip select a particular fuse, and enable you to program or read 
it. The exact method varies from device to device and manufacturer to 
manufacturer, and often was not available without an NDA (although the 
methods for the first PALs were published in the data sheets). 

This, in general is the only way to read out the contects -- the 
programming -- of the chip. In nromal mode, you can attempt to solve it 
as an unknown logic circuit, but rememebr there might be feedback terms 
or internal D-types (depending on the PAL) which will give sequential as 
opposed to combinatiorial behaviour.

And there is some bad news. There is a 'security fuse' in most, if not 
all, PALs. If that has been programmed, there is no (official) way to 
read out the fusr map. You can only use the device in normal, not 
programming, mode. It's a sort of copy-protection. 

If you know the device type and manufactuer and have a programmer that 
supposts that _exactly_, it won't hurt to try to read it out. But don't 
be suprised if you get a file of all 0's or all 1's.

-tony