Dumping the PALs

A project log for Reverse Engineering The Weather STAR 4000

The Weather STAR 4000: A Journey of reverse engineering the hardware to an iconic machine of the 1980s/90s.

techknighttechknight 03/10/2021 at 01:490 Comments

So the next battle in the journey is figuring out how to dump the PALs. So I did alot of googling and research. 

With combinational-only PALs, you can easily dump them like a ROM. but you have to know the inputs/vs/outputs. Luckily by looking at the schematic, its pretty straight forward to do so. 

But just dumping them as PROMs are only half the battle. You still have to figure out some way to reverse it back into logic. 

Every little PAL project I ran into, had its own way and method of reading chips. Plus, I didn't have any of the hardware that was necessary to dump a PAL to run through one of these programs. 

So, what I did was take a blend. of my own tools, and online tools. 

First thing is first though, We need to dump the PALs like a ROM. So, I used an Arduino Nano on a breadboard, I did not have a method to dump it with an external programmer. So I improvised. 

Like So:

Ok, That part is done. But now, I have to write a program. Now, dont laugh! I really at this point did not know how to write code in C just yet. But I have written code in VB/BASIC all my life, and thats how I used these little microcontrollers. 

Working with this machine actually taught me C. Surprisingly. 

Anyways, Time to write a program! 

Simple! well not really... i had to take some time to think, and then calculate how many iterations to read based on the number of inputs. i literally did this the hard way due to the reduced number of I/O on the Nano. I would read a block of bytes, then change the upper bits. i had 2 to 4 bits that the MCU didnt control. so i effectively made 16 dumps with 4 bits. Yeah, i could have used an arduino Mega and dumped the whole thing in one wack. 

But, Lazy..... 

Then you end up with the dump, something like this: 

That part is over. I dumped the rest of the chips and then stored the dumps for further processing. 

The fun part is yet to come. Deciphering the PAL logic!