Design and Program Your First PAL Circuit: A Hands‑On Guide for Hobbyists

You might think programmable logic is only for big labs or corporate engineers, but the truth is that a simple PAL Programmable Array Logic (PAL) chip can bring a whole new level of flexibility to a home‑brew project. Whether you’re building a custom keypad decoder, a tiny state machine for a robot, or just want to learn how digital logic can be “written” like software, this guide will walk you through every step—from picking the right chip to getting it to blink an LED.

Why PALs Still Matter in 2024

Modern FPGAs get a lot of hype, but they are often overkill for a hobbyist who just needs a few logic functions. PALs are cheap, low‑power, and easy to program with a simple hardware description language. They sit nicely between fixed logic gates and the heavyweight world of CPLDs. Plus, soldering a PAL onto a breadboard feels oddly satisfying—there’s something rewarding about seeing a single chip replace a whole row of discrete gates.

What You’ll Need

The Chip

A classic choice is the PAL16V8. It offers 16 inputs, 8 outputs, and a small amount of programmable “product terms” that let you create most combinational logic you’ll need for a starter project. If you want a bit more room, the PAL22V10 adds extra inputs and outputs without a big price jump.

Programming Hardware

You’ll need a PAL programmer. The Xilinx Impact or the open‑source PAL‑Prog kits are popular among hobbyists. They connect to your PC via USB and accept a simple binary file that describes the logic you want.

Software

The original PAL programming language is called PALASM. It’s a plain‑text language that looks a lot like a mix of Verilog and a truth table. You write a .asm file, run it through a PALASM compiler (many are free), and the compiler spits out a .jed file that the programmer can load onto the chip.

Breadboard and Basic Parts

• Breadboard and jumper wires
• Power supply (5 V regulated)
• A few LEDs and resistors (220 Ω) for testing
• Optional: a push‑button or two for input signals

Step 1: Sketch the Logic on Paper

Before you type a single line of code, draw a quick truth table for the function you want. Let’s start with a simple 3‑to‑8 decoder that lights one of eight LEDs based on a three‑bit binary input. The truth table looks like this:

Writing this down helps you see exactly which inputs drive which outputs. It also makes the later PALASM code much easier to verify.

Step 2: Write the PALASM File

Open any text editor (Notepad works fine) and create a file called decoder.asm. Here’s a minimal version for the PAL16V8:

TITLE 3to8 Decoder
DEVICE PAL16V8

; Define inputs
A2  PIN 1
A1  PIN 2
A0  PIN 3

; Define outputs
Y0  PIN 4
Y1  PIN 5
Y2  PIN 6
Y3  PIN 7
Y4  PIN 8
Y5  PIN 9
Y6  PIN 10
Y7  PIN 11

; Logic equations
Y0 = !A2 & !A1 & !A0 ;
Y1 = !A2 & !A1 &  A0 ;
Y2 = !A2 &  A1 & !A0 ;
Y3 = !A2 &  A1 &  A0 ;
Y4 =  A2 & !A1 & !A0 ;
Y5 =  A2 & !A1 &  A0 ;
Y6 =  A2 &  A1 & !A0 ;
Y7 =  A2 &  A1 &  A0 ;
END

A few notes for the uninitiated:

• ! means NOT (logical inversion).
• & means AND.
• Each line ends with a semicolon; the compiler treats everything before the semicolon as the equation.
• The PIN numbers correspond to the physical pins on the PAL package; you can find a pin‑out diagram in the chip’s datasheet.

Save the file and run the PALASM compiler (often a command like palasm decoder.asm). It will generate decoder.jed, the binary file the programmer understands.

Step 3: Load the Design onto the PAL

Plug the PAL into the programmer’s socket, connect the USB cable, and launch the programming software. Choose the decoder.jed file, verify the device type matches (PAL16V8), and click Program. The software will give you a green check if everything went well. If you see an error, double‑check the pin numbers and make sure the chip is oriented correctly.

Step 4: Wire It Up on a Breadboard

Now the fun part—hardware! Here’s a quick wiring checklist:

1. Power – Connect VCC (pin 20) to 5 V and GND (pin 10) to ground.
2. Inputs – Tie pins 1‑3 to three toggle switches or a binary counter. Pull each input low with a 10 kΩ resistor so the line isn’t floating when the switch is open.
3. Outputs – Connect each output pin (4‑11) to an LED through a 220 Ω resistor, then to ground.

When you flip the switches, exactly one LED should light up, matching the binary value you set. If nothing happens, double‑check the power pins and make sure the LEDs are oriented the right way (long leg = positive).

Step 5: Test and Tweak

A good habit is to verify each output one at a time. Use a multimeter or just watch the LEDs. If an LED stays dim, you might have a loose wire or a bad resistor. If two LEDs light at once, the logic equation may have a typo—go back to the PALASM file and look for a missing ! or &.

Once you’re confident the decoder works, you can expand it. Add a simple enable input that forces all outputs low when not active, or cascade two PALs to make a 4‑to‑16 decoder. The PAL’s limited product terms mean you have to be clever, but that’s part of the learning curve and makes the design process rewarding.

Tips for a Smooth First Project

• Keep the design simple. PALs have a finite number of product terms; trying to cram a huge state machine into a PAL16V8 will quickly hit a limit.
• Label your wires. A small piece of masking tape with “A2” or “Y5” saves a lot of head‑scratching later.
• Use a breadboard power rail. It keeps the 5 V and ground lines tidy and reduces the chance of a short.
• Back up your .jed file. If you ever need to re‑program the chip (and you will), having the compiled file handy saves a trip back to the computer.

Where to Go Next

Now that you’ve mastered a basic decoder, the sky is the limit. Try building a simple traffic‑light controller that cycles through red, yellow, and green based on a timer. Or design a keypad scanner that turns a 4×4 matrix into a single 4‑bit code. The PAL’s strength lies in turning repetitive gate logic into a single programmable chip, freeing up board space and making your design look cleaner.

If you ever feel stuck, swing by the Logic Circuit Lab forum. I love reading about the quirky ways hobbyists repurpose PALs—someone once used a PAL to drive a small stepper motor for a model train, and it worked like a charm.

Happy hacking, and may your first PAL light up more than just an LED!