7800_Eprom_cart.pdf

Creating a 32K Atari 7800 Custom Cartridge

Jeff Wierer

Introduction

With the recent availability of homes brews (Beef Drop, Frogger, and Q-bert) and hacks

(Pac-Man, Miss Pac Attack, and Hangly-man) for the Atari 7800 enthusiast are searching

for way to enjoy these on a real 7800 console. Sure the Cuttle Cart provides an easy

way to enjoy these games but, if you’re searching for an alternative or simply enjoy

having an individual cartridge to add to your collection there is a less expensive way!

Unfortunately, at the time of writing this, nobody has made a general purpose PCB or

cartridge shell available for the home-brewer to create your own cartridges. Then again

if there were we wouldn’t need this document… Unfortunately this leaves the only option

of cannibalizing a common 7800 32K cartridge. Thankfully there is an abundance of

these commons that can be picked off, even on eBay, for a relatively low cost.

Requirements

Before beginning make sure you have access to the following:

(1) 27256 or 27c256 32K EPROM

(1) 32K Atari 7800 Cartridge*

(1) 74LS04 (hex inverter)

EPROM Programmer (I use a Pocket Programmer 2 –

http://www.xtronics.com)

Solder Gun – if you’re not proficient, practice a bit before attempting

this.

Solder

Wire

Some double-sided tape (optional)

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Creating a 32K Atari 7800 Custom Cartridge

(1) 32-pin socket (optional)

* The type 1 & type 2 cartridges (see appendix for incomplete list) seem to work fine. In

this document we’ll look using at Food Fight. With minor modifications I’ve also used this

for Pole Position II.

Component Pin-Outs

Through the course of this document I’ll reference pins on the boards edge connector,

the hex inverter, and the ERPOM. Use the following as guides to better understand

which locations are being affected.

Read/Write from 6502, low = Write

Phase 2 clock from 6502

Halt to 6502

IRQ to 6502

D3 to/from 6502

Ground

D4 to/from 6502

D2 from 6502

D5 to/from 6502

D1 from 6502

D6 to/from 6502

D0 from 6502

D7 to/from 6502

A0 from 6502

A12 from 6502

A1 from 6502

A10 from 6502

A2 from 6502

10 A11 from 6502

A3 from 6502

11 A9 from 6502

A4 from 6502

12 A8 from 6502

A5 from 6502

13 +5 VDC

A6 from 6502

14 Ground

A7 from 6502

15 A13 from 6502

External Audio to system

16 A14 from 6502

A15 from 6502

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Creating a 32K Atari 7800 Custom Cartridge

27256 EPROM Pin-Out

LS7404 Hex Inverter Pin-out

Step-by-Step Instructions

Step 1:

A. Grab your least favorite 32K cartridge game (or your unwanted duplicate) and donate it to the process. In

this example we’ll look at using Food fight. A game I have a couple copies of.

B. Poke a hole in the center of the cartridge most, but not all, will have a screw that needs to be removed to

open the cart.

C. Gently flex and squeeze the cart from the sides to separate the two halves.

Step 2:

A. Remove the PCB (printed circuit board) from the cartridge.

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Creating a 32K Atari 7800 Custom Cartridge

B. Grab soldering gun and remove solder from each of the legs of the prom. Note: If you don’t care about the

prom you can cut the legs and quickly remove chip. I prefer to keep the chips to validate the board later in the

process

C. Gently remove the prom from the board. Do not force it as you may accidentally pull a trace or lift a solder

joint. Once removed it should look like the image to the right.

Step 3:

A. Remove any access solder from any of the holes. Once complete the backside should look like the image

to the right.

Step 4: (optional)

I recommend using a socket because it will allow you to easily remove the EPROM if it needs to be

reprogrammed with a newer release. 7800 cartridges generally have enough room for the socket and an

EPROM (unlike 5200 cartridges) so it does not prevent you from putting it back in the cartridge case it was

removed from in Step 1.

A. Take a 32-pin socket and solder it to the front of the board. Make sure you orientate the notch on the

socket properly. If inserted correctly the notch should be on the right side of the front of the board as in the

image to the right.

Step 5: (optional)

At this point we’ve only modified the board to contain a

socket and nothing else. To make sure we haven’t lifted a

trace you can insert the original PROM and test in your

Atari.

A. Re-insert the original prom into the socket on the PCB

board. If you forgot which way it goes just follow the notch

on the chip. It should be orientated just like your socket as

in the image to the right.

Step 6:

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Creating a 32K Atari 7800 Custom Cartridge

This is where we start to make the one major

modification to the board.

A. You will need to break the trace running

from pin 17 on the edge connector to OE (pin

22 on a 27256 EPROM). See the red line in

the image to the right.

B. Use a continuity tester to confirm that you

have broken the trace and it is now an open

circuit.

Step 7:

A. Using some double-sided tape, apply the tape to the back of your LS7404 (hex inverter) and adhere it to the

back of the PCB. The notch should be facing to the left (as seen in the next image).

B. Using a short wire, connect the ground pin on the hex inverter to ground of the board. In the picture this is

top right pin of the hex inverter running to trace that connects to pin 30 on the edge connector of the PCB.

C. Bend pin 4Y (this is an output pin) of the hex inverter and connect it to OE going to the EPROM

D. Bend pin VCC of the hex inverter and connect it to VCC of the EPROM

E. Scratch some of the protective film over the trace running to pin 17 of the PCB edge connector. In the image

this is the left most trace. We’ll need to do this so we can solder a wire to that trace.

F. Solder a wire to trace we just exposed in step 7E. and run it to pin 4A in the hex inverter

If all was done correctly, your board should look similar to the board below:

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