Log 3: Reading The EEPROM - An Atari's Perspective

A project log for SYNDRUM: An Atari 2600 Drum Machine

Yeah, so I wrote a little program and built a hardware interface that turned my Atari VCS into a MIDI-capable drum machine

john-sutleyJohn Sutley 09/27/2018 at 21:170 Comments

My code is now on a chip that was made in 2017! Great, now how do I get it to talk to a machine that was designed in the mid-70's?

No. Just stop.

After numerous attempts at trying to cram the EEPROM into the cartridge slot, I've gone for the meticulous route of designing a custom circuit and reproducing it using proto-board and cannibalized Atari cartridges.

Not wanting to go through the trouble of having to design and pay for a custom pcb, I used an old Combat cartridge pcb to handle the "plugging in to the Atari part", a zif socket to hold the EEPROM and a bunch of wires from an old LPT printer cable.

Here's what I was thinking electrical wise:

I marked the key differences between the two chips [left] and used that as the basis of the schematic [right]

The cartridge port acts as a pretty straight-forward bus for the Atari's Data lines (pink dots in the schematic) and Address lines (green dots). Pin 6 on the top row acts as the chip select, which had to be inverted since the Atari's original mask roms have the CS line active high rather than low like in the EEPROM. All other address lines on the EEPROM are pulled low.

And when I finally went to assemble everything according to the schematic, I goofed.

The Combat rom was only 2k.

SYNDRUM is 4k.

There was no A11 address line (and subsequently, no A11 pin) on the Combat cartridge. Instead of panicking and instinctively hurling a half-finished circuit board at the nearest brick wall, I instead super-glued a bit of metal where an A11 address line should have been and soldered it to it's respective bit of wire.

The purple wire that goes through that hole SHOULD connect to that A11 pin. So much for consistency between pics.

The result is something that's definitely not pretty to look at and works only if you have your tongue at the right angle, but it DOES work!

One step closer to taking over the world.