Monday March 27, 2017

Atari 2600 Working With 27C512 Eprom
  Posted by: AceBHound on Aug 30th, 2010 6:57 PM
I spent too long last night trying to get the 27c512 Eprom working with the Atari 2600. I forgot from when i was messing with Atari Eprom Carts about 8 years ago that most of these eproms need a 7404 hex inverter IC to change the high signal for the CE (chip enable) to a low signal. I had bought a whole bunch years ago but think I gave them away when I sold my 2732 eproms. If you look at the datasheet and see a line above the CE pin, that means it's inverted (ie. enabled when signal is low / 0 volts). The Atari cartridge PROMs had their CE high (exact opposite of what the eprom is expecting). So the hex inverter does just as its name implies and "inverts" the signal. High is low, low is high.

I spent a while thinking "how is it I NEVER have the parts I need no matter how many parts I buy". Then finally tracked down a 7404 in an old Rock-O-La logic board (don't ask). Hooked the input from the cart (CE high) to the input on the inverter, and the output of the inverter to pin 20 of the 27c512 on the breadboard. Burnt a game to the 27c512 and voila!! ..garbled junk. Didn't expect it to be easy :) Tried burning a 4k eprom too just in case (only reason I burnt 8k was that apparently the Atari 2600 Asteroids is one of the first games they used bank switching on to allow the system to access more data so figured start with the original game).

Played around with it some more this evening and saw that in grounding Vpp (/OE) on the chip I had also grounded the Vcc pin on the Atari cart too since pin 24 & 21 are tied together via a trace. Essentially created a short there. The OE on the eprom itself needs to be grounded since it requires a low signal (data sheet shows line above OE for that pin). Still didn't work. Finally decided to switch to a different set of pins on the 7404 hex inverter chip as sortof a last-ditch effort and voila! SUCCESS! Very snowy picture because of the length of the wires, but a 4k game is now running on a 27c512 eprom! I did try the 8k Asteroids eprom again and it displayed clearer garble, but something is still wrong with it. I'm happy getting a 4k game going at this time. When the DIP switches come in I'll attempt putting a few 4k games on a single 27c512.

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