motherboard differences

[eegad]

2 questions...

 

- I was just reading another post (http://www.atariage.com/forums/topic/170523-when-was-my-vcs-produced/) and was wondering about the photo of the heavy sixer motherboard. There's 3 main chips on the board, plus an "empty" chip space between the lower 2 chips. Was it ever discovered what that space was designed into the board for? Possibly extra RAM as a future upgrade - something like that?

 

- I've seen photos here and there at different times of the various 2600 motherboards/internals. Is there a collection all in one place that shows the full range of evolution of the 2600 boards? (ie-all different boards posted together in one topic or something like that)

[A.J. Franzman]

The unpopulated IC footprint was intended for a built-in 2K-sized game (such as Combat or Target Fun/Air-Sea Battle). However, the board layout is missing logic to disable the onboard ROM -- if you simply install a game chip there with no other changes, you will not be able to play any other games! There are not even any traces remaining of where any switching components would have been, so it would require a bit of electronics knowledge to convert the board to have a built-in game and still be able to play cartridges. Also, with a small additional modification, the built-in location could accept a 4K ROM instead of 2K. Adding an inverter would allow use of a programmed EPROM chip instead of an original masked ROM.

[alex_79]

Atari mask roms have 2 active-high chip-select pins (3 if the game is 2k), so the switching logic should only need a pullup resistor. The 1 chip junior also have an empty IC footprint for a built in game and there's a schematic on digitpress site that shows all the connections (Atari 2100 "JAN" single chip).

One of the CS pins is connected to cart port pin 24 and pulled high, while the other is connected to A12. When a cart is inserted it connects pin 24 to ground, so the internal rom is deselected. Eproms usually only have one active-low chip-select pin, so you need extra logic in that case (I posted a schematic of a clone console with built in games here).

 

Since a few carts do not have pin 24 and pin 12 connected togheter (harmony cart and a few activision games, for examaple), you should also add a switch between pin 24 of the cart port and ground so you can manually disable the internal rom, if needed.

Edited October 13, 2010 by alex_79