DISCLAIMER: I'd like to take full credit for this but must admit it was ripped off from Kevin Horton's work on Bankzilla. Full details here: Bankzilla
This page, by Curt Vendel of Atari Inc, is your starting point and I think the instructions are quite good: Atarimuseum Try your cartridge connector without this circuit first. Make sure the FB2 works in both modes. There's a lot that can go wrong along the way.
I had some fun poking around the FB2 board. Found some things out and hit some issues.
* The power supply reads about 13.6v with system off, 12.3v on.
* The 5v regulator is always connected. The power switch is actually on the 5v line.
* No heat sink for the 5v regulator. ICC readings were 32ma for built-in Adventure and 67 ma for the same game on a cart (versus 274 ma on a four switcher).
* The joystick connectors have a jumper/resistor pair so the 5v line can be connected instead of 3.3v. Guess it was for paddle or trackball experiments.
* The 5v line gets dropped down. I didn't find the component that caused it, but the 5v from the regulator reads about 4.24v on a cartridge. I think that it was a deliberate design decision to keep the cart from overdriving the 3.3v FB chips. Seems kinda low doesn't it?
* My approach was to mount the cart connector board to the bottom case with standoffs. This made it tricky because the FB2 board mounts to the top case. Screwing to the top or using hot glue are other approaches.
* The two difficulty switches really need LEDS. I might try changing them to slide switches on the back (which would have been a better design IMHO) and mounting a cart slot there instead.
* A jack to use the longer original 2600 power supply and longer AV cables would be good improvements too.
* When checking for shorts pins 12 and 24 will be shorted together because they are both ground (if you connect pin 24).
* The instruction "the other side of C4" is confusing because the black wire isn't connected to C4 to begin with. But it is connected to something shorted to C4, just trust your ohmmeter.
* The final results were about 3/4 of the 2600 carts I tried worked. I think there is a problem with my long wires (maybe 6 inches) or that 4.2v vcc, or the combination of the two. I plan to experiment more. I know the blue wires are super long, but those are for the autodectect which is only DC (I think).
PARTS LIST: (1) CD4016 or CD4066 CMOS Transmission Gates (2) 2k resistors CIRCUIT: 5v ________|________ | VCC | 3X -|1EN 1X|-- C4 (not ground side) | 1Y|-- 2Y, black wire from U2 | | ,-------|2EN 2X|-- gnd Cart p24___| | 2Y|-- 1Y, black wire from U2 | | 4016 | `-------|3EN 3X|-- 1EN,4EN, 2k pullup to 5v | 3Y|-- gnd | | 3X -|4EN 4X|-- to J3 (not gnd side) | 4Y|-- gnd | GND | |_________________| | gnd Give cart connector p24 a pullup resistor to 5v, I used 2 kohms for both pullups. Do NOT connect cart connector p24 to FB2 Cart connector p23 is 5v, p12 is ground for easy access to those signals. Here is pin order when looking down on cart end label: 24 23 22 21 20 19 18 17 16 15 14 13 ,------------------------------------, | | | Raider of the Lost Ark | | | `------------------------------------' 01 02 03 04 05 06 07 08 09 10 11 12
|Channel||Rom Mode||Cart Mode|
HOW IT WORKS:
The whole key to this is that a 2600 cart connector has two ground pins, but the system will work with just one, a cart connects both pins together anyway. Pin 24 is disconnected (or never connected) to the FB2 and is given a 5v pullup instead. With no cart in, this stays at 5v but plugging in a cart grounds it.
For the no-cart case, channel 2 is closed. This connects the black U2 wire to ground which enables the internal game rom. Also channel 3 is closed which opens channel 1 and 4. Channel 3 is just a poor man's inverter, easier than adding a 7404.
With a cartridge inserted, Pin 24 is grounded which opens channel 2 and 3. Channel 3 open makes channel 1 and 4 close. Channel 4 takes care of J3 and channel 1 connects the black wire path. Your cartridge should now be happy.
There is a couple other ways to try implementing all this. A DPDT relay might be able to do it with just one component. I didn't try it because a relay takes milliseconds to close, not nanoseconds like CMOS gates. Besides the relay would draw more current then the FB2 :) Level switches in the cart connector might have worked too but physical movement devices are always a pain to get aligned just right. Send an email if you try something though.
Quad analog switch pinout. +---+--+---+ 1X |1 +--+ 14| VCC 1Y |2 13| 1EN 2Y |3 12| 4EN 2X |4 4016 11| 4X 2EN |5 4066 10| 4Y 3EN |6 9| 3Y GND |7 8| 3X +----------+