SPECS: A. Gaming modes 1. 5200 2. 7800 3. 2600 through 7800 motherboard B. Compatiblity 1. 5200 games -- fully compatible 4-port motherboard 2. 7800 games -- fully compatible 3. 2600 games -- fully compatible, has expansion switch and cart extender C. Supported Controller Jacks 1. Standard controllers: both systems 2. Genesis/2600 Joysticks: both systems 3. VCS Paddles: 5200 and 2600 mode 4. SMS Lightgun: 7800 5. External Keypad: 5200 and 7800 (start/pause/select only on 7800) D. Power Supply 1. Both systems share jack for genesis-1 power supply E. AV outputs 1. S-video: both consoles 2. Stereo: 7800, (5200 is mono only) 3. Both consoles use the same set of rca output jacks
This bad boy was the second toughest project for me after the portable. A 5200 and 7800 were combined together into essentially one console, and a lot of nice extras were added along the way. It broke down into four steps:
Most times when I do a video mod, I don't install the RF shield back in (if it's so important, why doesn't the intv2 have one?). This gives more room, which came in handy here. The first step was to cut a big hole in the 5200 top case to fit the bottom part of a 7800. Then I cut down the bottom part of the 7800 until the wider part could sit flush on the 5200. The frontmost part of the 5200 has no area available so the plastic trim was cut at a slant and the front of the 7800 protrudes. Once all the electrical work was done, the 5200 top and bottom case were screwed together like normal, then the 7800 bottom case was screwed on. The 7800 top half was then attached with velcro tape because there was no way to reach the normal screwholes.
This was fairly easy, just wired a genesis-1 power supply to the inputs of both systems. It's the user's responsibility to only turn on one system at a time. I switched to a genesis supply because the 5200 one had a fuse which would blow anytime I shorted it to ground (which I did in the course of the project). A genesis supply is rated for less current, but I never plan to plug a 2600 adapter into the 5200 so it should have enough guardband. The 5200 was a 4-port modified for a direct power jack.
The 5200 was modified for s-video (from the FAQ instructions). The 7800 was also coverted to s-video using the procedure at Resqsoft Archive It was also modified for stereo (but I'm never sure what to do with the external audio signal in a stereo mod). All AV signals go to 5v relays. When a console turns on, its 5v closes the relays so its signals get connected to the output RCA jacks, kinda an internal automatic game switch.
This was the hardest part of the project because I wanted it to have everything. Added extra jacks on the bottom front of the 5200 and ran all signals down to circuit boards hiding under the shelf. First, both systems have adapter circuits for genesis controllers (info in console FAQs). Then added in another jack for a 2600 paddle set. The paddles also work with the 5200 because they have a switch mounted in them to swap the potentiometer connection. Another jack was added to support the SMS light phaser on 7800 gun games.
Finally a 4x4 keypad module was added to duplicate a 5200 keypad. It also controls the 7800 pause,select, and reset signals. In addition the module has a four position switch to select which controllers you want (normal jack, joystick, paddle, or lightgun). Most of the signals are in parallel anyway, only the 5200 pot lines and 7800 paddle lines need isolation between different controllers. Of course any normal controllers like the 5200 trackball will still work in the usual jacks. Still need to use the dreaded 5200 controllers for SW Arcade, Gorf, and maybe some others (never checked into the sports games much).
If the above wasn't hard enough, I also needed to make protection circuits so when one console puts voltage on the controllers and keypad, it can't reach the other console which is off. For this I used 4016 analog switch ICs. They were powered by a common 4.9v supply made by wiring in both 5v console signals through 0.1v Schottky diodes (either console can provide the juice). There were some controller signals like the 7800 joystick lines where current only went one way and diodes were used instead of 4016s.
This would have been so cool, the plan was to join 10 systems together into a mega-console. The merged atari would be the bottom part, but it would also have an odyssey 2 board right above the 5200 board, and an intellivision motherboard packed in with the 7800. On the sides of the 5200 would be black plastic supports that held a colecovision about a foot above. The CV case would have the left half hollowed out and both a SNES and top-loader NES motherboard mounted inside. The top of the CV case would have a CDX on the left and a jaguar on the right (that could be flipped up with a hinge to reach the CV cart slot. Then a TG-16 would be connected to the bottom of the CV upside down. All systems (except maybe the CV) would be powered by a 5200 supply and also have common AV jacks and wireless SNES controllers. There would also be a keypad controller for systems that used it plus a composite lightgun (SMS gun for SMS and 7800 that had an extra barrel which used the NES zapper circuit, common trigger). The 5200 and CV are both the same width, with black bodies and metallic trim, it would have been gorgeous.
What happened? Well the Atari part was obviously finished, and the rest of the bottom half was getting there. I made sure all 4 boards would fit, and even had an 02 board in there and working with the joystick for awhile. Its cart connector was moved to the 5200 storage area with a ribbon cable. However the motherboard got zapped one day when adding keypad support. O2s are cheap, but it got me concerned about ruining the toploader or CDX. I was also realizing how many ICs all the controller adapters and protection circuits would take, way too many (keypad signals, ugh!). So that project was canned and got transformed into Uber-rack instead.