7800 Genesis Ctrl Adapters

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7800 genesis adapter

Final adapter and dsub9 breakout board

I've built the standard genesis adapter circuit from the 2600 FAQ a couple times over the years, but was always curious about it because I can see NPN circuits in my head but not PNP. So one day I connected a 7800 controller through a dsub9 breakout board to analyze the signals:

7800 Signal Voltages
Action p1p2p3p4p5p6p9
R But5. (0.1)4.9 (0.1)0.1
L But5. (0.1)4.6 (0.1)

The voltages above are for a 7800 game plugged in. When a voltage for 2600 games is different, it is noted in parenthesis. Pin 7 is a constant 5v, and pin 8 is ground of course. It seems that the 7800 controller buttons short either pin 5 or pin 9 (both normally low) to pin 6 (normally high). When the console is in 7800 mode, pin 6 is strongly pulled up and will win the fight. In 2600 mode, pin 6 is weakly pulled up and loses.

I tried to improve on the FAQ circuit, took a wrong turn, and wound up with three different versions as shown below. The Autofire Circuit page mentioned using a $7 Radio Shack dsub9 null modem adapter for a project case, so I tried that. Turns out those things are tough to open and it got a little mangled, then at the end I needed long bolts to hold it together. However any of the three circuits will fit if you use surface mount chips and resistors. 30 guage wire will also help. It might be better to stuff the circuit into a genesis controller and use it only for your 7800 since gen controllers are $5 or less these days.

And don't forget the game list at The Atari 7800 Page. It tells you what games use both buttons.


7800 port                     Genesis controller
---------                         -------
pin 1-----------------------------pin 1
pin 2-----------------------------pin 2
pin 3-----------------------------pin 3
pin 4-----------------------------pin 4
pin 8 (GND)-----------------------pin 8
pin 7 (+5V)------------------+----pin 5
                             +----pin 7

FIRST VERSION:  The FAQ circuit with a PQ2907 PNP transistor array

ADVANTAGES:  Easier to get the connections right than with 3906s.  The
double collector pins really help soldering, especially with surface mount. 
If you build an adapter for player1 and 2, one chip is nicer than four

DISADVANTAGES: PQ2907 chips are harder to find than 3906s which you can
get at Cellphone Shack, uhh I mean Radio Shack.  However,
You-got-questions-we-got-cellphones Shack could probably order you some.

SIDENOTE:  The NPN version of this chip might make the 2600 chroma/luma
mod easier, hmmm......... 

                620            +---+--+---+
7800 pin 8-+---/\/\/--------C1 |1  +--+ 16| E1--7800 pin 6   1k
           |  7800 pin 9----C1 |2   P   15| B1-------------/\/\/--Gen pin 6    
           |  7800 pin 5----C2 |3   Q   14| E2--7800 pin 6
           +---/\/\/--------C2 |4   2   13| B2-------------/\/\/--Gen pin 9
                620         C3 |5   9   12| E3               1k
                            C3 |6   0   11| B3
                            C4 |7   7   10| E4
                            C4 |8        9| B4

SECOND VERSION: Using a 4066 analog switch

ADVANTAGES:  None really, unless you have a cd4016 or cd4066 lying around 

DISADVANTAGES: Lot of wiring.  I was halfway through before I 
realized the genesis controller needs its signals inverted, DOH!
(can you see the poor man's inverters made with channel 3 and 4?) 

 2k pulldown + 7800 pin 5---1X |1  +--+ 14| VCC-----vcc
               7800 pin 6---1Y |2       13| 1EN-------------+    
               7800 pin 6---2Y |3       12| 4EN--gen pin 6  |
 2k pulldown + 7800 pin 9---2X |4  4016 11| 4X--------------+-- and 2k vcc pullup
             4066 pin 8----2EN |5  4066 10| 4Y---gnd
              gen pin 9----3EN |6        9| 3Y---gnd
                     gnd---GND |7        8| 3X---to 4066 pin5 and 2k vcc pullup

THIRD VERSION: Using a 7404 inverter

ADVANTAGES:  Simple, almost foolproof 

DISADVANTAGES:  Will not work with 2600 games.  I even published 
this circuit before I realized it, double DOH!  However, if someone
wants to build this because it's so easy, they can always plug a
genny controller in directly for 2600 games (B is fire of course).

            +---------------------pin 9                             
pin 5----------+
            |  |
        +5V |  |
       |14 13 12 11 10  9  8 |
       |>        7404        |
         |  |              |
         |  |             GND
pin 9-------+
         +------------------------pin 6                             

Ratty looking at screen

Ratty and his lawyer Mr Mole look at the fine print for the first adapter
Mr Mole: "The guy wrote that if you make adaptors to sell, he wants a cut"
Ratty: "Get me a deal like that, but make them pay in cheese"

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