The following information can be found on the internet when looking for information about SEGA Genesis controllers:
Date: Tue, 2 Mar 93 08:10:28 CST
From: (Neal Patrick Howland)
Subject: Genesis joystick pinouts
Yes folks!  It is the much requested Genesis joystick pinout information!!!
<much applause>
First some background info:  The chip inside the controller is a 74HC157.
This is a high-speed cmos quad 2-line to 1-line multiplexer.  Basically how
this works is there are two inputs ( A and B ) for every output ( Y ).  There
are four groups like this.  There is one select signal for the whole chip.
When the select signal is low, the output ( Y ) is the same as input A.  When
the select signal is high, the output Y is the same as input B.  The pinout
for the chip is as follows:
               Pin 1 - Select           Pin 16 - Vcc (+5V)
               Pin 2 - 1A Pin 15 - G (? must be low)
               Pin 3 - 1B Pin 14 - 4A
               Pin 4 - 1Y Pin 13 - 4B
               Pin 5 - 2A Pin 12 - 4Y
               Pin 6 - 2B Pin 11 - 3A
               Pin 7 - 2Y Pin 10 - 3B
               Pin 8 - Gnd              Pin 9  - 3Y
All the controls are done with switches.  Up is a switch, Down is a switch,
etc. Now, I will be referring to the output of these switches later on.  What
I mean is that the output is usually high, that is when the switch isn't
When the button is pushed, the output goes low.  This is accomplished by 
connecting the output to +5V through a 10k resistor.  The button is then
attached between the output and ground.  It looks like this:
     +5V -----/\/\/------+--------- Output
               10k       |
                /        |
   Ground -----/  -------+
              (normally open)
For all of those who could actually decipher the above schematic,
I will now run down what lines from the plug are connected to what.
The line numbers are determined as follows, looking straight at the plug on
the front of the Genesis the numbers are:
               1 2 3 4 5
                6 7 8 9
  (For those of you who buy a joystick cable from radio shack the pin #'s to
   wire colors are as follows: 1-black 2-brown 3-red 4-orange 5-get cup from
sorry about that, lets start again  1-white 2-blue 3-green 4-brown 5-yellow
6-orange 7-red 8-black 9-gray,  )
anyway, line connections:
Line 1 - Up output
Line 2 - Down output    These are the only two direct connections
Line 3 - Pin 4 of the chip    output 1Y
Line 4 - Pin 7 of the chip    output 2Y
Line 5 - This line carries in +5V.  It is connected to the +5V bus line.
Line 6 - Pin 9 of the chip    output 3Y
line 7 - Pin 1 of the chip  this carries in a select signal from the Genesis.
                             This is a signal which varies rapidly and
                             controls which input goes through the output
Line 8 - Ground  This is connected to the Ground bus line.
Line 9 - Pin 12 of the chip   output 4Y
Now for the chips pin connections:
Pin 1 - Line 7 (select)
Pin 2 - Ground (1A)    don't ask me why they do this.  Maybe future expansion
Pin 3 - Left   (1B)
Pin 4 - Line 3 (1Y)
Pin 5 - Ground (2A)    again, possibly future expansion
Pin 6 - Right (2B)
Pin 7 - Line 4 (2Y)
Pin 8 - Ground  (GND)
Pin 9 - Line 6 (3Y)
Pin 10 - Button B  (3B)
Pin 11 - Button A  (3A)
Pin 12 - Line 9  (4Y)
Pin 13 - Button C  (4B)
Pin 14 - Start  (4A)
Pin 15 - Ground  (G)   This must be connected to ground
Pin 16 - +5V  (Vcc)   Power source for the chip