The PET was not initially designed for joysticks but a determined user had
  devised a dual-joystick interface employing the parallel port.  This interface
  allows you to plug in standard Atari/Commodore compatible joysticks....

  Interface Diagram  (use mono-spaced font to view properly)

  Looking at back of user port         1  1  1
            1  2  3  4  5  6  7  8  9  0  1  2
            =  =  =  =  =  =  =  =  =  =  =  =
           ### ######################### ######
            =  =  =  =  =  =  =  =  =  =  =  =
            A  B  C  D  E  F  H  J  K  L  M  N
            !     !  !  !  !  !  !  !  !     !
            !     !  !  !  !  !  !  !  !     !
  port 1->  8     1  2  3  4  !  !  !  !     !
                              !  !  !  !     !
                   port 2->   1  2  3  4     8


  Joystick pin diagram =>  5  [4] [3] [2] [1]
  looking at back of                    \ /
  male connector             9  [8]  7  [6]

  The 'fire' button (pin 6) is added by connecting joystick port pins 1 & 2
  to pin 6 with diodes, solder each diode from (already wired) pins 1 & 2