'Main Program
...
Enable Interrupts                                           'Turn on global interrupt ability
Enable TIMER1
On TIMER1 PL_tone_ISR
Waitms 1000   '<-- Trigger timer1 overflow and ISR

...
Do
Loop
End


PL_tone_ISR:
   TIMER1 = 19000

   If PLToneWavePosition = 3 Then                           'Wave position varies from 0 to 3.  At the top of the wave = 3
      PLToneWaveDirection = Down                            '0 = a wave heading down, 1 = up
      PLToneWavePosition = 2
      WalshBit1 = 1
      WalshBit2 = 0
   ElseIf PLToneWavePosition = 2 Then
      If PLToneWaveDirection = Down Then                    'Moving from wave position 2 to 1
         PLToneWavePosition = 1
         WalshBit1 = 0
         WalshBit2 = 1
      Else                                                  'Moving from 2 to 3
         PLToneWavePosition = 3
         WalshBit1 = 1
         WalshBit2 = 1
      End If
   End If
Return  '<-- Program execution returns from here to the very top of the file, processes 1 line, then jumps right back to the top of the ISR