 volatile int cycleNum = 666;
 volatile boolean mirrorTTL1 = true; // This is usually for varying systematics
 volatile boolean mirrorTTL2 = true; // Used for mirroring the Clock Bias field on/off
-volatile int mirrorTTL3source = 10; // 10 - source for beta shutter for polarized sequence
-                                    // 11 - source for beta shutter for unpolarized sequence
+volatile int mirrorTTL3source = pinTTL_IN3_pol; // 10 - source for beta shutter for polarized sequence
+                                                // 11 - source for beta shutter for unpolarized sequence
 
 void setup(){
   Serial_Init();

  pinMode(pinLCWaveplate, OUTPUT);
  pinMode(pinTTL_IN1, INPUT);
  pinMode(pinTTL_OUT1, OUTPUT);
  pinMode(pinClockTTL, OUTPUT);
  attachInterrupt(0, changeStartCOM, RISING);
  attachInterrupt(1, advanceCycleNum, RISING);

#define setPinTTL_OUT2_HIGH() PORTC |= 0b00000010;
#define setPinTTL_OUT2_LOW() PORTC &= 0b11111101;

#define pinTTL_IN3_pol 10
#define pinTTL_IN3_Upol 11
#define pinTTL_OUT3 12 //Source of mirrored TTL is set by mirrorTTL3source.
#define setPinTTL_OUT3_HIGH() PORTB |= 0b00010000;
#define setPinTTL_OUT3_LOW() PORTB &= 0b11101111;
