After spending quite some time looking for a way to get a second part cooling fan running off the PCB for the second extruder on my dual extruder Rigidbot without success I decided to take the matter into my own hands. So, here are the instructions for the modifications required to the firmware to run dual part cooling fans in sync, one off each extruder PCB.
DISCLAIMER: Slicer software (GCODE) only allows for software control of one part cooling fan. Therefore, this solution does not provide for independent software control of two part cooling fans, rather it runs the two fans in sync meaning whatever is happening at one is happening at the other. However, this means that rather than just having one fan ducted across two extruder heads you can have one fan ducted directly to each extruder.
You just add the three lines highlighted red below to the planner.cpp tab in the Marlin firmware file and re-flash the mobo.
#if defined(FAN_PIN) && FAN_PIN > -1
#ifndef FAN_SOFT_PWM
#ifdef FAN_KICKSTART_TIME
static unsigned long fan_kick_end;
if (tail_fan_speed) {
if (fan_kick_end == 0) {
// Just starting up fan - run at full power.
fan_kick_end = millis() + FAN_KICKSTART_TIME;
tail_fan_speed = 255;
} else if (fan_kick_end > millis())
// Fan still spinning up.
tail_fan_speed = 255;
} else {
fan_kick_end = 0;
}
#endif//FAN_KICKSTART_TIME
analogWrite(FAN_PIN,tail_fan_speed);
// Engage E0-FAN on pin 11 //
pinMode(11, OUTPUT);
analogWrite(11, tail_fan_speed);
#endif//!FAN_SOFT_PWM
#endif//FAN_PIN > -1
I've also created a printable dual fan mount bracket and fan outlet ducts which you can download from here http://www.thingiverse.com/thing:736563
I hope this is of use. If so, enjoy.