Problems with EEB configuration after adding Servo

When I added a servo to my EEB configuration, the servo goes crazy when the layer fans come on.

Modded A8 with RAMPS 1.4 and Marlin, with the following fan/servo pins configured:
FAN_PIN 5
E0_AUTO_FAN_PIN 4
E1_AUTO_FAN_PIN 4
SERVO0_PIN 11

I made a dual extruder setup, changed Marlin to EEB and used a fan extender board in the servo header to run the hotend and layer fans. I added a servo for the Z-probe (had to shift the extender board by 2 pins so it uses D4,D5 but that worked fine) and the servo deploys and retracts perfectly.

Problem comes when I hit layer 4 and the fan comes on... the servo goes mental and after a few seconds the computer loses comms with the printer. It then appears on another ttyACMx port.

I tried the following without success:
1. Moving the servo from D11 to D6 and changing the pin assignment in pins_RAMPS.h
2. Removing the Vcc/5v jumper and powering the servo connector directly from 5v (vie the remaining GND/5v pins in the servo header block)
3. Replacing the USB cable

Now I'm wondering whether Marling uses a different PWM mode for servos and for fans, and if so whether there's a workaround.

Also somewhat confused as to why the USB port would be affect, and why it then directly reappears as a new com port.

Any suggestions greatly appreciated.

Thanks,
Phil

Edited 1 time(s). Last edit at 02/01/2019 11:17AM by davies46.

I can now answer my own question - it might help someone

I finally got to swapping the two fans on the extender board (the hotend fans with the layer fans) and the corresponding pin definitions in Marlin. I tried this because the layer fans are PWM whilst the hotend fans aren't.

Seems to have totally cured it. Perhaps it was something to do with PWM modes having to be the same over groups of pins.

Have to say, the servo/microswitch beats the inductive sensor setup hands down for accuracy and repeatability!

-Phil