fix for Marlin/issues/23926

[aegelsky]

Requirements

Proposed fix for this issue:
MarlinFirmware#23926

Description

just repeting what I wrote in that issue:

when using in Configuration.h (attached below):

#define MOTHERBOARD BOARD_MKS_GEN_L
#define EXTRUDERS 2

it uses pins_MKS_GEN_L.h, which in turn uses pins_RAMPS.h,
where FET_ORDER_EEB is selected (which is right), but pins in RAMPS and MKS GEN L don't match in second extruder:

#elif FET_ORDER_EEB // Hotend, Hotend, Bed
#define HEATER_1_PIN MOSFET_B_PIN
#define HEATER_BED_PIN MOSFET_C_PIN

#elif FET_ORDER_EEB // Hotend, Hotend, Bed
#define FAN_PIN 4 // IO pin. Buffer needed

(from pins_RAMPS.h)

this leads to:
HEATER_1_PIN = 9
FAN_PIN = 4

actual MKS GEN L pins map:
https://github.com/makerbase-mks/MKS-GEN_L/blob/master/hardware/MKS%20Gen_L%20V1.0_008/MKS%20Gen_L%20V1.0_008%20PIN.pdf

pin D9 is fan pin and D4 is servo pin, so when enabling second extruder heater, fan turns ON and there is no way to power on fan.
(also I wouldn't power up extruder heater from fan socket, it very possibly might no handle 40W output)

adding this into pins_MKS_GEN_L.h (from line 40) fixes this issue:

// Hotend, Hotend, Bed + Fan on D9
#if FET_ORDER_EEB
#define MOSFET_B_PIN 7
#define FAN_PIN 9
#endif

Benefits

it sets proper pins for second extruder and fan when using two heaters, bed and fan

Configurations

Configuration.zip

Related Issues

MarlinFirmware#23926