This page describes the result of measurements, which pin is connected to where.

There is online not much information available about the ChiTu F X board. The following pdf is what I collected from a search through the internet:

• https://www.auditeon.com/xyz/Chitu_F_X.pdf
• https://de.aliexpress.com/i/4000935256869.html
• https://chitusystems.com/product/esp-01-wifi-module/ wifi module
• https://en.aliradar.com/item/4000935256869-3D-Printer-Motherboard-ChiTu-F-X-With-STM32-High-Stability

List of components:

• STM32F446: Microcontroller, https://www.st.com/resource/en/datasheet/stm32f446mc.pdf
• TMC2209: Stepper driver, https://www.trinamic.com/fileadmin/assets/Products/ICs_Documents/TMC2209_Datasheet_V103.pdf
• WSF40N06 N-Ch. Mosfet, https://datasheet.lcsc.com/lcsc/1809291408_Winsok-Semicon-WSF40N06_C86291.pdf
• 3.5“ TFT display module. ILI9341 based? If so, a similar pinout given here: https://www.raystar-optronics.com/3.2-3.5-3.9-tft-lcd-display/RFC350Y-AIW-DNS.html
  • XPT2046 Touchscreen controller: https://www.dzsc.com/uploadfile/company/785710/20111014212155973.pdf
  • (Original: TSC2046E-Q1 Touchscreen controller: https://www.ti.com/lit/ds/symlink/tsc2046e-q1.pdf)

Connection diagram
ChiTu F X series
V4.3
191205-FX

U8
 IC:STM32F446ZET6, LQFP 144 pin
  54 PF14 -> U6_E1,P.2 (E1-EN)
  47 PB1 -> U6_E1,P.16 (E1-STP)
  53 PF13 -> U6_E1,P.19 (E1-DIR)
  89 PG4 -> U6_E2,P.2 (E2-EN)
  82 PD13 -> U6_E2,P.16 (E2-STP)
  90 PG5 -> U6_E2,P.19 (E2-DIR)
  10 PF0 -> U6_X,P.2 (X-EN)
  04 PE5 -> U6_X,P.16 (X-STP)
  11 PF1 -> U6_X,P.19 (X-DIR)
  15 PF5 -> U6_Y,P.2 (Y-EN)
  21 PF9 -> U6_Y,P.16 (Y-STP)
  13 PF3 -> U6_Y,P.19 (Y-DIR)
  41 PA5 -> U6_Z,P.2 (Z-EN)
  42 PA6 -> U6_Z,P.16 (Z-STP)
  55 PF15 -> U6_Z,P.19 (Z-DIR)
  93 PG8 -> Q5,P.G (E1_HOT)
 110 PA15 -> Q20,P.G (E2_HOT)

U6_E1
 IC:TMC2209-LA
  01 OB2 (E1-MOTOR,P.2B)
  02 ENN (U8,P.54/PF14)
  03 GND
  04 CPO
  05 CPI
  06 VCP
  07 SPREAD
  08 5VOUT
  09 MS1_AD0
  10 MS2_AD1
  11 DIAG
  12 INDEX
  13 CLK
  14 PDN_UART
  15 VCC_IO
  16 STEP (U8,P.47/PB1)
  17 VREF
  18 GND
  19 DIR (U8,P.53/PF13)
  20 STDBY
  21 OA2 (E1-MOTOR,P.1B)
  22 VS
  23 BRA
  24 OA1 (E1-MOTOR,P.1A)
  25 .
  26 OB1 (E1-MOTOR,P.2A)
  27 BRB
  28 VS

U6_E2
 IC:TMC2209-LA
  01 OB2 (E2/EXT-MOTOR,P.2B)
  02 ENN (U8,P.89/PG4)
  03 GND
  04 CPO
  05 CPI
  06 VCP
  07 SPREAD
  08 5VOUT
  09 MS1_AD0
  10 MS2_AD1
  11 DIAG
  12 INDEX
  13 CLK
  14 PDN_UART
  15 VCC_IO
  16 STEP (U8,P.82/PD13)
  17 VREF
  18 GND
  19 DIR (U8,P.90/PG5)
  20 STDBY
  21 OA2 (E2/EXT-MOTOR,P.1B)
  22 VS
  23 BRA
  24 OA1 (E2/EXT-MOTOR,P.1A)
  25 .
  26 OB1 (E2/EXT-MOTOR,P.2A)
  27 BRB
  28 VS

U6_X
 IC:TMC2209-LA
  01 OB2 (X-MOTOR,P.2B)
  02 ENN (U8,P.10/PF0)
  03 GND
  04 CPO
  05 CPI
  06 VCP
  07 SPREAD
  08 5VOUT
  09 MS1_AD0
  10 MS2_AD1
  11 DIAG
  12 INDEX
  13 CLK
  14 PDN_UART
  15 VCC_IO
  16 STEP (U8,P.04/PE5)
  17 VREF
  18 GND
  19 DIR (U8,P.11/PF1)
  20 STDBY
  21 OA2 (X-MOTOR,P.1B)
  22 VS
  23 BRA
  24 OA1 (X-MOTOR,P.1A)
  25 .
  26 OB1 (X-MOTOR,P.2A)
  27 BRB
  28 VS

U6_Y
 IC:TMC2209-LA
  01 OB2 (Y-MOTOR,P.2B)
  02 ENN (U8,P.15/PF5)
  03 GND
  04 CPO
  05 CPI
  06 VCP
  07 SPREAD
  08 5VOUT
  09 MS1_AD0
  10 MS2_AD1
  11 DIAG
  12 INDEX
  13 CLK
  14 PDN_UART
  15 VCC_IO
  16 STEP (U8,P.21/PF9)
  17 VREF
  18 GND
  19 DIR (U8,P.13/PF3)
  20 STDBY
  21 OA2 (Y-MOTOR,P.1B)
  22 VS
  23 BRA
  24 OA1 (Y-MOTOR,P.1A)
  25 .
  26 OB1 (Y-MOTOR,P.2A)
  27 BRB
  28 VS

U6_Z
 IC:TMC2209-LA
  01 OB2 (Z-MOTOR,P.2B)
  02 ENN (U8,P.41/PA5)
  03 GND
  04 CPO
  05 CPI
  06 VCP
  07 SPREAD
  08 5VOUT
  09 MS1_AD0
  10 MS2_AD1
  11 DIAG
  12 INDEX
  13 CLK
  14 PDN_UART
  15 VCC_IO
  16 STEP (U8,P.42/PA6)
  17 VREF
  18 GND
  19 DIR (U8,P.55/PF15)
  20 STDBY
  21 OA2 (Z-MOTOR,P.1B)
  22 VS
  23 BRA
  24 OA1 (Z-MOTOR,P.1A)
  25 .
  26 OB1 (Z-MOTOR,P.2A)
  27 BRB
  28 VS

E1-MOTOR
 J:4 pin JST conn
  1B U6-E1,P.21
  1A U6-E1,P.24
  2A U6-E1,P.26
  2B U6-E1,P.1

E2/EXT-MOTOR
 J:4 pin JST conn
  1B U6-E2,P.21
  1A U6-E2,P.24
  2A U6-E2,P.26
  2B U6-E2,P.1

X-MOTOR
 J:4 pin JST conn
  1B U6-X,P.21
  1A U6-X,P.24
  2A U6-X,P.26
  2B U6-X,P.1

Y-MOTOR
 J:4 pin JST conn
  1B U6-Y,P.21
  1A U6-Y,P.24
  2A U6-Y,P.26
  2B U6-Y,P.1

Z-MOTOR
 J:4 pin JST conn
  1B U6-Z,P.21
  1A U6-Z,P.24
  2A U6-Z,P.26
  2B U6-Z,P.1

E1_HOT
 J:2-pin screw terminal 5.08mm
  E1 GND
  HOT <- Q5,P.D
 
E2_HOT
 J:2-pin screw terminal 5.08mm
  E2 GND
  HOT

Q5
 TR:WSF40N06, N-Ch. Mosfet (60v, 20mOhm, 50A)
  G <- R42,P.1 + U8,P.93 (PG8)
  D -> E1_HOT,P.HOT
  S <- +12/24V

R42
 R:01C (10k, 1%)
  1 Q5,P.G
  2 GND

Q20
 TR:WSF40N06, N-Ch. Mosfet (60v, 20mOhm, 50A)
  G <- R21,P.1 + U8,P.110 (PA15)
  D -> E2_HOT,P.HOT
  S <- +12/24V

R21
 R:01C (10k, 1%)
  1 Q20,P.G
  2 GND

Below is the pinout from the FPC cable (FPC-350C40119V1)

Connector U4

 1 VCC (+3.3v)
 2 VCC (+3.3v)
 3 VCC (+3.3v)
 4
 5
 6 GND
 7 GND
 8 GND
 9
10
11
12
13 XPT2046,P.16 (DCLK)
14 XPT2046,P.15 (_CS)
15 XPT2046,P.14 (DIN)
16 XPT2046,P.12 (DOUT)
17 XPT2046,P.11 (_PENIRQ)
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36 GND
37 
38 
39 
40 VCC (+3.3v)

 1 GND
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25 GND
26 YD
27 XL
28 YU
29 XR
30 K1
31 K2
32 K3
33 K4
34 K5
35 K6
36 A
37 GND
38 NC
39 NC
40 NC