Battery grip pins on Canon cameras

 Magic Lantern:

Found on battery grip in 50D

Can use this connection to gain access to EvShell 

With this access, you can communicate with DryOS

Found on 50D, 5D3, 700D and many others

EvShell is the "main" shell on ICU on DSLRs. You can access DryOS Shell using 'drysh' command from EvShell.

See https://www.magiclantern.fm/forum/index.php?topic=25662.0 for the general idea, but please note that linked thread is for Digic 8 so not everything will be applicable.

On MPU there's a separate shell like that:

https://www.magiclantern.fm/forum/index.php?topic=25661.0

Easy to distinguish due to different prompts :)

Numbers From screen side 1  to SD card side 6.

Canon 70D

Screen 1 | 2 | 3 | 4 | 5 | 6| SD card.

Pin 1 - ICU TX (must be connected to usb uart RX)

Pin 2 - ICU RX (must be connected to usb uart TX)

Pin 3 - GND. Connected directly (0 Om) to tripod mount and hot shoe base.

Pin 4 - Power (+). Camera off 3.1v, camera on 3.3v

Pin 5 - MPU TX (must be connected to usb uart RX)

Pin 6 - MPU RX (must be connected to usb uart TX)