M20 is a member of a class of M-codes which are used to access and manage the contents of the connected Smoothie's SD Card

Issuing an M20 causes the contents of smoothies SD card to be output to its serial port. A properly connected host will in turn receive and display the Smoothies SDcard directory to its console.

To verify and debug your Smoothie's serial connection try this: http://smoothieware.org/debug:computer-sees-board

{ARTHUR: Special instructions for Ethernet connections?}

M20-M36 are also members of this class of SD card commands:

• M21: Initialize SD card
• M22: Release SD card
• M23: Select SD file
• M24: Start/resume SD print
• M25: Pause SD print
• M26: Set SD position
• M27: Report SD print status
• M28: Begin write to SD card
• M29: Stop writing to SD card
• M30: Delete a file on the SD card
• M31: Select file and start SD print
• M32: Get the long name for an SD card file or folder
• M33: Stop and Close File and save restart.gcode
• M34: Set SD file sorting options
• M35: Upload firmware NEXTION from SD
• M36: Return file information

Go to other M-codes

The command is used as such : 

M20  [[S]|[[P|]]]

What it does: output contents of SD card to serial port while using to define the format and [[P|] defines the directory to list

The Shift parameter when used with Smoothie supports Style 0 only, which is the default. If you do not include it will assume [[S0|] The P parameter is not supported by Smoothie. Smoothie defaults to the SDcards root directory.

If your SDcard contents was:

• *config.txt

firmware.cur

Readme.txt
myGgcode.gcode**

This means if you command : 

M0 S0 

or

M0 S0 P1 

or

M0 S1 P1 

or

M0 P2 

….they will all print the following to the connected console: Ok config.txt firmware.cur Readme.txt myGgcode.gcode

The M0 command is not modal,

These resources are used as references:

• http://reprap.org/wiki/G-code#M20:_List_SD_card
• http://reprap.org/wiki/G-code#M-commands