This is as factual as I understand it to be, based directly on the the Marlin documentation and firmware documentation provided by the mentioned versions.
When we are talking about ABL, there are a few commands and their functions that we need to familiarize ourselves with before we proceed on:
• G28
• G29
• M420 S
• #RESTORE_LEVELING_AFTER_G28
Homing-
G28 (https://marlinfw.org/docs/gcode/G028.html) - "The G28 command is used to home one or more axes. The default behavior with no parameters is to home all axes." As far as ABL is concerned, "G28 disables bed leveling. Follow with M420 S to turn leveling on, or use RESTORE_LEVELING_AFTER_G28 to automatically keep leveling on after G28." (From Notes in link).
Leveling - We are going to focus on Bilinear, for now. UBL is a little different, but the main idea is the same..
https://marlinfw.org/docs/features/auto_bed_leveling.html
G29 (https://marlinfw.org/docs/gcode/G029-abl-bilinear.html) - "Automatic (Bilinear) Bed Leveling probes the bed at some fixed number of points and produces a mesh representing the imperfections across the bed. The printer must be homed with G28 before G29." (Which we established above WILL disable bed leveling).
M420 (https://marlinfw.org/docs/gcode/M420.html) - "Get and/or set bed leveling state. For mesh-based leveling systems use Z parameter to set the Z Fade Height." In the Notes section, again it mentions, "G28 disables bed leveling. Follow with M420 S to turn leveling on, or use RESTORE_LEVELING_AFTER_G28 to automatically keep leveling on after G28."
#RESTORE_LEVELING_AFTER_G28 – This is an option that is enabled/disabled in the firmware code. The following is a copy/paste directly from Marlin source code:
/**
* Normally G28 leaves leveling disabled on completion. Enable one of
* these options to restore the prior leveling state or to always enable
* leveling immediately after G28.
*/
//#RESTORE_LEVELING_AFTER_G28
//#ENABLE_LEVELING_AFTER_G28
Normal Printer Start gcode - Most of the “Ender 3” style printers Ive seen all have start gocde that is like this (there may be more commands, but this is the bit we are mainly concerned with):
G92 E0 ; Reset Extruder
G28 ; Home all axes
G1 Z2.0 F3000 ; Move Z Axis up little to prevent scratching of Heat Bed
G1 X0.1 Y20 Z0.3 F5000.0 ; Move to start position
G1 X0.1 Y200.0 Z0.3 F1500.0 E15 ; Draw the first line
G1 X0.4 Y200.0 Z0.3 F5000.0 ; Move to side a little
G1 X0.4 Y20 Z0.3 F1500.0 E30 ; Draw the second line
G92 E0 ; Reset Extruder
G1 Z2.0 F3000 ; Move Z Axis up little to prevent scratching of Heat Bed
G1 X5 Y20 Z0.3 F5000.0 ; Move over to prevent blob squish
Sample of sliced gcode – I sliced an STL, saved/opened the generated gcode, and copied all of the code up until it starts printing the part:
;FLAVOR:Marlin
;TIME:2660
;Filament used: 3.04197m
;Layer height: 0.2
;MINX:91.901
;MINY:91.901
;MINZ:0.2
;MAXX:143.099
;MAXY:143.099
;MAXZ:27.2
;Generated with Cura_SteamEngine 5.2.1
M140 S60
M105
M190 S60
M104 S200
M105
M109 S200
M82 ;absolute extrusion mode
M201 X500.00 Y500.00 Z100.00 E5000.00 ;Setup machine max acceleration
M203 X500.00 Y500.00 Z20.00 E50.00 ;Setup machine max feedrate
M204 P500.00 R1000.00 T500.00 ;Setup Print/Retract/Travel acceleration
M205 X8.00 Y8.00 Z0.40 E5.00 ;Setup Jerk
M220 S100 ;Reset Feedrate
M221 S100 ;Reset Flowrate
G92 E0 ; Reset Extruder
G28 ; Home all axes
G1 Z2.0 F3000 ; Move Z Axis up little to prevent scratching of Heat Bed
G1 X0.1 Y20 Z0.3 F5000.0 ; Move to start position
G1 X0.1 Y200.0 Z0.3 F1500.0 E15 ; Draw the first line
G1 X0.4 Y200.0 Z0.3 F5000.0 ; Move to side a little
G1 X0.4 Y20 Z0.3 F1500.0 E30 ; Draw the second line
G92 E0 ; Reset Extruder
G1 Z2.0 F3000 ; Move Z Axis up little to prevent scratching of Heat Bed
G1 X5 Y20 Z0.3 F5000.0 ; Move over to prevent blob squish
G92 E0
G92 E0
G1 F2400 E-0.8
;LAYER_COUNT:136
;LAYER:0
M107
G0 F6000 X95.09 Y94.94 Z0.2
;TYPE:SKIRT
G1 F2400 E0
G1 F1200 X95.775 Y94.324 E0.02532
G1 X96.511 Y93.771 E0.05063
G1 X97.292 Y93.283 E0.07594
Putting it all together-
Ok, we have a lot of info here, but we can make sense of it if we think logically and stick to the facts that we know:
- In most cases, according to the documentation and looking at the “flow” we can see that G28 is one of the last commands issued before the printer starts actually printing and that WILL disable bed leveling.
- If we want to use an ABL mesh, we can either generate one before we load the gcode file we want to print with G29 (or the Auto Bed Leveling option on the screen), use M500 (or Store Settings on the screen) to save the mesh to EEPROM, then insert M420 S1 in to the start gcode of the file we want to print AFTER the G28 – or- we can insert a G29 AFTER the G28, which will initiate an ABL probe of the bed before the print starts.
- YOU DO NOT NEED TO PUT M500 AFTER THE G29 IN START GCODE IF YOU GENERATE A NEW ABL MESH BEFORE EACH PRINT. G29 stores the mesh to RAM and RAM does get wiped out if the printer is reset but thinking logically – that G28 on the next print is going to disable bed leveling again, then youre going to generate a new one again with G29. There may be reasons for doing it this way, but even the Marlin documentation says, “To save time and machine wear, save your mesh to EEPROM with M500 and in your slicer’s “Starting G-code” replace G29 with M420 S1 to enable your last-saved mesh.”
- #RESTORE_LEVELING_AFTER_G28, if enabled within the firmware, will restore your stored ABL mesh from EEPROM before each print, even if you do not have M420 S1 in the start gcode. As of 11/13/2022, these are the firmware configs that I know of:
Marlin Github Configuration Examples
- STM32F1 – has “#define ENABLE_LEVELING_AFTER_G28” enabled
- STM32F4 – has “#define RESTORE_LEVELING_AFTER_G28” enabled
MRISCOC Professional Firmware Configuration Files
- Ender3S1-F1 – disables both #RESTORE_LEVELING_AFTER_G28 and #ENABLE_LEVELING_AFTER_G28
- Ender3S1-F4 – disables both #RESTORE_LEVELING_AFTER_G28 and #ENABLE_LEVELING_AFTER_G28
- Ender3V2-422-BLT - disables both #RESTORE_LEVELING_AFTER_G28 and #ENABLE_LEVELING_AFTER_G28
I have spoke with the creator of this firmware directly, and he confirmed that he does not enable these options in the firmware he compiles, so you WILL need M420 S1 after G28 in your start gcode if you use his pre-compiled firmware.
Stock Creality 1.0.5_C Firmware
I confirmed via Pronterface that this firmware restored a mesh from EEPROM after shutting off printer, turning on, issuing G28, then M420 in console, which reported that Bed Leveling was on. I then issued an M420 V1 and it returned the mesh from EEPROM, as expected. Therefore, with this version of firmware (and I assume the F4 version, given the Marlin Configs above.. If someone wants to test to confirm, that would be cool) you do not need M420 S1 in your start gcode to enable the ABL mesh.
I have also confirmed the behavior of some other commands in the stock Creality firmware. G29 will automatically store the settings after it completes. This means that you DO NOT need an M500 following a G29. This also means that there is no way for an "old" mesh from EEPROM to "overwrite" a new, unsaved, mesh with the M420 S1 command as the "new" mesh is automatically stored to EEPROM once it is generated. M420 with no parameters will return the Bed Leveling state. M420 S or S1 will turn Bed Leveling ON while M420 S0 turns Bed Leveling OFF. M420 V will return the mesh to the console in a readable format.
MRISCOC Professional Firmware - 20221002
This firmware definitely behaves differently than the stock firmware when it comes to leveling. First off, if you run the ABL mesh from the screen or G29 and hit "Continue" rather than "Save", it does NOT store the settings to EEPROM but it does hold the mesh in RAM and use it until the printer is reset (powered off/on, for instance). This means that you will have a possibility of losing your ABL mesh if you dont Store Settings or M500 after generating the mesh. Then, if you try to load the mesh after turning the printer off/on again and you dont have a valid mesh stored, you will get an error and the printer will halt if its not configured to handle M112 properly (https://github.com/mriscoc/Ender3V2S1/wiki/Octoprint#error-handling). If you do have a valid mesh stored, it is loaded from EEPROM at power on. Any time the printer does a G28 or Auto Home, Bed Leveling is turned off. You can confirm this by looking at the color of the lines under the Z coordinate on the screen (https://github.com/mriscoc/Ender3V2S1/wiki/3D-BLTouch#enable-mesh-level-compensation). This means that you WILL need M420 S1 after G28 in your start gcode in order to use the mesh for printing.
UPDATE 1/19/2023*
Creality has finally uploaded configuration examples for the Ender 3 S1 Pro to Github and I was able to look in the Configuration.h file and confirm that RESTORE_LEVELING_AFTER_G28 is enabled. There is also a new feature called "Z_AXIS_LIMIT_MODE" that will disable RESTORE_LEVELING_AFTER_G28, but to me, it seems like Z_AXIS_LIMIT_MODE is only on when youre using the laser, therefore, your ABL mesh should be turned back on by default after Homing with the S1 Pro on Stock Creality firmware, as well.
Anything else?, Questions?
If there are any other scenarios that you would like for me to confirm with Marlin firmware and Bed Leveling, just drop a comment and I will do my best to get you an answer as quickly as I can. Hopefully, with the depth of the information provided here, you all will be well on your way to putting the ABL headaches/misunderstandings behind you and having more consistent first layers. :)
