Similar to the current Arduino HardwareSerial
but with max. 256 byte buffer-size.
Deactivated by default.
The boards with AT90USB processor (USBCON) already use a TX-buffer.
Add an emergency-command parser to MarlinSerial's RX interrupt.
The parser tries to find and execute M108,M112,M410 before the commands disappear in the RX-buffer.
To avoid false positives for M117, comments and commands followed by filenames (M23, M28, M30, M32, M33) are filtered.
This enables Marlin to receive and react on the Emergency command at all times - regardless of whether the buffers are full or not. It remains to convince hosts to send the commands. To inform the hosts about the new feature a new entry in the M115-report was made. "`EMERGENCY_CODES:M112,M108,M410;`".
The parser is fast. It only ever needs two switch decisions and one assignment of the new state for every character.
One problem remains. If the host has sent an incomplete line before sending an emergency command the emergency command could be omitted when the parser is in `state_IGNORE`.
In that case the host should send "\ncommand\n"
Also introduces M108 to break the waiting for the heaters in M109, M190 and M303.
Rename `cancel_heatup` to `wait_for_heatup` to better see the purpose.
Move and extend axis_unhomed test to deploy/stow_z_probe().
Move and extend position store/restore to deploy/stow_z_probe().
Now all kinds of probes can use the 'E' parameter in G29/M48.
Allen key probes can be used now for grid and 3-point levelling.
Deploying the Allen Key probe uses big moves in z direction.
Too dangerous for an unhomed z-axis.
Throw a compile time error when we try to configure an
Allen Key probe homing to z-min and having no other z-min-endstop.
The `PROBE_IS_TRIGGERED_WHEN_STOWED_TEST` is able to detect if a probe is
successfully deployed/stowed.
It was part of the Allen Key probes code only.
Now the function is generally available.
Currently this test is compulsory for Allen Key probes to not alter the used behaviour
If you want to add the test for an other type of probe simply add
```
#define PROBE_IS_TRIGGERED_WHEN_STOWED_TEST
```
to your config.
