Real Time Commands

Real-time commands are single control characters that may be sent to RabbitGRBL to perform actions immediately, independent of the normal G-code stream.
They can be issued at any time, and RabbitGRBL will react within milliseconds.

Real-Time Commands Overview Table

Command	ASCII / Hex	Description
Ctrl+X	0x18	Soft Reset
?	0x3F	Status Report Query
~	0x7E	Cycle Start / Resume
!	0x21	Feed Hold
0x84	Extended ASCII	Safety Door
0x85	Extended ASCII	Jog Cancel
0x90	Feed Override	Set Feed = 100%
0x91	Feed Override	Feed +10%
0x92	Feed Override	Feed −10%
0x93	Feed Override	Feed +1%
0x94	Feed Override	Feed −1%
0x95	Rapid Override	Rapid 100%
0x96	Rapid Override	Rapid 50%
0x97	Rapid Override	Rapid 25%
0x99	Spindle Override	Spindle 100%
0x9A	Spindle Override	Spindle +10%
0x9B	Spindle Override	Spindle −10%
0x9C	Spindle Override	Spindle +1%
0x9D	Spindle Override	Spindle −1%
0x9E	Spindle Control	Toggle Spindle Stop
0xA0	Coolant Control	Toggle Flood Coolant
0xA1	Coolant Control	Toggle Mist Coolant

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General Characteristics of Real-Time Commands

A real-time command:

• Executes within tens of milliseconds
• Is a single raw character (ASCII or extended ASCII)
• Does not require newline (\n) or carriage return (\r)
• Is not part of the G-code streaming protocol
• Is intercepted immediately and never buffered
• Ignores additional real-time chars until the current one completes
• Can be bound to a physical button or switch
• Effect depends on the current machine state (IDLE, RUN, HOLD, JOG, etc.)

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ASCII Real-Time Commands

These are typeable from a keyboard and appear in the $ help message.

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0x18 (Ctrl+X) — Soft Reset

Immediately halts and safely resets RabbitGRBL without requiring a power cycle.

Behavior:

• Always accepted, at any time
• If triggered during motion:
  • Motion stops instantly
  • An ALARM is raised (lost position possible)
• If triggered while idle:
  • Position may remain valid; homing may not be required
• Clears planner buffers and modal states
• Commonly wired to a physical RESET/ABORT button

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? — Status Report Query

Requests an immediate status report from RabbitGRBL.

Reply Example:

<Idle|MPos:0.000,0.000,0.000,0.000|FS:0,0|Pn:XYZABCP|WCO:0.000,0.000,0.000,0.000>

Output typically includes:

• Machine state (Idle, Run, Hold, Alarm, etc.)
• MPos: Machine coordinates
• FS: Feed rate and spindle RPM
• Pn: Active input pins
• WCO: Work Coordinate Offset* (WCO is sent periodically)*

What is WCO?

WCO (Work Coordinate Offset) is the offset applied by the active work coordinate system (G54–G59).
It represents the difference between machine coordinates and work coordinates.

RabbitGRBL sends:

• MPos in every status report (deterministic)
• WCO periodically (not in every report)

To calculate work position:

WPos = MPos - WCO

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~ — Cycle Start / Resume

Resumes machine motion after a pause or safety-door event.

Resumes from:

• Feed Hold (!)
• Safety door suspend (when door is closed)
• Program pause (M0)

If parking logic is enabled:

• Re-enables spindle & coolant
• Returns to the stored pre-park position
• Continues execution automatically

May be wired to a Cycle Start button.

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! — Feed Hold

Decelerates the machine to a controlled stop, entering HOLD state.

Behavior:

• Works only in IDLE, RUN, or JOG
• If jogging:
  • Cancels current jog
  • Flushes queued jog motions
  • Returns to IDLE or DOOR (if door is open)
• Does not disable spindle or coolant — only motion is suspended
• May be wired to a Feed Hold button

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Extended-ASCII Real-Time Commands

Extended real-time commands use byte values 0x80–0xFF.
These are not typeable from a keyboard; GUIs send them as raw bytes.

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0x84 — Safety Door

Simulates/initiates the safety-door condition.

Behavior:

• Immediately suspends motion into DOOR state
• Spindle & coolant turn off
• If moving:
  • Machine decelerates to a stop first
• During homing:
  • Motion halts and a homing alarm is raised
• Supersedes HOLD or other suspend states
• If parking logic is enabled:
  • Machine moves to its predefined park position
• Jogging:
  • Jog is stopped
  • Remaining jog commands are flushed
  • Machine returns to IDLE when door is closed

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0x85 — Jog Cancel

Cancels the jog mode immediately.

Behavior:

• Triggers an internal feed hold
• Clears queued jog commands
• Only valid in JOG state
• After cancel:
  • Goes to IDLE or DOOR (if door open)

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Feed Override Commands

Modify the programmed feed rate in real-time.

• Affects only feed motions (G1, G2, G3)
• Does not affect rapid moves or jogs
• Typical allowed range: 10% — 200%
• Ignored if the override value does not change

Commands:

• 0x90 — Set feed override to 100%
• 0x91 — Increase feed by +10%
• 0x92 — Decrease feed by −10%
• 0x93 — Increase feed by +1%
• 0x94 — Decrease feed by −1%

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Rapid Override Commands

Modify the speed of rapid moves (G0/G28/G30).

Commands:

• 0x95 — Rapid override 100%
• 0x96 — Rapid override 50%
• 0x97 — Rapid override 25%

If the rapid override value does not change, the command is ignored.

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Spindle Speed Override Commands

Adjust spindle RPM in real-time.

• Applies whether spindle is ON or OFF
• Typical range: 10% — 200%
• Ignored if resulting override has no effect

Commands:

• 0x99 — Set spindle override to 100%
• 0x9A — Increase by +10%
• 0x9B — Decrease by −10%
• 0x9C — Increase by +1%
• 0x9D — Decrease by −1%

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0x9E — Toggle Spindle Stop

Toggles spindle enable/disable only while in HOLD state.

Behavior:

• Ignored during active motion (RUN/JOG)
• Prevents accidental spindle shutoff during cutting
• After resume (~):
  • Restores previous spindle state
  • Applies configurable delay (e.g., 4s) before motion continues
• Spindle override values remain active even while spindle is toggled off
• If safety door opens:
  • Door logic supersedes the spindle stop override

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0xA0 — Toggle Flood Coolant

Toggles flood coolant output.

Behavior:

• Valid in IDLE, RUN, or HOLD
• Updates coolant modal state (M8/M9)
• $G parser report will show M8 accordingly

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0xA1 — Toggle Mist Coolant

Requires compile-time option ENABLE_M7.

Behavior:

• Valid in IDLE, RUN, or HOLD
• Updates coolant modal state (M7/M9)
• $G report will display updated coolant state

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