LinuxCNC M66 Wait on Input

M66 pauses program execution until a selected digital or analog input reaches a specified state, or until a programmed timeout expires. The input is chosen with a P- word (digital) or an E- word (analog), the wait condition with an L- word, and the timeout in seconds with a Q- word. The input value read is stored in parameter #5399, which holds -1 if the timeout is exceeded.

Source checked11/11 values · 2026-10-26

Specification

1 source

Language	gcode
Code	M66
Group	M-Codes
Syntax	M66 P- \| E- <L->
Dialect	LinuxCNC

Parameters

Name	Description
P-	Digital input number from 0 to 3 (adjustable with the motmod num dio argument).
E-	Analog input number from 0 to 3 (adjustable with the motmod num aio argument).
L-	Wait mode type: 0 IMMEDIATE, 1 RISE, 2 FALL, 3 HIGH, 4 LOW.
Q-	Timeout in seconds for waiting; if exceeded, #5399 holds -1. Ignored when L is 0; a Q value of zero is an error when L is non-zero.

Study notes

Wait modes: 0 IMMEDIATE (no waiting, returns immediately), 1 RISE, 2 FALL, 3 HIGH, 4 LOW.
The current value of the input is stored in parameter #5399 in mode 0; on timeout #5399 holds -1.
Mode 0 is the only wait mode permitted for an analog input.
Programming M66 with both a P- word and an E- word (selecting both a digital and an analog input) is an error.
The number of I/O can be increased with the num_dio or num_aio parameter when loading the motion controller.
Example given: M66 P0 L3 Q5 waits up to 5 seconds for digital input 0 to turn on.
M66 stops further execution of the program until the selected event or the programmed timeout occurs.

Safety

In LinuxCNC these inputs are not monitored in real time and thus should not be used for timing-critical applications.
M66 will not function unless the appropriate motion.digital-in-nn pins or motion.analog-in-nn pins are connected in the HAL file to an input.