Under normal operating conditions, no symbol is displayed in front of the servo axis on the POS screen of a FANUC CNC system.
However, in some cases special characters such as *, D, I, L, S, or M may appear before the axis name. When these symbols are displayed, the machine may not operate normally or may move differently from the expected machining path.
Understanding these symbols helps operators quickly identify the axis status and troubleshoot potential issues.
This article explains the meaning of servo axis symbols displayed on the POS screen and how they relate to diagnostic signals in the CNC system.
Note: The example shown in this article is based on the FANUC 0i-F Plus system.
1. Normal State and Special State
Normal State
In normal conditions, no symbol appears before the servo axis on the POS screen.
Special State
When a symbol appears before the axis name, it indicates the axis is currently in a special status.
2. Axis in Motion
Symbol: *
When the symbol * appears before an axis, it indicates that the axis is currently moving.
The system status bar will display MTN (Motion).
3. Axis Detached
Symbol: D
When the symbol D appears before an axis on the POS screen, it indicates that the axis is in the control axis detached state.
This situation is commonly seen when a rotary table is removed from the machine.
Related Signal
| Signal | Address | Description |
|---|---|---|
| DTCH1 ~ DTCH8 | <Gn124.0 ~ Gn124.7> |
Control axis detach signals. When the signal value is 1, the corresponding axis has been detached. In this state, position control is not performed and the corresponding servo motor enters a de-energized (excitation off) state. |
4. Axis Lock
Symbol: I
This symbol indicates that the axis is locked and prohibited from moving.
Related Diagnostic Signals
| Signal | Address | Description |
|---|---|---|
| STLK | <G7.1> |
Start lock signal. Prevents axis movement during automatic operation modes such as DNC, MEM, and MDI. |
| *IT | <G8.0> |
All-axis interlock signal. When the signal value is 0, all axes are interlocked. |
| *CSL | <G8.1> |
Cutting block start interlock signal. Prevents the execution of program blocks containing movement commands other than positioning during automatic operation. Effective when the signal is 0. |
| *BSL | <G8.3> |
Block start interlock after program execution begins. Prevents the next program block from starting in automatic mode when the signal is 0. |
| *IT1 ~ *IT8 | <G130.0 ~ G130.7> |
Individual axis interlock signals. When the signal value is 0, the corresponding axis is locked. |
M-Series Systems
| Signal | Address | Description |
|---|---|---|
| +MIT1 ~ +MIT4 | <G132.0 ~ G132.3> |
Positive direction axis lock signals. Effective when the signal value is 1. |
| -MIT1 ~ -MIT4 | <G134.0 ~ G134.3> |
Negative direction axis lock signals. Effective when the signal value is 1. |
T-Series Systems
| Signal | Address | Description |
|---|---|---|
| +MIT1 | <X4.2> |
Individual axis lock signal (positive direction). Effective when the signal value is 1. |
| -MIT1 | <X4.3> |
Individual axis lock signal (negative direction). Effective when the signal value is 1. |
| +MIT2 | <X4.4> |
Individual axis lock signal (positive direction). Effective when the signal value is 1. |
| -MIT2 | <X4.5> |
Individual axis lock signal (negative direction). Effective when the signal value is 1. |
5. Machine Lock
Symbol: L
When the symbol L appears before the axis name, the machine is in machine lock mode, which prevents the machine from moving.
Related Diagnostic Signals
| Signal | Address | Description |
|---|---|---|
| MLK | <G44.1> |
Machine lock signal for the entire machine. When the signal value is 1, all feed axes are locked. |
| MLK1 ~ MLK8 | <G108.0 ~ G108.7> |
Individual axis machine lock signals. When the signal value is 1, the corresponding feed axis is locked. |
6. Servo Off
Symbol: S
This symbol indicates that the servo system is turned off.
In this condition:
-
Servo motor excitation is disabled
-
No current flows to the servo motor
-
Torque control is cancelled
-
Position detection remains active
7. Mirror Mode
Symbol: M
When mirror mode is active, the machine runs in reverse direction during automatic operation.
Axis coordinate behavior:
-
Absolute coordinates follow the program direction
-
Mechanical and relative coordinates move opposite to the program direction
Related Signal
| Signal | Address | Description |
|---|---|---|
| MT1 ~ MT8 | <G106.0 ~ G106.7> |
Mirror enable signals for each axis. |
Conclusion
Symbols displayed before servo axes on the FANUC POS screen provide valuable diagnostic information about the current axis status.
By understanding the meaning of symbols such as *, D, I, L, S, and M, operators and maintenance engineers can quickly identify issues such as axis movement, machine lock, servo shutdown, or mirror operation.
Checking the corresponding diagnostic signals can help determine the exact cause and ensure the machine returns to normal operation efficiently.
Reference Source: Beijing FANUC. This article is based on technical documentation provided by Beijing FANUC.
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