320-0FB8

Check limit switch wiring

Inspect all limit switch connections at the machine interface and verify continuity using multimeter. Check X16 connector pins for loose connections.

Ensure machine power is OFF before checking electrical connections

Test limit switch operation

Navigate to MOD > Machine Parameters > Input/Output and monitor limit switch status in real-time while manually actuating each switch.

Use the TNC's diagnostic screen to see live I/O status rather than relying on external meters

Verify switch mounting

Inspect physical mounting of limit switches for looseness, misalignment, or mechanical damage that could prevent proper actuation.

Replace faulty switches

Replace any switches showing intermittent or no signal response, ensuring proper NC/NO configuration matches machine parameter settings.

Access machine parameters

Navigate to MOD > Machine Parameters and locate MP75xx series parameters for the affected axis limit switch configuration.

Use Ctrl+F to quickly search for specific parameter numbers in the TNC parameter list

Verify limit switch enable parameters

Check MP7500.x (positive limit switch) and MP7501.x (negative limit switch) are properly enabled (set to 1) for the affected axis.

Incorrect limit switch parameters can lead to crashes - verify settings against machine documentation

Validate I/O assignment

Confirm MP7502.x and MP7503.x contain correct input numbers corresponding to physical limit switch connections on the control.

Check switch logic configuration

Verify MP7504.x and MP7505.x match the actual NC/NO configuration of the installed limit switches (0=NO, 1=NC).

Access PLC program

Navigate to MOD > PLC > Program to view the current PLC code and search for limit switch handling routines.

Look for FN16 SYSREAD ID270 calls which read limit switch status in the PLC

Check limit switch enable flags

Examine PLC markers and flags that control limit switch monitoring, typically found in initialization or safety monitoring sections.

Review recent PLC changes

Check PLC program modification date and compare against known good backup to identify any recent changes that may have disabled limit switches.

Re-enable limit switch monitoring

Modify PLC program to restore proper limit switch monitoring and cycle power to reload the corrected program.

Check I/O module status LEDs

Inspect the digital input module for error LEDs or missing status indicators that suggest hardware failure.

Only qualified personnel should work inside electrical cabinets with power applied

Test with known good inputs

Temporarily connect limit switch signals to different input channels to isolate whether specific input channels have failed.

Verify power supply to I/O

Check 24VDC supply voltage to I/O modules and ensure proper grounding of input commons using multimeter.

Replace I/O hardware

If hardware failure is confirmed, replace the affected I/O module and update machine parameters to reflect any address changes.