21 ALARM — Machine Side Detector No Signal 2

Check the servo parameter SV025 (MTYP) setting to ensure pulse-type detector parameters are not mistakenly set for a serial communication type detector. For spindle drives, check parameter SP037 (SFNC5). If encoder orientation is not used, set bit 0 and bit 8 to 0.

Inspect the machine side detector connector on the drive unit (CN3 for servo, CN6 for spindle) and on the detector itself. Disconnect and securely reconnect them by hand to ensure there is no loose connection.

Turn OFF the main circuit and control power. Verify the CHARGE lamp is completely off. Use a multimeter to test the detector cable for continuity and short circuits. If a connection fault or broken wire is found, replace the detector cable.

Inspect the detector's ambient environment for severe electrical noise, high temperature, or incomplete grounding. Ensure the drive unit is grounded to a single point at the PE terminal and improve communication cable shielding to prevent signal loss.

Connect the machine side detector to another known-good axis drive unit of the same capacity. If the alarm stays with the original drive unit, its input circuit is defective. If the alarm follows the detector, the detector itself is faulty.

If the alarm persists after adjusting parameters and cables, replace the faulty MDS-C drive unit or the machine side detector (linear scale or encoder). If an absolute position detector is replaced, re-establish the zero point, and contact authorized Mitsubishi service.