2026-09-05 · Technical Insights

Magnetostrictive Displacement Sensor Head LEDs: OK, No Magnet, Over-Range

On power-up, the head LEDs of a magnetostrictive displacement sensor tell you supply, magnet and range status before any meter is used. Read the electronics-head LEDs before unplugging. Analogue / SSI heads commonly have two lamps: green means the module is alive, red means the measuring condition is not met.

Electronics-head LED diagnostic table on a magnetostrictive displacement sensor — Head LEDs: green and red combinations and what each means

Lamp table

Lamps	Meaning	Next
Green on, red off	OK, magnet seen	Align reading and stroke; do not chase a missing supply
Green on, red on	No magnet	Fallen / reversed magnet, air gap, or left the valid stroke (over-range)
Both off	No supply, or module dead	Measure +24 V; see troubleshooting and multimeter 5 steps

Over-range and no-magnet often share the same lamps: the magnet entered a dead zone or left the rod. When both lamps are on, move the magnet to mid-stroke; if the lamps recover it was an over-range excursion, not a dead magnet. Lamps should return to green on / red off. If they stay dual-on at mid-stroke, it is the magnet or air gap.

With a multimeter

Both off: measure 20.4–28.8 V, polarity and connector.
Both on: move the magnet / piston before opening the head.
Lamps OK but the reading jumps: routing, shield, ripple — n9.

SSI (162 / 192) uses the same lamps; protocol: 162 vs 192. Fieldbus link lamps: drop-outs.

Engineer's notes

Fieldbus heads carry two sets of lamps. 194 (CANopen), 195 (Profibus DP), 197 / 198 (EtherCAT) and 199 (PROFINET) add a link lamp to the measuring lamps. A dropped node or lost frames is a link-lamp question first — measuring lamps being fine does not mean data reached the PLC.
Lamps report measuring conditions, not accuracy. Green means there is an echo to read. Jitter and non-linearity do not show up on lamps; those go to routing, shielding and ripple.
SSI uses the same lamp codes as analogue. 162 / 192 follow the table above; do not go looking for different lamp positions because the protocol changed.