How to Reference a Siemens SINUMERIK CNC (808D / 828D / 840D) — Reference Point & Absolute Encoder Calibration

On Siemens SINUMERIK controls the procedure depends on the encoder type. Machines with incremental encoders must perform a reference point approach at every power-up; machines with absolute encoders only need a one-time calibration (re-adjustment) after an encoder, motor or battery event — typically signalled by alarm 25000/25001 (hardware fault of active encoder) or Axis not referenced. This guide covers both, for 808D, 828D and 840D / 840D sl.

Incremental machines — reference point approach

1. Select JOG → REF POINT mode on the machine control panel.
2. Press the axis traverse key in the direction the builder configured (the axis symbol shows an unreferenced marker — a circle without the reference tick).
3. The axis moves to the reference cam, slows, and creeps to the encoder marker pulse. When done, the referenced symbol (Ⓡ tick) appears next to the axis and the machine coordinate is valid.
4. Repeat for every axis, in the order the builder specifies (usually Z first on machining centers to lift the head clear).

Absolute machines — encoder calibration (re-adjustment)

Needed after replacing a motor/encoder, a gearbox intervention, or losing the absolute datum. You will set machine data so the control learns the true axis position.

1. Set the access level to Manufacturer (password on 828D/840D: builder's; default SUNRISE on many systems).
2. Jog the axis to a precisely known position — the builder's alignment marks or a dial-indicated datum.
3. Open the axis machine data and enter the known position into MD34100 $MA_REFP_SET_POS (reference point value).
4. Set MD34210 $MA_ENC_REFP_STATE = 1 (enables calibration).
5. In JOG–REF mode, press the axis traverse key. The control does not move the axis; it stores the current absolute encoder reading against MD34100 and sets MD34210 = 2 (calibrated).
6. NCK reset (warm restart). Verify the axis shows referenced and the position matches the physical datum.

Common problems

• MD34210 falls back to 0 — encoder battery (if fitted) flat, or encoder fault; alarm 25000-series in the log confirms it.
• Alarm 10203 ‘Reference point approach aborted’ — axis started on the cam or NC-stop during approach; jog off the cam and repeat.
• Position wrong by a constant amount — wrong value entered in MD34100, or wrong datum used. Re-measure and recalibrate.
• Axis referenced but soft limits trip immediately — check MD36100/36110 (position limit switches) against the builder's sheet; a bad calibration shifts the whole travel window.

Related guides

• How to back up SINUMERIK 808D / 828D data
• Siemens 808D / 828D alarm list
• SINAMICS drive fault codes explained