Overview
On a SINUMERIK 808D ADVANCED turning controller (PPU161.x) paired with absolute encoders on linear axes, an operator pressing X+ or Z+ in Reference mode may observe that the axis does not automatically traverse to the reference point position the way it does on a SINUMERIK 828D. The machine data, the encoder class, and the referencing mode determine whether the controller commands an automatic traverse to the reference-point coordinate or waits for a manual jog. This reference explains the underlying behavior, the relevant machine data (notably MD34090 $MA_REFP_MOVE_DIST_CORR), the procedure for absolute encoders on the 808D platform, and field-proven verification steps for commissioning engineers.
Background: Reference Point Logic in SINUMERIK
The reference point (R) synchronizes the position measuring system with the machine coordinate system (MCS). On a turning center, the tool holder reference point (F) is then used to calculate tool offsets. Per the SINUMERIK 808D ADVANCED Programming and Operating Manual (Turning), the measuring system must be referenced before NC programs can run in AUTO or MDA with active geometry offsets.
Reference point approach is one of three referencing modes available in the SINUMERIK HMI:
- Reference point approach (REF) — drives the axis to the stored reference-point coordinate and synchronizes the measuring system.
- Reference point setting (SET) — declares the current actual position to be the reference-point coordinate without motion.
- Preset / Preset actual-value memory — sets a coordinate for the tool setter, with or without motion.
For incremental measuring systems, the controller requires a physical homing event — typically a BERO/zero-mark pulse combined with a reference cam. The HMI will command a directed traverse toward the reference cam and then decelerate to the zero-mark crossing.
For absolute measuring systems, the absolute position is retained in the encoder's non-volatile memory at power-down. The controller restores the absolute position at power-up; no physical homing traverse is required. This is where the 808D and 828D behaviors diverge.
Absolute vs Incremental Encoder Behavior
| Attribute | Absolute Encoder (EnDat / SSI) | Incremental Encoder (sin/cos + REF) |
|---|---|---|
| Position after power-up | Restored from non-volatile memory | Lost; axis isNotReferenced |
| Reference traverse required | No mechanical motion needed | Yes — traverse to cam + zero mark |
| Jog X+ / Z+ in REF mode | Manual traverse to ref-point coordinate | Automatic traverse to ref-point coordinate |
| Typical Siemens motors | 1FK7 / 1FT7 with EnDat 2.1/2.2 | 1FK7 / 1FT7 with sin/cos |
| Status after power-up | Position restored, needs re-sync only if MD modified | Position lost, full reference required |
On the SINUMERIK 808D, when an absolute encoder is fitted and the absolute-position adjustment has been performed correctly, the controller already knows the absolute machine position. The HMI may not command a directed reference-point traverse in REF mode because there is nothing to synchronize in the classical sense; instead it expects the operator to jog the axis to the reference-point coordinate. This explains why pressing X+/Z+ on a correctly referenced 808D appears to do nothing in REF mode — the system is already in a referenced state and the jog keys are treated as standard manual jog keys rather than reference-point traverse keys.
Why the 808D Behaves Differently from the 828D
The 828D platform supports the full Siemens homing routine, including a configurable reference-point traverse that can be commanded via jog keys in REF mode even when the encoder is absolute, provided the absolute-position adjustment is invalidated (for example after motor replacement, mechanical work, or a controlled reset via MD). The 808D ADVANCED firmware set is intentionally leaner:
- The 808D HMI does not expose the same jog-in-REF behavior when the absolute value is already valid.
- The referencing mode is bound to the encoder class through
MD34200 $MA_ENC_REFP_MODEandMD34100 $MA_REFP_SET_POS. - If the absolute value is lost (e.g., low battery on the encoder, controller replacement, MD reset), the system will request a full re-reference and only then will the jog-in-REF behavior be re-enabled.
Per the 808D Programming and Operating Manual (Turning), the reference-point-approach chapter explicitly directs users to the "Switching on and referencing" section; the manual confirms that the procedure for re-referencing after mechanical changes is identical to the first-time procedure.
Relevant Machine Data for Reference-Point Approach
| MD Number | Name | Typical Use on 808D Turning |
|---|---|---|
| MD34090 $MA_REFP_MOVE_DIST_CORR | Reference point offset distance | Mechanical compensation / offset added to ref-point traverse |
| MD34010 $MA_REFP_CAM_DIR_IS_MINUS | Reference cam direction | 0 = positive, 1 = negative approach direction |
| MD34020 $MA_REFP_VELO_SEARCH_CAM | Reference cam search velocity | mm/min — typically 5000–10000 |
| MD34040 $MA_REFP_VELO_SEARCH_MARKER | Zero-mark search velocity | mm/min — typically 200–500 |
| MD34060 $MA_REFP_MAX_MARKER_DIST | Max distance to zero mark | mm — guard against missed zero mark |
| MD34100 $MA_REFP_SET_POS | Reference point coordinate | Position value assigned to reference point (e.g., X=0, Z=0) |
| MD34200 $MA_ENC_REFP_MODE | Referencing mode for encoder | 0 = no reference, 1 = reference cam + zero mark, etc. |
| MD34210 $MA_ENC_REFP_STATE | Encoder referencing state | 0 = not referenced, 2 = referenced (read-only at runtime) |
| MD30240 $MA_ENC_TYPE | Encoder type | 4 = absolute EnDat, 1 = incremental |
For absolute encoders, the most relevant parameters are MD30240 (must be set to 4 for EnDat absolute) and MD34100 (the coordinate the controller assigns when the reference is set). MD34090 is used to compensate small mechanical offsets when the reference point is not exactly at the mechanical datum.
Reference-Point Approach Procedure on the 808D ADVANCED (Absolute Encoders)
- Power on the controller. Verify the HMI shows no Axis not referenced alarm (e.g., 21612, 20002).
- Switch the HMI to the Reference operating area (REF mode icon on the 808D turning HMI).
- For a first-time commissioning or after a controlled absolute-value reset:
- Use the axis-traverse keys (
X+,X-,Z+,Z-) to manually move each axis to the mechanical reference-point position. - Confirm the position by physical measurement (e.g., reference block, dial indicator, or known tool contact).
- Press the Set reference point softkey or activate the corresponding PLC signal (DB3x.DBX2.4 — refer to the 808D PLC ladder for axis-specific mapping).
- Confirm that the axis position field in the HMI now shows the value set in
MD34100 $MA_REFP_SET_POS. - Repeat for the second axis.
- Switch the operating mode to
JOGand verify the axes can be jogged without an alarm.
When to Re-Reference an Absolute Encoder
Per the 808D ADVANCED Programming and Operating Manual, re-referencing is required only when one of the following events has occurred:
- The motor has been removed from the machine or replaced.
- Mechanical work on the axis has changed the relationship between the encoder and the reference-point datum.
- The encoder battery has been disconnected long enough for the absolute value to be lost.
- The controller's commissioning archive was reloaded with absolute-encoder data cleared.
- MD
MD34210 $MA_ENC_REFP_STATEhas been manually reset via HMI commissioning or PLC signal.
If none of these conditions apply, the absolute value is retained across power cycles and the axes remain referenced indefinitely.
Commissioning Checklist
| Step | Action | Pass Criterion |
|---|---|---|
| 1 | Verify MD30240 = 4 (EnDat absolute) | HMI drive configuration shows "Absolute" |
| 2 | Verify drive telegrams match SINAMICS V60/V80 commissioning | Drive ready, no F- or A- alarms |
| 3 | Apply commissioning archive | All axes configurable |
| 4 | Manually traverse to reference datum | Position within tolerance of mechanical ref |
| 5 | Set reference point (HMI or PLC) | Position display equals MD34100 value |
| 6 | Apply MD34090 if mechanical offset is known | No alarm 21612 or 20002 |
| 7 | Power cycle and confirm axes remain referenced | MD34210 reads 2 for all axes |
| 8 | Run a test NC program in AUTO | Program executes without reference alarms |
Verification
After the reference-point set, perform the following checks:
- Switch to
JOGand traverse each axis a few millimeters. The controller must accept motion without raising alarm 21612 ("Axis %1 reference point not set") or 20002 ("Channel %1 axis %2 reference point not reached"). - Inspect the HMI service axis display: the "Referenced" indicator must be green for X and Z.
- Power down the controller for 30 seconds. Power up. The "Referenced" indicator must remain green without operator action.
- Execute a simple
G0 X0 Z0inMDAto confirm the controller can position to the reference coordinate. - Run a representative turning program in
AUTOwith safe air-cut parameters; the part program must run to completion without reference-loss alarms.
Troubleshooting Matrix
| Symptom | Likely Cause | Corrective Action |
|---|---|---|
| Jog X+/Z+ in REF mode does nothing | Absolute encoder already referenced; no traverse needed | Verify MD34210 = 2; this is normal behavior |
| Alarm 21612 at power-up | Absolute value lost or MD30240 wrong | Check encoder battery, set MD30240 = 4, re-reference |
| Alarm 20002 during NC program | Reference coordinate outside software limits | Verify MD34100 against machine coordinate system |
| Axis drifts after reference set | Encoder mounting slip or mechanical backlash | Inspect coupling, re-verify mechanical ref position |
| Encoder value differs from mechanical position after power-up | Encoder battery exhausted | Replace battery, perform full re-reference |
| Reference set on one axis but not the other | PLCSignal not applied to second axis | Verify PLC interface, re-trigger Set Ref Point softkey |
| HMI shows "Incremental" for absolute encoder | MD30240 set to 1 instead of 4 | Correct MD, power cycle, re-reference |
Differences from 828D Behavior (Summary)
| Aspect | 808D ADVANCED (PPU161) | 828D |
|---|---|---|
| Absolute encoder re-reference trigger | Manual jog + Set Ref Point | Can be auto-triggered via jog-in-REF or PLC |
| Jog X+ in REF after valid absolute | Treated as JOG motion (no traverse to ref) | Treated as reference traverse (axis moves to ref coord) |
| ASUP for referencing | Not required for standard absolute reference | Optional, used for OEM-specific sequences |
| Manual reference procedure | Same as first-time procedure | Same as first-time procedure |
| MD34090 usage | Mechanical compensation offset | Mechanical compensation offset |
The 828D's richer referencing interface stems from its larger MD set and the availability of PLC-controlled referencing sequences via DB3x interface. The 808D intentionally omits the jog-in-REF-when-valid behavior to keep the HMI state machine simple; the operator is expected to use the Set Reference Point softkey for any re-referencing scenario.
Field-Proven Cautions
- Do not modify
MD34100 $MA_REFP_SET_POSafter the first commissioning without re-running the full reference sequence; the value is written into the absolute-encoder adjustment on the 808D. - Do not rely on the 828D's behavior expectation when migrating a program or operator procedure to the 808D; the reference semantics differ even though the physical hardware is similar.
- When replacing an absolute encoder, always restore both the encoder and the controller's commissioning archive together; mismatched absolute values will produce alarm 21612 and may show position jumps if the system is allowed to continue.
- When the 808D HMI displays Absolute in the axis status, do not interpret the lack of motion on jog-in-REF as a fault — verify via the service display that
MD34210reads 2. - Battery-backed encoders (EnDat 2.1) require battery voltage monitoring; a low battery may not immediately invalidate the absolute value but will on the next power cycle, leading to surprising alarm 21612 on Monday morning start-up.
Standards and Document References
- SINUMERIK 808D ADVANCED Programming and Operating Manual (Turning) — Section "Switching on and referencing"
- SINUMERIK 808D ADVANCED Commissioning Manual — page referenced (p. 237) for machine-data details
- SINUMERIK 808D Function Manual — HMI operating area description for Reference mode
FAQ
Why does pressing X+ or Z+ in REF mode not move the axis on my 808D turning machine?
On a SINUMERIK 808D ADVANCED with absolute encoders that have already been referenced, the jog keys in REF mode are treated as standard manual jog keys, not as reference-traverse keys. The controller already holds the absolute position in MD34210 $MA_ENC_REFP_STATE = 2 and does not command a reference traverse. This is normal behavior; use the Set Reference Point softkey if a re-reference is required.
Do I need an ASUP (asynchronous subroutine) to reference an absolute encoder on the 808D?
No. The standard reference-point set for an absolute encoder is performed via the HMI Set Reference Point softkey or the corresponding PLC signal. ASUPs are only required for OEM-specific referencing sequences, not for the built-in absolute-encoder reference procedure documented in the 808D Programming and Operating Manual.
What machine data should I check when the 808D reports the axis is not referenced after power-up?
Verify MD30240 $MA_ENC_TYPE = 4 (EnDat absolute), MD34100 $MA_REFP_SET_POS matches the desired reference coordinate, and the encoder battery is healthy. Then read MD34210 $MA_ENC_REFP_STATE — a value of 0 indicates the absolute value was lost and a re-reference is required; a value of 2 means the axis is referenced.
What is MD34090 $MA_REFP_MOVE_DIST_CORR used for?
MD34090 adds a mechanical compensation distance to the reference-point traverse. Use it to correct small known offsets between the electrical zero of the encoder and the physical reference-point datum. It does not replace the standard reference-point procedure; it adjusts the position at which the reference is declared complete.
How does the 808D referencing behavior differ from the 828D?
The 828D supports a richer set of jog-in-REF behaviors and PLC-controlled referencing sequences through DB3x, including a directed traverse to the reference coordinate on a valid absolute encoder. The 808D intentionally omits this — once the absolute value is valid, the system is considered referenced and the jog keys in REF mode act as normal jog keys. The re-reference procedure is otherwise identical between the two platforms.