S7-1500 TO_ExternalEncoder: Configure Absolute Encoder Position

Problem Description

When commissioning a SIMATIC S7-1500 CPU with a TM PosInput 2 module and a TO_ExternalEncoder technology object, the actual position read by the user program does not survive a power cycle of the PLC even though the connected encoder is an absolute device. The diagnostics view of the TM PosInput module shows a correct, updated absolute position immediately after restart, but the application DB exposes a value that resets to 0.0 on every STOP-to-RUN transition. This breaks applications that rely on a retained absolute position, such as rotary tables, hoists, or tracking systems where the mechanical reference cannot be re-acquired cheaply after every PLC restart.

Observed Symptom Pattern

• Encoder hardware reports a valid absolute value (for example 4000 increments) in the TM PosInput diagnostics.
• Mechanically rotating the encoder while the PLC is powered off produces the expected change in the diagnostic view once the PLC powers back up.
• The user program's Actual position tag in the TO_ExternalEncoder instance DB starts at 0.0 after every restart.
• Calling MC_Home with mode 7 (Absolute encoder adjustment) overwrites the position to 0.0 instead of adopting the absolute value from the encoder.
• Attempting to change the relevant configuration parameter in the technology object online produces no effect; the parameter appears greyed out or unavailable.

Affected Hardware and Software

Understanding the Data Flow

Before applying the fix, it helps to understand how encoder data flows from the physical device to the application program.

1. Encoder hardware: The physical encoder maintains its absolute position internally. Single-turn resolution is the number of increments within one mechanical revolution. Multi-turn resolution adds additional bits for the number of full revolutions. Common resolutions are 13-bit single-turn (8192 increments per revolution) and 25-bit multi-turn (33 million revolutions).
2. TM PosInput 2 module: The module reads the encoder data over its native protocol (SSI, sin/cos with EnDat, or pure PROFIdrive) and translates it into the standard PROFIdrive telegram 83 format. The module does not lose the absolute information; it simply packs the position into Gx_XIST1 and the fine resolution into Gx_XIST2.
3. PROFINET/PROFIBUS: Telegram 83 travels from the TM PosInput 2 to the CPU via PROFINET IO or PROFIBUS DP. The full 32-bit position plus 12-bit fine resolution is delivered each cycle.
4. TO_ExternalEncoder: The technology object receives the telegram data. If the object is configured for absolute operation, it stores the position as a true absolute value and updates the user-visible Actual position tag accordingly. If not, it treats Gx_XIST1 as a relative counter and resets the position on every STOP-to-RUN transition.
5. Application program: The user program reads the Actual position tag from the technology object's instance DB and uses it for control decisions.

Root Cause Analysis

The TM PosInput 2 correctly reads the absolute position from the encoder and publishes it to the technology object through PROFIdrive telegram 83. The technology object, however, treats the encoder as a non-absolute source unless its configuration explicitly marks it as absolute. In non-absolute mode, the technology object treats the incoming Gx_XIST1 as a relative counter and clears the position reference to zero on every CPU restart and every STOP-to-RUN transition.

There are three compounding causes that, together, produce the symptom:

1. Technology object configuration: The Operating mode parameter of the TO_ExternalEncoder is not set to support absolute evaluation of the connected encoder.
2. PROFIdrive interpretation: The associated telegram data is interpreted as incremental, so Gx_XIST1 is treated as a relative counter rather than an absolute position.
3. Online edit limitation: The configuration change cannot be performed online. Configuration data of technology objects in TIA Portal is only applied when the project is recompiled and downloaded. This is why the parameter appears unavailable when attempted from the online view.

Why MC_Home Mode 7 Cannot Rescue a Misconfigured Encoder

Calling MC_Home with mode 7 (Absolute encoder adjustment) writes a home position offset based on the current position value returned by the technology object. If the technology object has already cleared the absolute reference on restart, the offset is computed against 0.0 and the home position snaps to 0.0 as well. Homing cannot rescue a configuration that has been told to ignore the encoder's absolute capability. The encoder evaluation flag must be set first; only then will MC_Home mode 7 behave as documented.

Prerequisites

• Installed TIA Portal project containing the S7-1500 station, the TM PosInput 2, and the TO_ExternalEncoder.
• Project consistent and successfully downloaded at least once prior to the fix.
• CPU accessible from the engineering station via PROFINET or PROFIBUS.
• User rights to edit the technology object configuration in the project.
• Encoder manufacturer datasheet available, with single-turn resolution, multi-turn range, and protocol (SSI or PROFIdrive) documented.
• Online backup of the current project for rollback if the new configuration produces unexpected results.

Encoder Selection in TIA Portal

When you add a TM PosInput 2 module to the device configuration, TIA Portal offers a list of supported encoders and protocols. The selection determines the default telegram and the default fine resolution.

If the encoder is not in the list, the User-defined entry allows manual configuration of telegram and fine resolution. Always cross-check the manual configuration against the encoder datasheet; mismatched fine resolution is one of the most common causes of position jumps in the commissioning phase.

Solution: Configure the TO_ExternalEncoder for Absolute Operation Offline

The configuration changes must be made offline, then compiled, then downloaded. Online-only modifications to technology object configuration data are silently rejected by TIA Portal, which is why the parameter appears unavailable when changed from the online view. This is a feature, not a bug: the technology object configuration is part of the project source of truth and must be reproduced from the offline project at every download.

Step-by-Step Procedure

1. Open the project in TIA Portal and switch to the project tree.
2. Navigate to Technology Objects > TO_ExternalEncoder_1 > Configuration.
3. Open the Hardware interface section and select the encoder channel assigned to the TM PosInput 2.
4. In the Encoder configuration page, set the encoder operating mode to support absolute position evaluation. For an SSI or PROFIdrive absolute encoder, this is typically the Absolute encoder parameter or the Encoder type selector in the configuration dialog.
5. Switch to the Position parameters section and enable absolute value handling so that the technology object reads the encoder's absolute position on every restart instead of treating the counter as relative.
6. If the application uses a circular range, leave Modulo enabled. Verify the modulo range matches the encoder's single-turn or multi-turn range to avoid wrap-around jumps in the displayed position.
7. Under Fine resolution, confirm the values Gx_XIST1 = 0 bit and Gx_XIST2 = 12 bit match the encoder datasheet. The 12-bit fine resolution means the technology object can resolve the position to 1/4096 of one encoder increment.
8. Confirm the encoder's direction setting (positive or negative count direction) matches the mechanical setup.
9. Save the project.
10. Right-click the S7-1500 station and select Compile > Hardware and Software (rebuild all).
11. Download the compiled configuration to the CPU. The CPU may briefly transition to STOP during download; this is expected for configuration data.
12. After download, switch the CPU to RUN and observe the technology object diagnostics.

Solution: Apply MC_Home Mode 7 in the User Program

Once the technology object is configured for absolute operation, MC_Home with mode 7 (Absolute encoder adjustment) performs an offset correction so the displayed actual position matches the configured home position. Unlike modes 0 to 6, mode 7 does not overwrite the encoder's mechanical reference; it only applies a software offset.

Structured Text Example

// Trigger absolute encoder adjustment
#instHome.Execute := TRUE;
#instHome.Mode := 7;               // Absolute encoder adjustment
#instHome.Position := 0.0;         // Target home position in user units
#instHome.Encoder := "TO_ExternalEncoder_1";
MC_Home_DB(#instHome);

// Wait for Done
IF #instHome.Done THEN
    #instHome.Execute := FALSE;
END_IF;

// Evaluate error
IF #instHome.Error THEN
    // Log ErrorID for diagnostics
    #diagErrorID := #instHome.ErrorID;
END_IF;

Conditions for a Successful Adjustment

• The technology object must be enabled (MC_Power Status = TRUE).
• The encoder must report a valid absolute value (check the technology object's StatusWord for the absolute-encoder bit).
• The job must be triggered after the configuration download has been acknowledged and the CPU is in RUN.
• If the home position is configured in the technology object as a non-zero value, either change the Position parameter in the MC_Home call to match, or set the home position in the technology object to 0.0 if the application requires a zero-referenced axis.

Interpretation of MC_Home Modes

Modulo Configuration Considerations

The modulo setting tells the technology object that the position is circular within a specified range. For a rotary encoder with 4096 increments per revolution, setting the modulo to 4096 user units produces a position display that wraps from 4095 back to 0 with no discontinuity. Mismatches between modulo range and encoder resolution cause unexpected behavior:

• If modulo is smaller than the encoder's single-turn range, the position wraps mid-turn.
• If modulo is larger than the encoder's single-turn range, the position accumulates over multiple turns and breaks the wrap-around semantics.
• If modulo is set to 0 (off), the position accumulates over the encoder's full multi-turn range, which is appropriate for linear axes but rarely for rotary axes.

Verification Procedure

1. Download the modified project to the CPU and let it transition to RUN.
2. In the technology object diagnostics, confirm that the encoder is reported as absolute and that the Actual position value matches the encoder's mechanical position.
3. Rotate the encoder to a known mechanical position (for example 4000 increments).
4. Monitor the Actual position tag in the online watch table and confirm it follows the mechanical movement without jumps or oscillations.
5. Switch the CPU to STOP, then back to RUN. Verify that the Actual position tag in the user program now retains the last known position instead of resetting to 0.0.
6. Mechanically rotate the encoder by 90 degrees while the CPU is off, then power the CPU back on. The Actual position should reflect the new mechanical position immediately after restart.
7. Execute MC_Home mode 7 once and verify that the absolute position is adopted as the configured home position without snapping to 0.0.
8. Disconnect the encoder cable, re-connect, and verify the encoder value reappears correctly without a STOP-to-RUN transition.
9. Cycle the CPU power three times in succession and confirm the position is retained across every cycle.

Troubleshooting Matrix

Reference: PROFIdrive Telegram 83 Layout

Gx_XIST1 carries the absolute encoder position with the configured fine resolution of the lower bits stripped. Gx_XIST2 carries the additional fine-resolution bits, allowing the technology object to reconstruct the full position with sub-increment precision. With the 12-bit fine resolution specified in this application, the technology object resolves 4096 sub-increments per encoder increment.

Reference: Technology Object Status Word (TO_ExternalEncoder)

Best Practices for Absolute Encoder Projects on S7-1500

• Always edit technology object configuration data offline and download the compiled project. Do not rely on online parameter patching for technology objects; the configuration data set is only loaded at project download.
• Cross-check the encoder's single-turn and multi-turn resolution against the configured modulo range. A mismatch produces silent wrap-around at unexpected angles and corrupts the position reference.
• Use a Watch table with the technology object's StatusWord and ErrorID tags during commissioning. These tags surface issues before they propagate into the application logic.
• Document the exact firmware version of the TM PosInput 2 module and the CPU. Different firmware versions change the available configuration options and the handling of multi-turn encoders.
• Add a power-on initialization block that calls MC_Home mode 7 once after MC_Power reports ready. This guarantees the application sees a defined home reference on every cold start.
• Never use MC_Home mode 0 (direct homing absolute) with an absolute encoder in production unless you deliberately want to override the mechanical reference. Mode 7 preserves the encoder's true absolute position while still providing the application's preferred zero reference.
• For multi-turn encoders with battery-backed turn counters, include the battery voltage in your preventive maintenance schedule. A depleted battery silently breaks multi-turn capability while leaving single-turn operation intact.
• Keep the encoder cable shield grounded at one end only. Splitting or floating the shield introduces noise that the technology object reports as intermittent EncFault flags.
• When scaling user units, compute the conversion factor once and store it in a constant tag. Inline scaling arithmetic in the application obscures the relationship between encoder increments and user units.

Field-Proven Caveats

Several recurring failure modes show up in commissioning and can be diagnosed from the symptoms alone:

• Mixing incremental and absolute configuration data: If the encoder itself is incremental but the technology object is set to absolute, the position drifts every power cycle. Always match the encoder's intrinsic type to the technology object setting.
• Modulo mismatch with the encoder's mechanical range: An SSI encoder with 4096 increments per turn combined with a modulo range of 360000 user units produces wrap-around at unexpected mechanical angles. Either match the modulo range to the encoder's physical range or scale to the application's needs.
• Fine resolution applied twice: If the encoder already provides its fine resolution in Gx_XIST1 and the technology object is configured with non-zero fine resolution, the position will appear doubled. Confirm the encoder's actual fine resolution before setting the technology object value.
• Direction inversion triggered by cable swap: Swapping the A and B signals (or their digital equivalents) on the encoder cable inverts the count direction. Configure the inversion parameter correctly and verify by monitoring the count direction during a test rotation.
• Configuration parameter silently ignored: A common symptom is that the engineer changes the absolute flag, sees the online view update, but the next power cycle resets the position. The change was never written to the offline project; it was a temporary online patch that the next STOP-to-RUN transition discarded.

Related Siemens Documentation

For deeper coverage of absolute encoder adjustment and the MC_Home instruction, refer to the following Siemens Industry Online Support entries:

• Absolute encoder adjustment with TO_ExternalEncoder: Siemens support article 109739589
• MC_Home instruction and homing modes: Siemens support article 109486133

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