Configuring Auto-Reset for MotL Blocks in SIMATIC PCS 7 V8.2

Overview

The MotL (Motor, two feedback signals for speed and current) block from the SIMATIC PCS 7 Advanced Process Library (APL) provides three distinct command inputs that govern how a motor reacts to fault, inhibit, and pre-condition signals: Protect, Permit, and Interlock (Intlock). Operators frequently encounter a situation where a motor is tripped by an upstream MCC fault and, after the fault is acknowledged and physically cleared, the motor will not restart until an operator presses the Reset pushbutton in the faceplate. This behavior is not a bug: it is the documented contract of the Protect input. The remedy is to use the correct input for the intended safety function and, where appropriate, adjust the block feature bits to enable automatic restart once the inhibit condition has cleared.

This article explains the difference between the three protection commands, identifies which input to use to obtain auto-reset behavior, and walks through the configuration changes required in PCS 7 V8.2 with the APL installed. The procedures reference the standard PCS 7 Advanced Process Library Function Manual (Entry ID 109739941) and the MotL Functions Reference (Entry ID 109812806).

The Three Protection Commands on MotL

Every MotL instance exposes three separate Boolean input structures on the CFC chart. Each performs a distinct role within the IEC 61131-3 / ISA-88 batch and safety hierarchy, and each behaves differently when its signal returns to healthy.

The contract is explicitly stated in section 6.2.3 (Interlocks) and section Feedbacks 1.1.3.4 ff of the APL function manual:

"When you use the 'Intlock' command, you don't need to reset the motor first. After the Intlock is healthy again ('1' signal), the motor will start automatically when the automatic command is still '1'."

Conversely, the Protect input requires a deliberate operator action through the faceplate Reset pushbutton before the motor becomes startable again. This is the correct behavior for safety functions – the operator must consciously confirm the cause of the trip has been investigated and cleared.

Root Cause: Why the Motor Stays Off After an MCC Fault

Typical MCC faults (contactor overload, breaker trip, fuse failure) are wired to a digital input that maps to the Protect structure of the MotL block. When the MCC fault clears, the Protect input returns to 1, but the MotL block has internally latched the trip state. The latch is cleared only by:

1. An operator pressing the Reset pushbutton in the WinCC faceplate, or
2. A rising edge on the Reset input at the CFC block, or
3. Forcing a reset via the standard reset input of the upstream operator panel.

If the application logic should restart the motor without operator intervention once the MCC reports healthy, the MCC fault must be re-mapped to the Intlock input – not Protect. The Interlock input has no latched trip state; it is evaluated continuously and the motor is allowed to start as soon as it returns to 1 while an automatic start command is present.

Prerequisites

Before reconfiguring the block, confirm the following:

• PCS 7 V8.2 (or compatible V8.2 SPx) installed with the matching Advanced Process Library version that ships on the PCS 7 V8.2 media. APL revisions are version-locked to the PCS 7 release.
• The CFC chart containing the MotL instance is open in the engineering station.
• The SFC / SCL compile and download procedures are coordinated with operations; live re-wiring of protection commands on a running plant must be performed during a maintenance window.
• The signal source for the MCC fault is identified in the I&C hardware documentation (which DI module, which channel, which terminal).
• WinCC Explorer is available for faceplate reconfiguration (if message classes are touched).
• Cross-reference the actual parameter set against the official APL Function Manual (Entry ID 109739941) – chapter 6 documents the MotL block and its command structures.

Step-by-Step: Enabling Auto-Reset on a MotL Block

The procedure below converts an existing Protect-based shutdown to an Interlock-based auto-restart. It assumes the MCC fault signal is a clean Boolean that returns to 1 once the MCC is healthy.

Step 1 – Identify the Fault Source

Open the CFC chart and locate the MotL block. Inspect the input named Protect on the block. The signal currently wired to it is the source of the latching behavior. Right-click the input and select "Go to interconnection point" to trace the origin in the signal database.

Step 2 – Move the Signal to the Interlock Input

1. Delete the connection to the Protect input.
2. Wire the same Boolean signal to the Intlock input of the same MotL block.
3. Compile the CFC program (Charts → Compile → Charts as program).
4. Download to the AS in Stop / Run mode using the standard PCS 7 download dialog.

Step 3 – Verify Behavior in Automatic Mode

With the input rewired:

• Force the Intlock input to 0 to simulate the MCC trip. The motor must stop and the faceplate must show a process interlock alarm (not a Protect trip).
• Force the Intlock input back to 1. Within the configured monitoring time the motor must start automatically, provided the automatic command is still latched at 1.
• Confirm the Reset pushbutton is no longer required for this path.

Step 4 – Adjust Feature Bits (If Required)

The MotL block exposes two Feature inputs (Feature.Bit1 through Feature.Bit8 on each Feature structure) that control start/stop, reset, and feedback behavior. The following bits are commonly relevant for auto-reset and pushbutton operation. Always verify the exact assignment in the APL version installed in your plant – the values below are documented in the MotL function reference (Entry ID 109812806) and may be subject to revision.

Per the MotL function reference: "In pushbutton operation (Bit 4 = 0) the automatic commands in 'automatic' mode are latching, in other words StartAut, StopAut can be reset to 0". When Feature Bit 1 Bit 4 is set to 1, the block operates in continuous automatic mode: the StartAut and StopAut inputs are evaluated as level-sensitive, which is the normal condition for auto-restart logic.

Step 5 – Update the WinCC Faceplate (Optional)

When the protection source is moved from Protect to Interlock, the faceplate status icon and message class should also be reviewed:

1. Open the WinCC graphics designer and load the relevant faceplate from the PCS 7 master library.
2. Confirm the status icon for the motor now displays the interlock symbol rather than the protect symbol.
3. Re-compile the OS and perform an OS download.

Pushbutton vs. Automatic Mode Behavior

The MotL block supports two command-input modes selected by Feature Bit 1, Bit 4. The choice directly impacts how automatic commands and the Reset button behave.

For an auto-restart scheme, Automatic mode (Bit 4 = 1) is the correct setting. In Pushbutton mode (Bit 4 = 0), the automatic commands latch in the on state, and the operator Reset button is used to clear the latch – this is rarely the desired behavior for an auto-restart interlock.

Handling the Permit Input Alongside Interlock

The Permit input is evaluated only while the motor is stopped. Once the motor is running, a transition of Permit from 1 to 0 does not stop the motor. This is the correct ISA-style "pre-start permissive" semantics. For applications that require both a one-time pre-start check and a continuous process interlock:

• Use Permit for the pre-start check (e.g., maintenance bypass key-switch).
• Use Intlock for the continuous process interlock (e.g., low flow, MCC healthy feedback).

Both inputs can be active simultaneously. The motor will start only when Permit = 1, Intlock = 1, and the start command is present. If either goes to 0 while running, the motor stops, and Intlock = 1 will permit an automatic restart.

Commissioning Checklist

Common Pitfalls

• Wiring the MCC fault to Protect and then asking for auto-reset. The block will never auto-reset on Protect. Re-wire to Intlock.
• Modifying Feature Bit 1.4 without recompiling / re-downloading the chart. Feature bit changes do not take effect until the program is downloaded to the AS.
• Replacing a genuine safety (SIL) signal with Intlock. Do not. Safety functions must always be wired to Protect and require operator reset by procedure.
• Forgetting to verify the StartAut signal is still latched at 1 after the fault clears. If StartAut is pulsed or momentary, the auto-restart will not occur even with Intlock healthy. Use Automatic mode (Bit 1.4 = 1) for level-sensitive evaluation.
• Mixing up Permit and Interlock semantics. Permit is evaluated only when the motor is stopped. Wiring a continuous process permissive to Permit will not interrupt a running motor – that is the Intlock's job.
• Using the Operator Reset button to clear an Intlock-based shutdown. Reset only clears latched trips (Protect). For Intlock, the automatic restart is the normal path; the operator can always issue a fresh Start command from the faceplate.

Verification Procedure

1. Open the WinCC runtime and navigate to the affected motor faceplate.
2. Confirm the motor is in the desired Automatic operating mode and the automatic start command is latched at 1.
3. From the maintenance / engineering station, force the Intlock input to 0 in the CFC online view. The motor must stop and the faceplate must show the interlock state.
4. Force the Intlock input back to 1. Within the configured feedback-monitoring time the motor must start automatically.
5. Repeat the test with the actual field signal source (e.g., simulated MCC fault) to validate end-to-end behavior.
6. Document the test result and the configuration (input source, feature bit values) in the plant's PCS 7 change-log.

Related Documentation

Refer to the following Siemens official entries for the authoritative description of the MotL block, its inputs, and feature bits:

• PCS 7 Advanced Process Library – Function Manual (Entry ID 109739941) – chapters 6.2.3 (Interlocks) and Feedbacks 1.1.3.4 onward.
• MotL Functions – SIMATIC PCS 7 Advanced Process Library (Entry ID 109812806) – function reference for the MotL block including feature-bit behavior.