Implementing PTE400 Command Interruption and Resumption Logic in PCS 7 APL

In SIMATIC PCS 7 Advanced Process Library (APL), replicating the classic PTE400 library "Befehlsfreigabe" (command enable / interruption) behavior on motor and valve blocks (MotL, VlvL) is achieved by utilizing the Permit (execution enable) input parameter combined with APL feature bits. In PTE400, when command enable drops to FALSE, the drive de-energizes while its operational state remains ACTIVE, resuming automatically when the signal recovers. In APL, wiring the enabling signal to Permit and configuring Feature.bit2 = 0 ("Do not reset commands for starting") and Feature.bit27 = 0 ensures that Automatic mode and target running states are preserved during interruption, with clear visual padlock indications on operator faceplates.

PTE400 Command Enable Philosophy vs Standard APL Interlocks

In legacy SIMATIC PCS 7 systems utilizing the classic PTE400 (Power & Process Engineering) library or customized CEMAT standards, drives and valves featured a distinct functional state known as Befehlsfreigabe (Command Enable / Command Interruption).

The operational principle of Command Interruption requires:
1. When the external enabling condition drops to FALSE (e.g., a process gate closes or an auxiliary ventilation fan stops), the motor de-energizes immediately.
2. However, the controller does NOT trip the drive into a latching fault state, and does NOT reset the supervisory sequence to Inactive.
3. The drive retains its ACTIVE target operating state in Automatic mode.
4. On the operator faceplate, an interruption symbol alerts the operator that the drive is waiting for command release.
5. As soon as the enable condition returns to TRUE, the drive automatically restarts without operator intervention.

APL Interlock Types: Permit vs Interlock vs Protect

In the Advanced Process Library, interlocking logic is partitioned into four distinct categories:

APL Interlock InputName / FunctionEvaluated Before Start?Evaluated During Running?Operating State ReactionPermitExecution EnableYes (Blocks start if FALSE)Yes (De-energizes output if FALSE)Non-latching interruption; auto-resumesIntlockOperating InterlockYes (Blocks start if FALSE)Yes (Trips motor to STOP)Dependent on Feature bits; can require resetProtIntProtection InterlockYes (Blocks start if FALSE)Yes (Emergency trip to STOP)Latching safety trip; requires operator reset

To replicate PTE400 Command Enable, the enabling signal must be wired to Permit, not to ProtInt or Intlock.

Configuring Feature Bits for Bumpless Interruption and Auto-Resumption

To ensure that MotL (FB1850) or VlvL (FB1899) does not abort its automatic running state when Permit drops:

Feature.bit2: Resetting the commands for starting the motor.
- Set Feature.bit2 = 0.
- When set to 0, if Permit drops and subsequently recovers, the internally queued start command from the automatic sequence remains active, allowing the motor to restart immediately.
Feature.bit27: Interlocking condition resets the operating state to default.
- Set Feature.bit27 = 0.
- Prevents the block from reverting to Manual mode or resetting internal target states.
Feature.bit30: Resetting depending on the operating mode.
- Set Feature.bit30 = 0 (or 1 depending on whether an explicit reset is desired during manual starts).

Operator Faceplate Visualization and Padlock Indicators

When Permit is utilized for command interruption:
* The standard APL motor faceplate displays an explicit Permit button on the standard view.
* When Permit.Value = 1, the padlock icon displays an open green padlock (Enabled).
* When Permit.Value = 0, the padlock icon transitions to a closed blue padlock (Interrupted / Waiting for Enable).
* Operators in the control room immediately understand that the motor is not faulted, but temporarily suspended by an active process condition.

Verification

To verify command interruption and auto-resumption:
1. In CFC, open the chart containing MotL online in test mode.
2. Put the motor into Automatic mode (AutModLi = 1) and command StartAut = 1. Verify MotL.Start.Value = 1.
3. Drop the command enable input Permit.Value = 0.
4. Confirm that MotL.Start.Value transitions to 0 (motor stops), but the block remains in Automatic mode without generating trip alarms.
5. In WinCC Runtime, verify that the faceplate shows the closed blue padlock on the Permit button.
6. Restore Permit.Value = 1; confirm that the motor automatically starts (Start.Value = 1) without operator reset.

Limitations and Operational Precautions

Never route personnel safety or machine-critical protection devices (such as emergency stop pull cords, motor overloads, or high-high pressure switches) through Permit. Automatic restarting after an interruption presents extreme physical hazards if used for safety trips. Critical safety protection devices must strictly be connected to ProtInt (Protection Interlock), which enforces mandatory latching and manual operator acknowledgment.

Engineer FAQ

Can Command Interruption be implemented in Manual mode as well?

Yes. If the motor is started manually and Permit drops, the motor de-energizes. Whether it restarts automatically when Permit returns in Manual mode depends on Feature.bit2 and Feature.bit30.

What is the difference between Permit and Intlock in APL?

Permit is designed as an activation condition that does not force latching alarms. Intlock is an operating interlock that can be configured to latch and generate process alarms.

Can the label 'Permit' on the faceplate be renamed to 'Befehlsfreigabe'?

Yes. In Process Object View under the 'Block Icons / Texts' tab, the interlock text attribute for Permit can be customized to any facility-specific label.