Configuring Automatic-to-Manual Stop Transition on APL MotL and VlvL Blocks: Overriding Bumpless Transfer

In SIMATIC PCS 7 Advanced Process Library (APL), motor and valve blocks (MotL, VlvL) execute a bumpless transfer by default when switching from Automatic to Manual mode, tracking the active automatic run command so the field device does not abruptly shut down. To configure a drive or valve to immediately transition to the OFF (stopped/closed) state when switched to Manual mode, engineers must implement edge-triggered reset logic in CFC. Detect the rising edge of the manual mode output (ManAct) using an R_TRIG or Pulse block, and connect the pulse to StopMan (for MotL) or CloseMan (for VlvL). This immediately de-energizes the device upon entering Manual mode.

Bumpless Transfer Mechanics in APL MotL and VlvL Blocks

In continuous process automation, unexpected equipment stops can trigger pressure surges, thermal shocks, or process trips. Consequently, the SIMATIC PCS 7 Advanced Process Library (APL) is designed with bumpless changeover as the fundamental default behavior. When a technological block such as MotL (FB1850) or VlvL (FB1899) is running in Automatic mode (AutAct = 1) and the operator or a sequence switches the block to Manual mode (ManAct = 1), the block algorithm tracks the current output state into the manual command registers. As a result, a running motor remains running, and an open valve remains open.

Why APL Retains the Running State During Auto-to-Manual Transitions

The internal state machine of MotL executes the following logic during mode change:

While in Automatic mode, the output Start is driven by StartAut.
When the mode shifts to Manual, the block transfers StartAut into internal manual command state StartMan.
This ensures that the transition itself does not create a process disturbance.

However, in specific plant operating philosophies (such as hazardous chemical transfer lines, packaging units, or burner purging systems), the customer safety requirement mandates that switching to Manual implies the operator must take conscious, affirmative action to restart or restroke the unit. Leaving the device running in Manual mode is considered an operational hazard.

The Role of Feature Bits in Mode Change and Command Resetting

APL provides several Feature bits that influence mode switching:

Feature.Bit2 (Resetting commands for changing the mode): Resets incoming control signals (AutModLi, ManModLi) to prevent conflicting latched inputs, but does not alter the underlying drive state.
Feature.Bit17 (Bumpless switchover to automatic mode): Controls the reverse transition—preventing switchover from Manual to Automatic unless manual and automatic commands match.
Feature.Bit1 (Reaction to the out of service mode): Resets commands only when entering Out of Service.

Because no single Feature bit forces a motor to trip simply upon entering Manual mode, external CFC logic coordination is standard engineering practice.

Implementing Automatic-to-Manual Stop Logic in CFC Charts

To force an immediate stop when switching to Manual mode:

Monitor Mode Outputs: Use the block output ManAct (Manual Mode Active) and AutAct (Automatic Mode Active).
Detect Positive Transition: Connect ManAct to the input of an edge detection block (R_TRIG or standard library pulse block Pulse).
Condition with Prior Auto State: To ensure this only triggers when transitioning from Automatic mode (and not on initial CPU startup), gate the edge detection with AutAct stored from the preceding cycle.
Pulse the Stop Command: Connect the resulting single-cycle pulse directly to input pin StopMan (on MotL) or CloseMan (on VlvL).
Upon entering Manual mode, the pulse fires for one execution cycle, causing the block algorithm to set Start = 0 (or Open = 0).

Comparative Behavior Matrix: Standard vs Forced-Stop Auto-to-Manual Transfer

The following table compares runtime behaviors between standard APL and the forced-stop CFC circuit:

Operational ScenarioStandard APL MotL/VlvLForced-Stop CFC CircuitSafety / Process ConsequenceMotor Running in Auto -> Switch to ManualMotor continues running (Start = 1)Motor de-energizes immediately (Start = 0)Prevents unattended manual operationValve Open in Auto -> Switch to ManualValve remains open (Open = 1)Valve strokes to closed (Open = 0)Isolates process pipe automaticallyMotor Stopped in Auto -> Switch to ManualMotor remains stopped (Start = 0)Motor remains stopped (Start = 0)No behavioral differenceOperator Restart in ManualOperator clicks Stop/StartOperator must click StartManForces intentional operator actionInterlock Trip during Mode SwitchInterlock trips motorInterlock trips motor + forced stop pulseFailsafe state maintained

Step-by-Step CFC Configuration for Drive and Valve De-energization

Follow this configuration procedure in the CFC chart:

Open the CFC chart containing MotL (or VlvL).
Insert an R_TRIG (Rising Edge Detection) block from the Standard Library or CFC Elements library.
Interconnect MotL.ManAct to R_TRIG.CLK.
Insert a Boolean AND block. Connect R_TRIG.Q to Input 1. Connect a helper memory or previous cycle AutAct to Input 2.
Interconnect the output of the AND block to MotL.StopMan (for motors) or VlvL.CloseMan (for valves).
If the block is controlled via interconnectable inputs (ModLiOp = 1), ensure StopLi is asserted concurrently with ManModLi by the coordinating SFC or logic block.
Compile CFC charts with changes and download to the AS controller.

Verification

Verify the forced-stop behavior in CFC Test Mode and WinCC Runtime:

Start in Auto: Put the motor in Auto mode and assert StartAut = 1. Confirm the motor starts and feedback confirms running (FbkRun = 1).
Switch to Manual via Faceplate: Open the MotL faceplate in WinCC. Click the Manual mode button.
Verify Immediate Stop: Confirm that output Start drops to 0 immediately, the motor run contact opens, and the faceplate indicates Stopped.
Manual Restart Verification: In the faceplate, click Start. Verify the motor can be started manually without residual error or latched stops.
Repeat for Valves: Perform the identical test on VlvL, confirming the valve drives to the closed position.

Limitations and Operational Precautions

Keep these operational precautions in mind:

Unintended Tripping During Nuisance Toggles: If an operator accidentally clicks Manual on a critical running pump or compressor, the unit will trip immediately. Ensure operations teams are fully trained on this deviation from standard APL behavior.
SFC Coordination: If a Sequential Function Chart (SFC) manages the drive, ensure that unexpected manual de-energization transitions the SFC into a Held or Paused state rather than causing step transition timeouts.
Commercial Accuracy Disclaimer: Software order numbers (MLFBs), package structures, and hardware capacity prerequisites vary across PCS 7 versions and hardware platforms. System integrators must confirm exact software entitlements and licensing requirements against the official Siemens Catalog CA 01 and obtain an authorized Siemens distributor quotation prior to purchasing.

Engineer FAQ

Can the operator restart the motor immediately after the forced stop?

Yes. The stop pulse only lasts for a single execution cycle (e.g. 100 ms). Once the pulse clears, the operator can click StartMan on the faceplate normally.

Does pulsing StopMan violate APL architecture standards?

No. Connecting external logic pulses to StopMan or CloseMan is a supported method for implementing project-specific operational interlocks and safety policies.