Suppressing APL MotL Start-Stop Pulse Outputs in Local Mode on PCS 7

In SIMATIC PCS 7 V8.1, the Advanced Process Library (APL) MotL (Motor - Large) block continues generating impulse outputs (P_Start and P_Stop) when feedback run (FbkRun) toggles while in Local mode (LocalLi = 1, LocalSetting = 2) because the internal state machine calculates transitional command pulses to maintain internal alignment. In PCS 7 V9.0 and later, Siemens introduced Feature.bit20 ("Disable calculation of impulse controls in local setting 2 and 4"), which forces P_Start and P_Stop to static zero during local operation. In PCS 7 V8.1 installations where Feature.bit20 is unavailable, engineers must suppress pulse commands by gating P_Start and P_Stop with the inverted local state (NOT LocalAct.Value) in CFC before routing signals to digital output driver blocks.

MotL Operating Modes and LocalSetting Configurations

In the SIMATIC PCS 7 Advanced Process Library (APL), the standard motor block MotL (FB1850) supports three primary operating modes: Automatic mode, Manual mode, and Local mode. When switched to Local mode—either via the operator faceplate or through the interconnection input LocalLi = 1—control of the motor is transferred from the supervisory automation program to a local control station located adjacent to the physical equipment.

The exact behavior of Local mode is governed by the configuration parameter LocalSetting:

LocalSetting ValueMode Name / ConfigurationControl Command OriginFeedback Tracking BehaviorLocalSetting = 1Local Control via SwitchDriven by block outputs based on local switchMotor commanded via block outputs Start / StopLocalSetting = 2Local Control via PushbuttonsField signals wired to LocalOp, LocalStart, LocalStopBlock tracks motor state; outputs pulses during feedback transitionsLocalSetting = 3Local Control via Switch (No Interlocks)Local switch controls motor directly in fieldBlock tracks motor state without evaluating interlocksLocalSetting = 4Local Control via Pushbuttons (No Interlocks)Local pushbuttons wired in field; interlocks inactiveBlock tracks motor state; impulse calculation occursLocalSetting = 5Local Tracking ModeLocal hardware controls motor directlyBlock passively tracks run state; commands forced to zero

In many field architectures utilizing LocalSetting = 2, the local stop and start pushbuttons are wired directly to the motor starter or Variable Frequency Drive (VFD) in the field, while the PLC only receives the run feedback (FbkRun). Engineers expect that the PLC block will remain entirely passive in this mode, issuing no output commands.

Mechanism of Pulse Calculation During Local State Transitions

In PCS 7 V8.1 APL implementations of MotL, the block state machine evaluates state changes in order to ensure bumpless transfer when transitioning from Local mode back to Manual or Automatic mode.

When the motor is started locally at the field station, the physical contactor closes and asserts FbkRun = 1. In PCS 7 V8.1, the internal logic detects the rising edge of FbkRun and calculates an internal start event, which momentarily asserts the pulse output P_Start = 1 (impulse start command). Conversely, when the motor stops and FbkRun drops to 0, the block momentarily asserts P_Stop = 1.

If the PLC's P_Start and P_Stop outputs are wired to interposing relays or field contactor coils in parallel with the local field circuit, these spurious PLC pulses can cause unexpected contactor latching, double-triggering of VFD run circuits, or conflicting command signals.

Feature.bit20 Functionality in PCS 7 V9.0 and Later

Recognizing this limitation in earlier library releases, Siemens introduced a dedicated feature bit in PCS 7 V9.0 APL (APL V9.0 SP1 and later):

Feature.bit20: Disable calculation of impulse controls in local setting 2 and 4.

When Feature.bit20 is parameterized to 1:
1. If the block is operating in Local mode with LocalSetting = 2 or LocalSetting = 4, the calculation of impulse outputs is completely inhibited.
2. When FbkRun transitions from 0 to 1, the static output Start transitions to 1 to reflect motor tracking, but the pulse output P_Start remains strictly 0.
3. When FbkRun transitions from 1 to 0, the static output Start resets to 0, but the pulse output P_Stop remains strictly 0.

Engineering Workaround for PCS 7 V8.1 and Earlier

In PCS 7 V8.1 and earlier library versions where Feature.bit20 does not exist within the Feature parameter structure, engineers must suppress pulse commands directly in the Continuous Function Chart (CFC):

Identify the Active Local State: The MotL block outputs the boolean structure LocalAct (LocalAct.Value = 1 when the block is actively operating in Local mode).
Implement Logic Gating in CFC:
- Insert standard boolean logic blocks from the PCS 7 Basic Library or Elementary library (e.g., And02 / Not01 from SIMATIC H-Lib or BitLogic).
- Connect MotL.LocalAct.Value to an inverter (Not01).
- Feed the inverted signal into one input of an And02 block, and connect MotL.P_Start.Value to the second input.
- Route the output of the And02 block to the digital output driver block (Pcs7DiOu).
- Replicate the identical gating structure for the stop pulse output P_Stop.
Alternative: Utilize LocalSetting = 5: If local control is entirely hardwired in the electrical switchgear and the PLC only tracks status, configure LocalSetting = 5. In setting 5, the block operates purely in tracking mode, suppressing all command outputs.

Verification

To verify that pulse commands are successfully suppressed during local operation:
1. In the CFC Editor, open the chart containing the MotL instance and switch to online test mode.
2. Set the motor to Local mode (LocalLi = 1 or via faceplate). Verify LocalAct.Value = 1.
3. Force or toggle the run feedback input FbkRun from 0 to 1.
4. Verify that MotL.Start.Value updates to 1 for status indication, but the gated output to the field digital output channel remains strictly 0.
5. Toggle FbkRun from 1 to 0; confirm that no stop pulse is transmitted to the physical output module.

Limitations and Operational Precautions

When gating P_Start and P_Stop in CFC, ensure that safety interlocks are not bypassed. While Local mode allows field-level operation, critical interlocks (such as Intlock, SupInt, or emergency stops wired to Permit) must be evaluated according to plant safety requirements. In LocalSetting = 2, standard interlocks remain active, whereas in LocalSetting = 4, interlocks are disabled. Always verify interlock functionality during factory acceptance testing.

Engineer FAQ

Why does MotL issue a start pulse when the motor is started locally in V8.1?

In PCS 7 V8.1, the internal state machine calculates command pulses upon feedback state changes to keep internal status registers synchronized for bumpless mode switching.

Is Feature.bit20 available if I upgrade from PCS 7 V8.1 to V9.0?

Yes. Upgrading the APL library to V9.0 or later provides Feature.bit20, allowing software suppression of impulse controls in Local settings 2 and 4 without CFC logic workarounds.

What is the difference between LocalSetting 2 and LocalSetting 5?

In LocalSetting = 2, the block evaluates local start/stop commands and interlocks. In LocalSetting = 5, the block acts purely as a passive tracking block, ignoring commands and disabling all command outputs.