Automating Control Module Mode Initialization to Auto on CPU Startup in PCS 7

To force all Control Modules (CMs: valves, motors) to automatically initialize in Automatic mode upon CPU startup or fresh downloads while preserving manual operator override capability (ModLiOp = 0), engineers must implement a transient mode transfer pulse. Setting Feature.Bit0 = 1 retains previous modes across warm restarts but cannot force Auto mode after fresh block downloads. The optimal architecture uses the STARTING sequence of ISA-88 Sequential Function Charts (SFC / SFC Types) or an OB100 startup pulse block in SCL. During the initialization phase, the logic temporarily forces ModLiOp = 1 and AutModLi = 1, and upon entering RUN, releases ModLiOp = 0, leaving all equipment in Auto mode with full manual operator authority.

The Operational Conflict: Auto Initialization vs Operator Manual Authority

During plant commissioning and testing in SIMATIC PCS 7, software engineers execute numerous full or changes-only downloads to S7-400 controllers (e.g. CPU 410-5H 6ES7410-5HX08-0AB0 or CPU 416-3 PN/DP 6ES7416-3ES07-0AB0). A persistent operational challenge arises: newly downloaded or initialized Advanced Process Library (APL) technological blocks (MotL, VlvL, PIDConL) default to Manual mode (ManMod).

While setting Feature.Bit0 = 1 (Set startup characteristics) instructs the block to retain its last valid operating mode across an OB100 Warm Restart, it does not solve two common commissioning scenarios:
1. Following a full download or initialization, historical memory is cleared, defaulting all blocks to Manual.
2. Commissioning engineers must manually click through hundreds of WinCC faceplates to switch valves and motors back to Auto mode.

Attempting to permanently hardwire AutModLi = 1 requires setting ModLiOp = 1 (Link mode), which completely locks out the WinCC operator faceplate, preventing operations personnel from taking manual control during plant emergencies.

APL Operating Mode Control: ModLiOp, AutModLi, and ManModLi Logic

In the Siemens Advanced Process Library, operating mode selection is governed by a strict priority architecture:

ModLiOp (Mode Linked vs Operator Priority):
When ModLiOp = 0 (Operator mode), the block responds exclusively to operator commands from the WinCC faceplate (AutModOp, ManModOp). Input AutModLi is completely ignored by the block algorithm!
When ModLiOp = 1 (Linked mode), the block responds exclusively to program logic via inputs AutModLi (switch to Auto) and ManModLi (switch to Manual). Operator mode buttons on the faceplate are greyed out and locked.

Therefore, to initialize all equipment into Automatic mode while leaving ModLiOp = 0 for normal operation, the control logic must execute a dynamic handshake: temporarily assert ModLiOp = 1 and AutModLi = 1 for a few controller execution cycles, verify that the block switches to Auto (AutAct = 1), and then release ModLiOp back to 0.

Comparison of Control Module Auto-Initialization Methods

The following table compares different architectural methods for automating Control Module mode initialization:

Initialization MethodImplementation LocationOperator Lockout?Effective After Full Download?Engineering ComplexitySFC Starting Sequence (ISA-88)Unit / Equipment Phase SFCNo; released in RUN stepYes; executes whenever SFC startsLow; standard PCS 7 design patternSCL Startup Pulse Function BlockOB35 with OB100 handshakeNo; pulse expires after ~2 secondsYes; executes on every controller restartModerate; requires custom SCL blockFeature.Bit0 = 1 (Retentive Mode)Block parameterizationNo; operator retains controlNo; resets on initial download/memory wipeLowest; standard APL feature bitPermanent Link Mode (ModLiOp = 1)Hardwired in CFCYes; operator completely locked outYesUnacceptable; violates plant safety guidelinesWinCC Script Bulk SwitchGlobal Script VBS on OSNo; runs from HMI clientYesHigh; creates network burst on Terminal Bus

Step-by-Step Procedure to Implement the SFC Starting Mode Transition

In standardized PCS 7 plants adhering to ISA-88 batch or continuous sequencing, the SFC Starting sequence is the industry-recommended pattern:

Open the Equipment Module or Unit SFC:
- Open the SFC or SFC Type coordinating the process unit in the SFC Editor.
Configure Actions in the STARTING Step:
- In the SFC state machine, navigate to the STARTING sequence step (executed prior to RUN).
- In the Step initialization action, add commands for all subordinate CMs (valves, motors, pumps):
- CM_Valve_01.ModLiOp = 1
- CM_Valve_01.AutModLi = 1
- CM_Pump_01.ModLiOp = 1
- CM_Pump_01.AutModLi = 1
Configure Step Transition to RUN:
- In the transition condition leading from STARTING to RUN, verify that all equipment has successfully acquired Auto mode:
- CM_Valve_01.AutAct = 1 AND CM_Pump_01.AutAct = 1
Release Operator Control in the RUN Step:
- In the initialization action of the RUN step, immediately release linked mode:
- CM_Valve_01.ModLiOp = 0
- CM_Valve_01.AutModLi = 0
- CM_Pump_01.ModLiOp = 0
- CM_Pump_01.AutModLi = 0
- All control modules are now operating in Automatic mode, and operator manual controls on WinCC faceplates are 100% unlocked.

Alternative SCL Startup Pulse Implementation via OB100

For continuous plants that do not utilize SFCs, a dedicated SCL startup block provides identical functionality:

Create SCL Block with OB100 Detection:
- Create a function block (e.g. FB520 - CM_AutoInit) executed cyclically in OB35 (100ms cyclic interrupt).
- Use system function RD_SINFO (SFC6) or an OB100 startup flag to detect when the CPU has transitioned into RUN.
Generate Transient Pulse:
- Upon detecting startup, set output Pulse_ModLiOp = TRUE and Pulse_AutModLi = TRUE for a duration of 3.0 seconds (using a standard IEC timer or cycle counter).
- After 3.0 seconds, reset both outputs to FALSE.
CFC Interconnection:
- In CFC, connect Pulse_ModLiOp to ModLiOp and Pulse_AutModLi to AutModLi across the required technological blocks.

Verification

Verify initialization behavior in a staging or simulation environment:
1. Place all valves and motors in Manual mode via their WinCC faceplates.
2. Start the coordinating SFC or trigger a planned CPU restart.
3. Observe the SFC transition: verify that during STARTING, ModLiOp and AutModLi assert, causing faceplates to transition to Auto mode.
4. Confirm that once the sequence enters RUN, ModLiOp returns to 0.
5. Open an individual valve faceplate in WinCC Runtime and click Manual Mode. Verify that the block allows the operator to switch freely to Manual without being blocked by program interlocks.

Limitations and Operational Precautions

Process Hazard of Simultaneous Equipment Energization: Forcing dozens of motors and pumps to Automatic mode simultaneously can trigger massive electrical inrush currents or unintended fluid hydraulic hammer. Stagger initialization sequences where necessary.
Interlock Prioritization: Operating mode transitions must never bypass technological interlocks (Intlock, Protect). If an interlock is tripped, the block will refuse Auto mode and remain in a safe state.
Faceplate Lock Indication: Operators should be informed that during the brief 2-second startup window, faceplates will temporarily show the 'Linked' padlock symbol before returning to operator control.

Engineer FAQ

Why does connecting AutModLi have no effect when ModLiOp is 0?

In the Siemens APL architecture, ModLiOp = 0 assigns absolute priority to the operator faceplate. In this state, the block completely ignores inputs AutModLi and ManModLi by design to prevent programmatic logic from conflicting with manual operator control.

What does AutAct indicate on an APL block?

AutAct is a digital output indicating that the technological block is currently actively operating in Automatic mode. It is used in transition logic to confirm that mode changes have executed before progressing recipe sequences.

Can I use this method to switch blocks to Manual mode instead of Auto?

Yes. By pulsing ModLiOp = 1 and ManModLi = 1 (instead of AutModLi), you can automatically force equipment into Manual mode upon specific shutdown sequence events.