Configuring Operator Step Transitions and Step Control Modes in PCS 7 SFC Types (Auto vs Manual Constraints)

In SIMATIC PCS 7, an operator cannot bypass or force an unsatisfied step transition while an SFC Type or SFC chart is running in Automatic mode. By process control safety design, Automatic mode enforces strict adherence to configured technological interlocks and process criteria. Step control modes that incorporate operator intervention-such as 'T or O' (Transition OR Operator confirmation), 'T and O' (Transition AND Operator confirmation), or 'O' (Operator confirmation only)-can only be activated or altered when the SFC is switched to Manual mode. If automatic sequence progression requires operator confirmation or conditional override during normal operation, engineers must configure technological operator request flags (e.g., via OpRead / OpB block contacts) directly within the transition logic in the SFC Type editor.

Executive Summary

Sequential Function Charts (SFC Types and SFC instances) execute mission-critical sequential logic in SIMATIC PCS 7 plants. During plant operations, production scenarios arise where an operator needs to acknowledge a step completion or force a transition forward when field sensor conditions are temporarily unavailable. However, when operators open the standard SFC faceplate while the sequence is executing in Automatic mode, they find that step control options such as 'T or O' or manual step advance buttons are locked out. This article explains the architectural safety principles separating Automatic and Manual execution in PCS 7 SFCs, details the five standard step control modes, and provides proven engineering methodologies for integrating operator confirmation directly into automated transition logic.

Architecture and Fundamental Mechanics

SFC Execution Architecture and Step Control Mechanics:

Operating Modes and Safety Boundaries:
SIMATIC PCS 7 SFC architecture establishes rigid boundaries between operating modes:
1. Automatic Mode (AUTO): The sequence is driven strictly by programmed automation logic or supervisory batch recipes (SIMATIC BATCH). Transitions are evaluated exclusively based on AS process conditions. Manual step forcing, jumping, and step mode overrides are locked out to prevent accidental process hazards or out-of-sequence actuator operation.
2. Manual Mode (MAN): The sequence is controlled by human operating personnel. The operator can pause, resume, halt, jump to arbitrary steps, or force transition execution.
Step Control Modes in SFC:
SFC sequencers support five standardized step control modes:
- T (Transition only): Step advances automatically when transition conditions evaluate to TRUE. Mandatory in AUTO mode.
- T or O (Transition OR Operator): Step advances if either transition conditions become TRUE OR the operator confirms.
- T and O (Transition AND Operator): Step advances only when transition conditions are TRUE AND the operator confirms.
- O (Operator only): Step advances solely upon operator confirmation, completely ignoring process transition conditions.
- T / T and O (Step-Specific): Individual steps define whether operator acknowledgment is required.
Crucial Product Rule: As per Siemens PCS 7 design, changing the step control mode from T to T or O, T and O, or O is restricted exclusively to Manual mode. In Automatic mode, the step control mode is fixed to T.
Engineering Operator Interaction in Automatic Sequences:
When an automatic sequence requires operator approval or conditional override, engineers should not attempt to switch step control modes. Instead, the transition condition itself should be engineered to include a technological operator request flag (e.g. an OpB or OpRead block contact) that an operator can toggle from WinCC.

Step-by-Step Implementation Procedure

Performing Manual Step Advance via SFC Faceplate (Manual Intervention):
* If a process obstruction requires an operator to force advance a step:
* Open the SFC faceplate in WinCC Runtime.
* Switch operating mode from Auto to Manual (requires Process Controlling user authorization).
* In the Step Control dropdown, select T or O or O.
* Click the Advance (Step >) confirmation button on the faceplate toolbar.
* The active step clears immediately and execution proceeds to the next step.
* Return operating mode to Auto once the sequence is past the obstruction.
Engineering Operator Confirmation into SFC Type Transition Logic (Automatic Mode):
* Open the Master Data Library in SIMATIC Manager.
* Open the SFC Type in the SFC Type Editor.
* In Characteristics -> Inputs/Outputs, create a new Boolean parameter: OpReq_NextStep (Direction: IN, Attribute: S7_m_c := 'true').
* Open the sequencer and select the Transition following the hold step.
* Configure the transition logic using Boolean operators:
- For Optional Operator Bypass: Transition := (Process_Temp_OK = TRUE) OR (OpReq_NextStep = TRUE);
- For Mandatory Operator Confirmation: Transition := (Process_Temp_OK = TRUE) AND (OpReq_NextStep = TRUE);
Auto-Resetting Operator Request Flags in Target Step Actions:
* In the Target Step following the transition, open the Action Editor.
* Under the Initialization or Processing tab, add an assignment action:
OpReq_NextStep := FALSE;
* This guarantees that the operator request bit automatically resets upon advancing, preventing unintended fall-through in subsequent cycles.
WinCC Faceplate Integration:
* In Graphics Designer, place a standard APL prompt block icon (e.g. OpDi01) or button linked to OpReq_NextStep.
* Configure operator authorization level 5 (Process Controlling) on the control element.
Compile and Test Sequence:
* Save and compile the SFC Type, update block types in the multiproject, and perform OS compilation.
* In runtime test mode, confirm that triggering the operator prompt advances the transition while the SFC remains in Auto.

Verification

Verify SFC Step Control and Transition Behavior:
1. In the WinCC SFC faceplate, confirm that when mode is set to 'Auto', the Step Control selector is greyed out and fixed to 'T'.
2. Switch mode to 'Manual'; confirm that Step Control options ('T or O', 'T and O', 'O') become selectable.
3. In Automatic mode, simulate an unsatisfied process condition; verify that toggling the engineered OpReq_NextStep bit successfully evaluates the transition to TRUE and advances the sequence.

Limitations and Operational Precautions

Operational Precautions and Process Safety Constraints:
- Never engineer operator bypass logic that overrides critical personnel or environmental safety interlocks. Emergency shutdowns and safety interlocks must be hardwired or implemented in certified fail-safe controllers (S7 F-Systems) outside the SFC sequencer.
- Forcing step transitions in Manual mode bypasses normal step exit actions and transition monitoring times; verify field equipment status before commanding manual advances.

Engineer FAQ

Why can't operators change step control modes in Automatic mode?

Automatic mode enforces programmed process interlocks without human deviation. Step control modes requiring operator confirmation (T or O, O) can only be activated when the SFC is explicitly placed into Manual mode.

What does the 'T or O' step control mode do?

'T or O' (Transition OR Operator) advances the active step if either the programmed process conditions evaluate to TRUE OR the operator clicks the advance confirmation button.

What is the difference between 'T and O' and 'O'?

'T and O' requires both process conditions to be met AND operator confirmation. 'O' advances solely on operator confirmation, completely ignoring process conditions.

How do I make an automatic sequence wait for operator approval?

Incorporate a dedicated operator request parameter (e.g. OpReq with S7_m_c := 'true') into the transition logic, and reset the bit in the actions of the subsequent step.

Can SIMATIC BATCH command SFC step transitions?

SIMATIC BATCH commands recipe phases via the BATCH interface (UNIT_PLC / BMOD), but it does not bypass internal SFC transition conditions unless explicitly programmed in the phase recipe.