Operational Prerequisites and Parameter Validation for Starting SFC Type Instances in CFC

In SIMATIC PCS 7, an SFC Type instance in a CFC chart cannot be started simply by forcing the START command input while in Manual mode or while parameter validation is incomplete. To successfully start an SFC Type instance from CFC logic: 1) The instance must be in Automatic mode by setting AUT = 1 and MAN = 0 (confirmed by output AutAct = 1). In Manual mode (ManAct = 1), automatic commands are rejected. 2) Parameter checking must be validated: if setpoints or control strategies (CS) exist, input PARAM must be set to 1 (or pulsed) to verify limit boundaries; unvalidated parameters keep output QDIS_START = 1, blocking starts. 3) Interlocks must be healthy (ENSTART = 1, INTLOCK = 0). Once AutAct = 1 and QDIS_START = 0, pulsing START = 1 transitions the SFC from Idle to Starting and Run.

Executive Summary

During plant testing and commissioning in SIMATIC PCS 7, control engineers frequently attempt to test SFC Type instances directly from the CFC Editor by going online and toggling control inputs (such as START, STOP, HOLD, ABORT). A recurring frustration occurs when forcing START = 1 produces no reaction: the SFC instance remains frozen in the Idle state. This occurs because SFC Types enforce a rigid operating state logic and safety interlock model. Automatic procedural commands are strictly ignored if the SFC is in Manual mode or if internal parameter validation has not been satisfied. This guide details the mandatory execution conditions and parameter handshakes required to control SFC Type instances programmatically.

Architecture and Fundamental Mechanics

SIMATIC PCS 7 SFC Types operate according to the ISA-88 state model managed by an internal operating system state machine:
1. Operating Mode Arbiters:
- Manual Mode (ManAct = 1): Commands originate exclusively from operator faceplates or SFC visualization in WinCC. In Manual mode, automatic control inputs (START, COMPLETE, HOLD, ABORT) from CFC chart interconnections are explicitly locked out to prevent logic from overriding human operators.
- Automatic Mode (AutAct = 1): Commands originate from external automation logic (such as SIMATIC BATCH, higher-level Equipment Phases, or CFC interconnections). To transition from Manual to Auto via logic, the engineer must assert AUT = 1 while MAN = 0 (or toggle ENAUT = 1).

Parameter Validation and Start Disabling (PARAM and QDIS_START):
- When an SFC Type includes Control Strategies (CS) or Setpoints, the SFC operating system checks whether target setpoints fall within configured physical high/low limits.
- The output QDIS_START (Start Disabled) acts as a hardware/software interlock. Whenever parameters are modified, QDIS_START asserts 1.
- The input PARAM serves as the parameter validation trigger. Setting PARAM = 1 instructs the SFC to evaluate all setpoints and control strategies. If all limits are satisfied, the block clears QDIS_START to 0, arming the start trigger.
- If an SFC Type contains no setpoints, PARAM must still be asserted to clear start-disable registers.
Execution Triggers:
Once AutAct = 1, QDIS_START = 0, and start enable is healthy (ENSTART = 1), a rising edge on input START transitions the SFC sequence from Idle (State = 1) through Starting (State = 2) into Run (State = 3).

Step-by-Step Implementation Procedure

Verify SFC Instance Operating Mode in CFC:
a. Open the CFC chart containing the target SFC Type instance.
b. Go online in test mode (Debug -> Test Mode).
c. Inspect block outputs AutAct and ManAct. If ManAct = 1, the SFC is locked in Manual mode.
d. To switch to Automatic mode via logic, set input parameter AUT = 1 and ensure MAN = 0. Confirm that output AutAct transitions to 1.
Clear Start Disabling via Parameter Validation:
a. Monitor output parameter QDIS_START on the SFC instance.
b. If QDIS_START = 1, start execution is blocked.
c. Locate input parameter PARAM (Check Parameters / Setpoints).
d. Set PARAM = 1 (or apply a high pulse). The SFC evaluates configured control strategies and setpoints.
e. Confirm that output QDIS_START transitions to 0.
Check Interlocks and Start Enable Flags:
a. Verify that input parameter ENSTART is set to 1 (Start Enabled).
b. Confirm that interlocking inputs (INTLOCK, INTLK_CS) evaluate to healthy zero states.
c. Ensure no active reset or abort requests are latched.
Execute Start Command:
a. With AutAct = 1 and QDIS_START = 0, apply a rising edge to input parameter START (toggle from 0 to 1).
b. Observe the internal state: the SFC Type advances from Idle to Starting and enters Run.
c. Reset START back to 0 to prepare for subsequent cycles.

Verification

In CFC online test mode, verify that output State changes from 1 (Idle) to 3 (Run).
Open the SFC Type chart in the SFC Editor: confirm the active step advances through the initial step into the cyclic operational sequence.
Open the SFC Faceplate on the WinCC OS Client: verify that the faceplate indicates Automatic mode and displays the active step name.
Apply a pulse to input HOLD: verify the SFC responds by transitioning through Holding into Held.

Limitations and Operational Precautions

If the SFC Type is instantiated inside an Equipment Module (EM) controlled by a SIMATIC BATCH Equipment Phase (EPH), direct manipulation of CFC inputs will be overridden by the batch allocation manager. Setting PARAM = 1 permanently bypasses runtime setpoint limit checking; ensure application setpoints are verified prior to starting production equipment.

Engineer FAQ

Why does forcing START have no effect on an SFC Type instance?

The instance is either in Manual mode (AutAct = 0), parameter validation is incomplete (QDIS_START = 1), or start is disabled by interlocking logic (ENSTART = 0).

How do I switch an SFC Type to Auto directly from CFC?

Set input parameter AUT = 1 while ensuring MAN = 0; the block transitions to Automatic mode as indicated by AutAct = 1.

What does the output QDIS_START indicate?

QDIS_START indicates that starting the SFC is disabled, typically because setpoints have not been validated by asserting PARAM = 1 or an interlock is active.