Understanding Internal Commands, Acquisition Logic, and State Execution in PCS 7 SFC Types

Internal Commands in SIMATIC PCS 7 SFC Types (such as 'INTSTART', 'INTHOLD', 'INTABORT', 'INTSTOP', and 'INTRES') are programmatic control inputs that allow external CFC logic, higher-level equipment phases, or interlocks to command the SFC state machine directly. Unlike operator commands ('OPCMD') from the WinCC faceplate which are subject to manual mode authorization, internal commands are evaluated by the SFC runtime engine regardless of whether the block is in Manual or Automatic mode, taking precedence to ensure deterministic automated execution and emergency abort coordination.

Executive Summary

When designing complex procedural automation and equipment phases in PCS 7, control engineers must choose between operator commands, interconnected automatic commands, and internal commands (INTCMD). A frequent point of confusion is how internal commands interact with faceplate controls, operating mode switches (ENAUT/ENMAN), and ISA-88 state transition boundaries. This guide explains the internal architecture of SFC Type command evaluation, the behavioral distinctions between internal commands and operator requests, and best practices for interlocking and equipment coordination.

Architecture and Fundamental Mechanics

SFC Type Command Processing Hierarchy and State Logic Engine:

Command Channels in SFC Types:
1. Operator Commands (OP_...): Initiated by the operator via the WinCC faceplate (e.g., Start, Hold, Reset, Abort). Valid only when the block is in Manual mode (MAN) and operator authorization privileges are satisfied.
2. Interconnected Automatic Commands (START_LI, HOLD_LI, ABORT_LI, etc.): Evaluated only when the block is in Automatic mode (AUTO) and linked via CFC signal logic.
3. Internal Commands (INTSTART, INTHOLD, INTABORT, INTSTOP, INTRES, INTCOMP):
- Evaluated by the SFC state engine in both Automatic and Manual modes.
- Designed for technological interlocking, emergency equipment shutdown, and coordination between nested SFC charts and Control Module Types (CMTs).
Priority Evaluation and Conflict Resolution:
If an operator attempts to issue a Resume command while an INTHOLD or INTABORT signal is active, the internal command wins; the SFC remains held or aborted.
State Transition Hierarchy: Abort commands have absolute priority over Stop, Hold, Pause, and Start.

Step-by-Step Implementation Procedure

Applying Internal Commands in CFC Equipment Logic:

Inspect Available Internal Command Inputs in SFC Type:
- Open the SFC Type in the SFC Editor.
- Under the Inputs container, locate standard internal command pins:
- INTSTART: Command to Start
- INTHOLD: Command to Hold
- INTABORT: Command to Abort
- INTSTOP: Command to Stop
- INTRES: Command to Reset
- INTCOMP: Command to Complete
Interconnect Interlocks to Internal Commands in CFC:
- Open the CFC chart containing the SFC Type instance.
- For process safety interlocks (e.g., high tank pressure, emergency trip), connect the interlock output to INTABORT or INTHOLD.
- Because internal commands are pulse-triggered in the state engine, ensure the trip signal asserts as an edge or maintained pulse until the state transition acknowledges.
Coordinate Multi-Unit Sequencing:
- When Unit A finishes reaction, connect UnitA.QSTATE = Completed to UnitB.INTSTART to initiate downstream charging automatically.
Verify Feedback and Status Outputs:
- Monitor QSTATE (numeric state), QHOLD (held flag), and QABORT (aborted flag) to ensure downstream interlocks react promptly.

Verification

To verify internal command operation:
1. In WinCC, place the SFC faceplate in 'Manual' mode.
2. In CFC test mode, assert a TRUE value on INTHOLD; confirm that the SFC immediately transitions to the 'Holding' and then 'Held' state despite being in Manual mode.
3. Attempt to click 'Resume' on the faceplate while INTHOLD remains asserted; verify that the resume command is rejected.
4. Clear INTHOLD and assert INTRES; confirm the SFC resets to 'Ready to start'.

Limitations and Operational Precautions

Operational precautions:
- Because INT commands override Manual mode, improper wiring can prevent field operators from taking manual control during plant emergencies.
- Do not permanently tie INT command pins to TRUE; internal commands should be asserted as pulses or conditioned signals. A stuck INTSTART pin can cause continuous restart attempts.
- In plants managed by SIMATIC BATCH, batch recipe phases use dedicated Batch commands. Do not interconnect raw INT pins to Batch phases without arbitrating through BATCH_EN.

Engineer FAQ

Do Internal Commands work when the SFC is in Manual mode?

Yes. Internal commands (INTSTART, INTHOLD, etc.) are processed in both Manual and Automatic modes.

What is the difference between `INTABORT` and `ABORT_LI`?

ABORT_LI is the interconnected automatic abort input, evaluated only when AUTO mode is active. INTABORT is evaluated in both Auto and Manual modes.

Can an operator override an internal hold command from the faceplate?

No. As long as the condition asserting the internal command is active, faceplate operator commands will be rejected.

Should internal commands be maintained or pulsed?

Internal commands are evaluated on signal edges or until the state transition completes; maintaining an internal command permanently can block subsequent state changes.

What output indicates the active state of an SFC Type?

The QSTATE integer output reports the active state according to the ISA-88 state model.

Technical References

Related Tags

SFC Type
Internal Commands
INTSTART
ISA-88 State Logic
Interlocking

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Understanding Internal Commands, Acquisition Logic, and State Execution in PCS 7 SFC Types