Process Simulation Architectures in SIMATIC PCS 7: Comparing CFC Driver Simulation, S7-PLCSIM, and SIMIT

Simulating process behaviors in SIMATIC PCS 7—such as automatically driving digital input feedback signals when an output command activates—is accomplished through three distinct architectural levels. At the block level, standard channel drivers (CH_DI, CH_AI, FbAnIn, FbDiIn) feature built-in simulation parameters (SimOn = TRUE, SimPV_In) allowing loopback simulation charts directly within CFC. At the controller level, S7-PLCSIM emulates S7-400 / CPU 410 firmware execution on a PC. For comprehensive plant digital twin simulation, Siemens SIMIT (formerly SIMBA) connects seamlessly to S7-PLCSIM or Virtual Controllers, providing standardized behavioral components (valves, motors, actuators) that automatically calculate travel times, pressure dynamics, and sensor feedback loops without modifying controller application code.

Executive Summary

Virtual commissioning and process simulation are essential methodologies in modern process automation, enabling engineering teams to thoroughly validate control logic, test interlocks, verify WinCC faceplates, and train plant operators long before physical hardware is energized. A fundamental requirement in any simulation model is behavioral loopback: when an automation controller commands a motor to run or a valve to open, the simulation system must automatically drive the corresponding process inputs (run feedback, open limit switch, fluid flow rate) after realistic physical delay times. Within SIMATIC PCS 7, engineers can implement simulation across three progressive layers: internal CFC driver simulation, software controller emulation with S7-PLCSIM, or comprehensive multi-physics modeling with SIMIT. This guide provides an architectural comparison and implementation workflows for each approach.

The Simulation Pyramid: From Basic Loopback to High-Fidelity Digital Twins

Process simulation in PCS 7 spans three distinct tiers of capability and complexity:

Tier 1 (Internal CFC Logic): Lightweight simulation logic embedded directly inside the AS application program. Uses standard driver simulation parameters and basic CFC delay blocks (TimerP).
Tier 2 (Controller Emulation - S7-PLCSIM): Replaces physical S7-400 or CPU 410 hardware with a software emulator executing compiled controller code on an engineering station, interfacing directly with WinCC OS runtime.
Tier 3 (Process Modeling - SIMIT Platform): Completely external simulation engine that interacts with the controller's I/O image. SIMIT models real-world plant physics—actuators, sensors, pipe networks, hydraulic pressures, and chemical reactions—providing realistic closed-loop feedback without modifying a single line of production AS code.

Level 1: Native Channel Driver Simulation in CFC (SimOn / SimPV_In)

For basic loop testing on an active engineering system without external software:

APL Driver Capabilities: Every Advanced Process Library channel driver block (such as FbAnIn, FbDiIn, CH_AI, CH_DI) includes dedicated simulation parameters:
* SimOn (BOOL): When set to TRUE, disconnects the block algorithm from physical I/O hardware.
* SimPV_In (BOOL/REAL): The simulated process variable value passed to the block algorithm.
Implementing Loopback in CFC:
* An engineer places an auxiliary simulation chart containing a timer block (e.g. TimerP).
* When the technological valve block (VlvL) energizes its output command Ctrl = 1, the timer starts counting (e.g. 3-second stroke time).
* Upon timer expiration, the timer output drives CH_DI.SimPV_In = 1 for the open limit switch.
Drawback: Requires modifying production CFC charts and adding artificial logic that must be rigorously purged before plant startup.

Level 2: Virtual Controller Emulation via S7-PLCSIM

S7-PLCSIM provides virtual controller execution without physical hardware:

Native Integration: In SIMATIC Manager, clicking the Simulation On/Off icon starts S7-PLCSIM.
Program Download: HW Config, CFC charts, and NetPro connections download directly into PLCSIM as if it were a physical CPU.
Memory Tables: Engineers can force inputs, monitor outputs, and verify alarm generation.
Limitation: PLCSIM emulates only the CPU; it does not automatically model physical plant dynamics or provide closed-loop automatic feedback without external scripting or SIMIT.

Level 3: Comprehensive Virtual Commissioning with SIMIT Simulation Platform

Siemens SIMIT (formerly SIMBA) is the certified simulation platform for SIMATIC PCS 7:

Non-Invasive Architecture: SIMIT operates entirely outside the automation station. Production CFC charts remain 100% identical to commissioning code; zero simulation blocks are placed in the AS program.
Standard Component Library: SIMIT provides pre-built, drag-and-drop behavioral models for standard PCS 7 equipment:
Drives: Direct-on-line motors, variable frequency drives, reversing starters with built-in run feedback and fault simulation.
Valves: Discrete open/close valves, modulating control valves with realistic stroke times, travel hysteresis, and end-position switches.
Sensors: Thermocouples, RTDs, flowmeters with noise generation and failure modes.
Automated I/O Coupling: SIMIT reads the PCS 7 HW Config and symbol table, automatically mapping controller output addresses to simulation inputs, and driving corresponding simulated inputs back into the controller process-image partition.

Modeling Automatic Input/Output Feedback in SIMIT

Configuring automated closed-loop feedback in SIMIT requires minimal engineering effort:

Import Hardware Configuration: In SIMIT, create a new coupling (e.g. PLCSIM Coupling or Virtual Controller Coupling). Import the PCS 7 project export file; SIMIT automatically populates all I/O tag addresses.
Instantiate Equipment Component: Drag a standard Valve component from the SIMIT library onto a simulation diagram.
Connect Tags:
* Connect the controller output tag Valve_Open_Cmd (e.g. Q 4.0) to the component input Open.
* Connect the component outputs Opened and Closed to the controller input tags Valve_Open_Feedback (e.g. I 4.0) and Valve_Closed_Feedback (e.g. I 4.1).
Parameterize Physical Properties: Set the valve parameter Moving Time to 4.0 seconds.
Execute Simulation: Start SIMIT. When the PCS 7 program commands the valve to open, SIMIT automatically delays 4.0 seconds before asserting input I 4.0, providing perfectly realistic process feedback to the AS and WinCC faceplate.

Process Simulation Tools Comparison and Sizing Matrix

The following table compares simulation options available for SIMATIC PCS 7 installations:

Simulation ApproachSimulation ScopeAS Code Modification Required?Dynamic Physics ModelingTypical Industrial Use CaseCFC Driver Sim (SimOn)Single I/O channelYes (Temporary CFC blocks)Very limited (Manual timers)Quick loop checks during commissioningS7-PLCSIM AloneController executionNo (Standard download)None (Static manual forcing)Logic sequence and interlock checkingSIMIT + PLCSIMFull plant I/O loopbackNo (Zero code modification)Standard behavioral dynamicsVirtual commissioning, factory acceptance tests (FAT)SIMIT + Virtual ControllerMultiple AS emulationNo (High-capacity execution)Advanced multi-physics & thermodynamicsHigh-fidelity operator training simulators (OTS)

Verification

To verify that process simulation operates with high fidelity:

Verify Non-Invasive Coupling: In SIMIT, start the coupling to S7-PLCSIM. Confirm that all imported I/O addresses report green communication status without addressing mismatches.
Trigger Valve Command: From the WinCC OS faceplate, issue an 'Open' command to a motor or valve.
Monitor Feedback Dynamics: Observe that the output command transitions to TRUE in CFC. Confirm that SIMIT indicates travel progression, and after the parameterized moving time, the open limit input in CFC transitions to TRUE.
Verify Alarm Suppression: Confirm that the technological block (VlvL) transitions to the open state without triggering actuator feedback monitoring alarms (e.g. 'Opening time exceeded').

Coupling Methods: SIMIT Shared Memory, Virtual Controller, and PROFINET Emulation

SIMIT provides scalable coupling technologies depending on project scale:

SIMIT Virtual Controller (VC): Emulates S7-400 and CPU 410 firmware execution independently of Step 7, allowing multi-AS simulation across distributed PC networks with high cycle accuracy.
SIMIT UNIT (Hardware Box): Replaces physical distributed I/O racks on live PROFIBUS and PROFINET networks, allowing real physical AS controllers to communicate with SIMIT models in real-time.
Shared Memory / OPC DA / OPC UA: Facilitates co-simulation with external third-party chemical, thermodynamic, or hydrodynamic modeling tools.

Limitations and Operational Precautions

Never leave SimOn = TRUE active on physical production controllers running live plant processes. If simulation logic was added directly into CFC charts, conduct a mandatory pre-commissioning audit to ensure all simulation interconnections are removed.

When sizing SIMIT software packages, tag count licenses, or SIMIT UNIT hardware emulators, all specifications must be validated against official Siemens catalogs and confirmed through an authorized Siemens distributor quotation.

Engineer FAQ

Can SIMIT simulate analog transmitter drift and noise?

Yes. Standard SIMIT sensor components include parameters for Gaussian white noise, drift rates, calibration offset, and failure modes.

Does S7-PLCSIM support communication with WinCC OS Servers?

Yes. WinCC OS connects directly to S7-PLCSIM using the 'PLCSIM.MPI' or 'PLCSIM.TCPIP' communication channel without specialized drivers.

Can I run multiple S7-400 CPUs simultaneously in classic PLCSIM?

Classic S7-PLCSIM V5.x only supports a single active CPU instance; multi-AS simulation requires SIMIT Virtual Controller or multiple virtual machines.

What is the difference between SIMBA and SIMIT?

SIMBA was the original hardware-based fieldbus simulation tool from Siemens, which was completely evolved and rebranded into the modern SIMIT Simulation Platform.

Process Simulation Architectures in SIMATIC PCS 7: Comparing CFC Driver Simulation, S7-PLCSIM, and SIMIT