Architectural Differences Between SFC Types and Equipment Module Types in PCS 7

In SIMATIC PCS 7, an SFC Type and an Equipment Module Type (EMT) are not synonyms; they operate at different abstraction levels within the technological engineering hierarchy. An SFC Type is an executable state machine and sequential function chart type that compiles into a Step 7 Function Block (FB). In contrast, an Equipment Module Type (EMT) is a complete technological CFC template that encapsulates an SFC Type alongside auxiliary function blocks, standard Control Module Types (CMTs), technological I/O definitions, global commands, and role assignments that can be synchronized across the plant hierarchy.

Executive Summary

Beginning in SIMATIC PCS 7 V8.1 and expanded comprehensively in V9.0 and V9.1, Siemens transitioned from classical tag-based engineering toward technological object-oriented engineering aligned with ISA-88 standards. In this framework, engineers frequently encounter the terms 'SFC Type' and 'Equipment Module Type' (EMT), often wondering whether they represent identical concepts under different names.

While both artifacts facilitate reusable sequence control, an SFC Type is strictly the internal algorithmic sequencer (the state machine governing steps and transitions), whereas an EMT is an expansive technological container. An EMT embeds the SFC Type inside a CFC chart and links it to lower-level Control Module Types (CMTs) using standardized technological commands and role assignments. This article provides a comprehensive architectural breakdown of both concepts, explaining when to deploy pure SFC Types versus complete Equipment Module Types.

Architecture and Fundamental Mechanics

The technological architecture of SIMATIC PCS 7 structures procedural automation across multiple layers:

The SFC Type (Algorithmic Sequencer Layer):
- An SFC Type is created in the Component View or Master Data Library as an S7 program object.
- It defines sequence logic: operating states (Running, Holding, Held, Aborting), sequencers, steps, transitions, setpoints, control strategies, and internal bit memories.
- When compiled, the SFC Type generates a standard Step 7 Function Block (e.g., FB1025) and an associated OS faceplate (@PG_SFC_TYPE.pdl).
- Limitation: A raw SFC Type cannot contain external CFC function blocks, cannot encapsulate auxiliary logic, and must be manually interconnected pin-by-pin to field drivers or motor blocks within an instance chart.
The Equipment Module Type (EMT - Technological Template Layer):
- An EMT represents a physical plant subsystem (such as a reactor dosing system, CIP skid, or mixer group) according to ISA-88.
- It is created as a specialized CFC chart in the Master Data Library with 'Plant Types' enabled.
- Inside the EMT chart, an SFC Type instance provides sequence control, but the chart also contains auxiliary function blocks (e.g., interlock logic, calculation blocks, totalizers) and defined 'Role Assignments'.
- Role Assignments and Technological I/Os: The EMT defines abstract roles (e.g., 'Inlet_Valve', 'Agitator_Drive'). CMTs (Control Module Types) are assigned to these roles. The embedded SFC Type commands these CMTs using high-level technological commands (e.g., Open, Start) rather than hardwired bit interconnections.
- Centralized Synchronization: Modifying an EMT in the Master Data Library allows batch-synchronizing all instances across the multiproject (including CFC logic, SFC sequencers, and technological connections) using the 'Plant Types > Synchronize' wizard.

Step-by-Step Implementation Procedure

Follow these steps to configure an Equipment Module Type (EMT) utilizing an SFC Type in SIMATIC PCS 7:

Create the Underlying SFC Type in the Master Data Library:
- In SIMATIC Manager, open your Master Data Library (MDL).
- In the Component View, expand the Charts folder.
- Insert a new 'SFC Type' (e.g., SFC_Dosing_Unit).
- Open SFC_Dosing_Unit in the SFC Editor.
- Configure sequencers (e.g., RUNNING, HOLDING), steps, transitions, and setpoints.
- Save and compile the SFC Type to generate the underlying Function Block.
Create the Equipment Module Type (EMT) Chart:
- In the Master Data Library Plant View, navigate to the template folder.
- Insert a new CFC chart and rename it EMT_Dosing_Skid.
- Open EMT_Dosing_Skid in the CFC Editor.
- Instantiate the SFC Type: drag SFC_Dosing_Unit from the catalog into the EMT chart.
- Add any required auxiliary blocks (e.g., an Intlk02 interlock block, mathematical totalizers, or monitoring blocks).
Configure Technological I/Os and Roles in the EMT:
- In the CFC Editor, select 'View > Technological I/Os' to open the technological tree.
- Right-click the root node and configure it as an 'Equipment Module'.
- Define 'Roles' representing the field equipment controlled by the sequence (e.g., Role: Dosing_Valve, Role: Feed_Pump).
- Insert the target Control Module Types (e.g., CMT_Valve, CMT_Motor) into the template chart or establish role bindings.
Configure Technological Commands and Statuses:
- Under the defined roles, create technological commands (e.g., Cmd_Open, Cmd_Start).
- In the SFC Type steps within the EMT, call these technological commands in the step actions instead of creating direct graphic interconnections between block pins.
Instantiate and Synchronize Across the Multiproject:
- Copy EMT_Dosing_Skid from the Master Data Library into the plant hierarchy folders in your project.
- Whenever updates are made to the EMT in the Master Data Library, execute 'Options > Plant Types > Synchronize...' from SIMATIC Manager.
- The synchronization wizard updates all EMT instances across the entire plant project simultaneously.

Verification

In SIMATIC Manager Plant View, verify that the instantiated EMT displays the Equipment Module technological icon.
Open an instantiated EMT chart in CFC: verify that the embedded SFC Type instance correctly binds to the local Control Modules via technological role assignments.
Execute an AS compile ('Changes only') and download to the controller.
Open WinCC OS Runtime: verify that the EMT faceplate allows operating mode transitions (Auto/Manual) and that triggering a step command initiates the assigned CMT actions in the field.

Limitations and Operational Precautions

Converting an existing classical project utilizing standalone SFC Types into EMT architectures requires structured planning; direct conversion of instantiated SFC charts into EMTs cannot be performed automatically if underlying CMTs lack standardized technological command interfaces. EMT architectures require PCS 7 Engineering System licenses that support Plant Automation Accelerator / Advanced Engineering features. Verify all engineering tools and library options with an authorized Siemens distributor or the Siemens Industry Mall.

Engineer FAQ

Are SFC Types being replaced by Equipment Module Types?

No. Equipment Module Types do not replace SFC Types; they encapsulate them. An EMT uses an SFC Type as its internal state machine engine while adding technological role assignments and CFC block logic.

Can I use an SFC Type without creating an EMT?

Yes. Classical PCS 7 projects can instantiate standalone SFC Types directly into CFC charts. However, you miss out on the advanced role assignment, global command modeling, and unified synchronization capabilities provided by EMTs.

What is the difference between an EMT and an EPHT?

An EMT (Equipment Module Type) represents a functional equipment unit (such as an agitator group), whereas an EPHT (Equipment Phase Type) represents an autonomous process phase that can dynamically acquire and control multiple lower-level EMs/CMTs.

Do EMT instances consume more Process Objects (PO) than SFC Types?

No. Process Object licensing is booked based on the technological blocks instantiated (e.g., the SFC Type instance and underlying motor/valve blocks), not by the EMT wrapper itself.