Siemens PCS 7 CMT: Multiple Outputs to One CFC Input

Siemens PCS 7 CMT: Connecting Multiple Outputs to One CFC Input

Control Module Types (CMTs) in SIMATIC PCS 7 provide a powerful method for standardizing and reusing automation logic. A recurring engineering question in plant design is whether multiple block outputs can be interconnected to a single input inside a CMT — for example, gathering the QBAD (bad quality) flags from a pair of driver blocks (open/close) into one CSF or PERMIT input of a higher-level block such as an interlock.

The short answer is that the SIMATIC CFC editor never permits a direct fan-in of two sheet outputs onto one sheet input. The CFC chart editor enforces a one-to-one relationship for interconnections on the sheet itself. However, the Control Module Type editor exposes an optional interconnection mechanism through the Technological I/O tab that gives the same end result at chart instance level, and the Advanced ES (AdvES) variant concept lets you expose different upstream sources per plant unit without duplicating the CMT.

1. Why CFC Forbids Direct Output-to-Output Merging

CFC charts compile into a sequence of PLC operations. An interconnection represents a single data assignment. If two outputs were tied to the same input directly on the sheet, the compiler would have to define an arbitration rule (last-writer-wins, AND, OR, priority) and an evaluation order. To keep the chart semantics deterministic, the CFC editor simply disallows the operation and reports a placement error.

The restriction is enforced at three levels:

• Chart editor (runtime view): The drag-and-drop cursor shows the "no entry" symbol when a second source is dropped on an occupied input pin.
• Compiler: The CFC compiler in PCS 7 V9.x / V10.0 returns error Cannot connect output of block <name> to input of block <name>: input already interconnected.
• CMT Technological I/O editor: Each optional input is configured with a single canonical source per CMT, but the canonical source can be reassigned per instance through variants.

The Siemens manual CFC for SIMATIC S7 and the PCS 7 Compendium describe this behavior. For deeper details on the optional interconnection flag, refer to the official Siemens document Control Module (CM) Technology - Efficient Engineering in SIMATIC PCS 7 V10.0.

2. Architectural Background: Control Module Types and Variants

A Control Module Type is a CFC chart that has been declared as a type in the plant view. It carries a Technological I/O interface — a list of named parameters that act as the public contract between instances of the CMT and the surrounding automation code. Each parameter has a flag called Optional.

The Optional flag is the cornerstone for implementing "either/or variants" — for instance, a valve CMT that is sometimes driven by an INTERLOCK_2 block and sometimes by an INTERLOCK_8 block, depending on the safety integrity level of the loop. The PCS 7 documentation uses the term either/or variants exactly for this case (CM Technology manual, section 5.4).

3. The Virtual Multi-Source Interconnection Pattern

The pattern that solves the "many outputs to one input" question uses the CMT itself as the merger point. Instead of trying to drag two sheet outputs onto one sheet input inside the CMT chart, both source parameters are promoted to the Technological I/O, and one of them is then connected to the destination input inside the CMT. The second source is exposed only at the variant level.

3.1 Concrete example: motor interlock with driver quality flags

Consider a motor CMT that contains two driver FBs, an open driver and a close driver (typical for reversing starters or double-acting actuators). Each driver exposes a QBAD output. The higher-level interlock block has a PERMIT input that must be deactivated if either driver is in bad quality.

Inside the CMT chart, only QOPEN_BAD is wired to PERMIT. The variant editor then offers two variants:

• Variant A — OPEN active, CLOSE inactive: external interconnections connect QOPEN_BAD to the actual QBAD source of the open channel; QCLOSE_BAD remains unconnected (or tied to constant 0).
• Variant B — both channels active: external interconnections from the open-channel QBAD land on QOPEN_BAD, and the close-channel QBAD lands on QCLOSE_BAD. The internal sheet now shows two virtual sources to PERMIT, one through QOPEN_BAD and one through QCLOSE_BAD; the AdvES generator reconciles them so the PERMIT input sees whichever variant's wiring is active.

4. Step-by-Step Procedure in the CMT Editor

The procedure below assumes PCS 7 V9.0 SP6 / V9.1 / V10.0 with the Advanced ES add-on installed. Equivalent steps apply for PCS 7 V8.2 with the CMT engineering package.

4.1 Prerequisites

• PCS 7 engineering station with CFC V9.x or V10.0 installed.
• Advanced ES license (AdvES) for variant authoring. Without AdvES, the CMT can still use Optional flags, but variant generation is limited to a single variant.
• A control module that has already been declared as a CMT (right-click chart → Control module technology → Declare as type).
• Plant view must show the CMT icon under AS Objects → Charts.

4.2 Promoting parameters to Technological I/O

1. Open the CMT chart in CFC. Switch to the Technological I/O tab at the bottom of the editor (or open it via View → Technological I/O).
2. Drag the QBAD output of FB_OPEN from the sheet onto the Technological I/O list. The editor prompts for a symbolic name; enter QOPEN_BAD.
3. Repeat for the QBAD output of FB_CLOSE; name it QCLOSE_BAD.
4. Select the newly created Technological I/O entries and set the Optional checkbox to Yes in the properties dialog.
5. Confirm with OK; the chart sheet now shows the two parameters as floating input/output nodes with the same name as in the Technological I/O list.

4.3 Interconnecting inside the CMT

1. Drag a connection from the QOPEN_BAD sheet node to the PERMIT input of the interlock FB. This is now a normal intra-sheet interconnection.
2. Leave QCLOSE_BAD floating on the sheet — it has no intra-sheet destination. The compiler accepts this because Optional = Yes.
3. Compile the chart with Chart → Compile → Chart as CMT. The compiler issues no error because every mandatory input has been satisfied.

4.4 Creating variants in AdvES

1. Switch to the plant view and right-click the CMT instance folder. Choose Advanced ES → Create variant.
2. Name Variant A as OPEN_only. In the variant editor, drag a real source block (for example, the diagnostic block of the open-driver channel) onto the QOPEN_BAD Technological I/O pin. Leave QCLOSE_BAD bound to a constant FALSE or disconnected.
3. Create Variant B DUAL_channel. Map the open-channel QBAD source to QOPEN_BAD and the close-channel QBAD source to QCLOSE_BAD.
4. Generate the variant instance charts from the AdvES dialog (Generate instances → from CMT → apply variant). Each generated instance carries the appropriate interconnections.
5. Compile and download the AS program. The instances now physically carry the multi-source — > single-input structure at runtime, even though the CMT itself shows a single source.

5. Runtime Semantics of the Implicit OR Merge

Even though the CFC compiler refuses to merge two outputs on a sheet, the variant mechanism effectively produces an OR-of-quality pattern. The runtime rule is determined by the destination block, not by the editor:

• Boolean PERMIT inputs: Most interlock blocks treat a QBAD rising edge as a "channel not healthy, do not allow operation" signal. If either virtual source is high, the channel is unhealthy. The pattern is therefore OR by design.
• Word/DWord quality status: When the destination is a status word, the last writer wins; this is typical for diagnostic aggregators. The AdvES variant approach gives one writer per variant, so the writer selection is unambiguous.
• Analog values: If two analog outputs are merged, take the higher of the two for safety interlock (fail-safe direction), unless the destination explicitly defines an averaging policy.

6. Verification Checklist

After applying the multi-source pattern, verify the result with the following steps:

1. Compile CMT without errors. In CFC, choose Chart → Compile → Check consistency. Expected: no errors, optional parameters are listed in the warnings as "not interconnected at CMT level".
2. Generate both variants. Open Advanced ES → Variant overview; both variants should report status Generated with no unresolved references.
3. Cross-reference the Technological I/O. In the variant instance chart, every Technological I/O pin must show the expected incoming or outgoing line according to the variant definition.
4. Download and run. Use SIMATIC Manager PLC → Download; on the AS, the PERMIT input of each instance must show the value wired by the active variant.
5. Force a QBAD condition on one of the upstream driver blocks (for example, by simulating channel fault in the diagnostic FB). The destination interlock must react as expected: for a safety-related interlock, the operator command must be inhibited; for a permissive, the permissive must drop.
6. Audit the safety case. If the loop is SIL 2 or higher, document in the SRS that the multi-source pattern is implemented via CMT variant rather than direct OR logic, and reference the CMT version and the AdvES variant IDs.

7. Common Errors and How to Recover

8. Alternative Approaches

If the AdvES variant mechanism is not available, two manual workarounds exist.

8.1 Aggregate block pattern

Create a small custom FB that takes N QBAD inputs and exposes a single aggregated output (for example, QANY_BAD) implemented as OR of all inputs plus the standard PCS 7 quality propagation. Place the FB inside the CMT and connect the upstream driver QBAD outputs to its inputs. The destination interlock then receives a single QANY_BAD input. This works without Optional flags but requires a small block development effort and re-import into the master data library.

8.2 External merger chart

Define a small shared CFC chart that takes two upstream QBAD signals, merges them with an explicit OR gate (AND block with negated inputs, or a custom OR FB), and exposes the merged signal as a chart output. The CMT then interconnects to the merged output of this external chart. This keeps the CMT single-variant and avoids modification of the standard library, at the cost of an additional chart in the S7 program.

9. Comparison of Patterns

10. Engineering Best Practices

• Reserve Optional flags for genuine either/or choices. A CMT where every Technological I/O entry is Optional defeats the standardization purpose of the CMT.
• Document variants in the project manual. Each variant should have a one-line description visible in the AdvES dialog (right-click → Properties → Comment). Engineers commissioning a new loop must be able to read the variant intent without opening the chart.
• Limit fan-in to two sources. When three or more sources need to feed one input, prefer an aggregator FB to keep the CMT readable. Variants of fan-in degree greater than two are difficult to maintain.
• Lock CMT names after first variant generation. Renaming Technological I/O pins after variants exist forces a full regen. Add a project-specific naming convention review step before the first cut.
• Run the cross-reference after every variant change. The cross-reference is the only tool that proves the instance chart matches the variant definition. Treat any discrepancy as a release blocker.

11. FAQ