AS-AS Communication Limits for CPU 410-5H in PCS 7 V8.2

Overview of AS-AS Communication in PCS 7

In a SIMATIC PCS 7 plant the Automation Station (AS) is the level where process I/O, control blocks, and interlocks execute. When more than one AS must exchange process values—alarms, interlocks, sequencing handshakes, or operator setpoint routing between redundant subsystems—engineers refer to this traffic as AS-AS communication. The communication path is built on top of S7 connections configured in the PC station's WinCC / PCS 7 engineering environment, and every AS-AS pair consumes a defined share of the CPU's connection table, operand bandwidth, and OS connection budget.

The CPU 410-5H (MLFB 6ES7410-5HX08-0AB0) is the high-availability controller introduced for PCS 7 V8.x. It ships as part of the AS 410 Tx system and runs the PCS 7 firmware bundle that is paired with the engineering system release. The CPU executes the S7-400 instruction set extended with PCS 7 function blocks (APL or successor libraries), supports H-CPU redundancy through fiber-optic sync modules, and provides the connection resources that all AS-wide traffic must compete for alongside operator station connections, engineering connections, and S7 routes to subordinate stations.

Engineers sizing a PCS 7 V8.2 project frequently ask three questions:

1. How many S7 connections can a CPU 410-5H terminate simultaneously?
2. How much payload can each AS-AS connection carry without breaching the cyclic OB1 / OB35 budget?
3. What is the maximum number of AS-wide interconnections per AS?

The exact numerical ceiling for each item is not a single product spec—it depends on the connection variant (S7 single, S7 fault-tolerant), the engineering configuration (Compendium Part A rules), and the firmware revision. This reference walks through each factor and points to the official Siemens documents where the binding values are published, so that sizing can be repeated across projects with predictable results.

CPU 410-5H Hardware Profile and Firmware V8.2

The AS 410 controller family was extended with the CPU 410-5H (6ES7410-5HX08-0AB0) for PCS 7 V8.x deployments. The controller is a 1oo2 high-availability CPU; both subsystems are synchronized through a fiber-optic pair and run identical firmware images. The "-5H" suffix denotes the PROFIBUS DP master interface count and the PN/IE capabilities. The controller slots into the UR2-H rack alongside its redundant partner and the I/O modules of the AS.

Key attributes of the CPU 410-5H platform relevant to AS-AS sizing:

• Form factor: S7-400 1oo2 H-CPU module, plugs into the UR2-H rack.
• Firmware alignment: PCS 7 firmware V8.2 is the binding feature set for this configuration.
• Instruction set: Compatible with S7-400 CPU 41x family; supports SCL, FBD/LAD, GRAPH, CFC, SFC.
• Connection resources: Documented in the product manual entry for 6ES7410-5HX08-0AB0 under Communication functions → Number of connections.
• Redundancy behavior: Connection table is mirrored between H-subsystems; loss of one half causes a transparent switchover for S7 connections flagged as fault-tolerant.
• Variant note: Firmware V8.2 can be loaded onto CPU 410-5H and onto the related CPU 410E expansion rack variants.

Firmware V8.2 introduces the following communication-relevant enhancements per the official Siemens function manual Process automation with the SIMATIC PCS 7 CPU 410-5H controller:

• Extended PROFINET IO support, including media redundancy (MRP) and shared device.
• Extended diagnostic buffer entries for AS-AS connection faults.
• Improved hot-restart performance after redundant sync link loss, reducing the impact of a reconnection storm after switchover.
• Optional reuse of CPU 410E expansion racks under newer CPU 410-5H firmware.

Engineers upgrading from older PCS 7 V7.x projects to V8.2 must verify that all HMI panels, engineering PCs, and subordinate stations accept the new AS connection endpoints. The PCS 7 software update procedure automatically re-translates the PC station's S7 connections, but the manual cross-check is mandatory for fault-tolerant connections because the local/partner port assignments change between major firmware revisions.

Communication Resource Architecture

The CPU 410-5H exposes three physical interfaces that can carry S7 connections:

1. PROFIBUS DP / MPI (X1) — typically reserved for field devices and engineering; not used for AS-AS unless migrating legacy projects.
2. PROFINET / Industrial Ethernet (X5) — primary AS-AS carrier in new projects.
3. PROFINET / Industrial Ethernet (X8) — secondary interface for plant-bus segmentation or redundant ring topologies.

The CPU's connection table is partitioned by type. The official product page 6ES7410-5HX08-0AB0 product information lists each partition under Communication functions. The partitions that matter for AS-AS sizing are:

• S7 connections — used for AS-wide interconnections, OS-AS handshakes, and engineering routes.
• H connections (fault-tolerant) — reserved for H-tagged S7 connections; consumes two S7 slots per logical channel.
• HTTP / OPC UA connections — not applicable for AS-AS but compete for socket memory on the PN interface.

The total connection count is the sum across all three interfaces. AS-AS traffic typically uses only the PN interfaces (X5 / X8), so when counting headroom, engineers must subtract any connections already reserved for OS, ES, and routes to subordinate SIMATIC stations.

Connection Resources by Type

PCS 7 V8.2 does not publish a single global "AS-AS connection max" value. The binding limit is the sum of:

• Maximum total S7 connections per CPU 410-5H (firmware-dependent).
• Maximum simultaneously-active fault-tolerant S7 connections.
• Maximum AS-wide interconnections per AS (Compendium Part A rule).
• Practical payload per AS-AS connection (Compendium Part A rule).

The first two values are published on the Siemens product support page for the CPU. Engineers must:

1. Open the product page 6ES7410-5HX08-0AB0.
2. Select the Technical data tab.
3. Open Communication functions in the left navigation.
4. Read Number of connections, S7 connections, and Fault-tolerant S7 connections.

For a typical PCS 7 V8.2 AS 410-5H configuration, AS-AS traffic commonly uses:

• Plain S7 connections when both AS are non-redundant.
• Fault-tolerant S7 connections when either side is redundant or the application demands bumpless switchover.
• S7 routes for engineering diagnostics.

A sizing spreadsheet should track each of the following categories independently so that the engineer can prove headroom at commissioning:

Fault-Tolerant S7 Connections

A fault-tolerant S7 connection is a single logical connection that is realized as two physical channels — one on each H-subsystem of the CPU 410-5H. From the partner AS's perspective it is one endpoint ID; internally, the firmware maintains two TCP/ISO sessions and selects the active one per cycle. When a switchover occurs, the firmware drops the surviving channel forward without renegotiating the connection ID with the partner, which is what makes the handover transparent to the application.

The behaviour is documented in section 17.11.2 of the PCS 7 Process Control System manual (entry ID 109748473). Engineers must read this section before sizing because:

• Each fault-tolerant S7 connection consumes two entries in the local connection table.
• The maximum number of simultaneously-active fault-tolerant connections is lower than the total S7 connection count.
• A redundant CPU switchover may force a brief reconnect storm if many FT connections are active.
• Fault-tolerant S7 connections cannot be mixed with raw S7 routes on the same partner ID — the connection type is a property of the logical endpoint.

When commissioning an AS-AS pair where both sides are H-CPUs, prefer fault-tolerant S7 connections because the single logical endpoint survives the redundancy switchover transparently. The Compendium Part A guideline explicitly recommends fault-tolerant S7 connections for AS-wide interconnections between redundant AS pairs.

Practical rules when configuring fault-tolerant AS-AS links:

1. Mark the connection as Establish connection actively on the master side only; the partner side stays passive.
2. Set the Operating mode to Fault-tolerant S7 connection in NetPro.
3. Verify in the online connection table that Active = yes appears for both H-subsystems of the CPU 410-5H.
4. Confirm the connection ID is unique across the project; duplicate IDs cause sporadic drops on switchover.
5. Verify the connection resources on both the local and the partner CPU 410-5H; a low-resource partner causes the FT connection to fail to establish even when the local CPU is healthy.

AS-Wide Interconnections Data Volume

The PCS 7 engineering concept of an AS-wide interconnection is distinct from a raw S7 connection. An AS-wide interconnection is a single signal (BOOL, INT, REAL, BYTE, WORD, DWORD, or a structure) that the engineer wires in CFC across AS boundaries. The CFC compiler bundles many AS-wide interconnections into a smaller number of AS-AS S7 connections, depending on the data type and the packing rules of the firmware.

Two limits apply:

1. Per AS: The Compendium Part A publishes a maximum count of AS-wide interconnections per AS, expressed as a recommended ceiling rather than a hard stop. Exceeding the recommendation is allowed but requires that the engineer validate the cyclic time and connection budget manually.
2. Per connection: The payload carried by each AS-AS S7 connection must remain inside the cyclic OB budget (OB1 for cyclic, OB35 / OB36 for faster update groups). The Compendium Part A publishes a recommended maximum bytes per AS-AS connection.

The exact numbers are firmware-revision dependent. To retrieve them for PCS 7 V8.2 firmware:

1. Open the PCS 7 Compendium Part A — Configuration Guidelines manual in the PCS 7 documentation set.
2. Navigate to the AS resource utilization section.
3. Locate the table titled Maximum AS-wide interconnections per AS and the Recommended data volume per AS-AS connection.
4. Cross-check the firmware revision listed in the table heading against the running CPU 410-5H firmware.

Data-type packing rules used by the CFC compiler (typical V8.2 behaviour):

Because BOOLS are bit-packed, an array of 240 BOOLS becomes roughly 15 bytes on the wire, not 240 bytes. Engineers who skip the bit-packing step in their sizing spreadsheet will over-estimate the AS-AS payload and may request additional AS-AS connections that the firmware cannot use efficiently.

Configuration Guidelines from PCS 7 Compendium Part A

The Compendium Part A is the canonical sizing reference for any PCS 7 V8.x deployment. For AS-AS sizing it provides:

• Recommended maximum number of AS stations per project (OS-side consideration).
• Recommended maximum number of AS-wide interconnections per AS.
• Recommended maximum number of AS-AS connections per AS.
• Recommended maximum data volume per AS-AS connection (bytes per cycle).
• Rules for using fault-tolerant vs plain S7 connections.
• Guidance on CFC block selection (CH_AI, CH_AO, CTRL_PID, INTERLOK, MOT_SPEED, etc.) for cross-AS signals.
• Cycle-time guidance for OB1, OB35, and OB36 relative to AS-AS payload.

The Compendium values are not absolute limits. They are recommended ceilings derived from lab validation against typical CPU 410 cycle times. Field deployments that exceed the recommendations are sometimes accepted by Siemens support, provided the engineer demonstrates — via trace and connection diagnostics — that the AS still meets the cycle time and the OB1 / OB35 deadlines.

Typical recommended ranges from the Compendium Part A table (firmware V8.2 — confirm against the live PDF):

These row labels are the binding items; the live Compendium Part A PDF published with the PCS 7 engineering DVD provides the numeric values for each firmware revision.

Sizing Methodology and Calculation

The proper sizing sequence for a new PCS 7 V8.2 project is:

1. Inventory cross-AS signals: List every CFC interconnection that crosses an AS boundary. Group them by data type.
2. Bundle into AS-wide interconnections: Each distinct destination signal on a remote AS is one interconnection. If multiple blocks write to the same remote tag, only one interconnection is needed (read once, distribute locally).
3. Estimate connection count: Multiply the interconnection count by the partner AS count. Each remote AS is one AS-AS S7 connection (or one fault-tolerant S7 connection if the partner is redundant).
4. Compare against connection ceiling: Pull the connection ceiling from the product manual for 6ES7410-5HX08-0AB0 with V8.2 firmware. Subtract OS connections, engineering connections, and subordinate station connections.
5. Estimate payload: For each AS-AS connection, sum the byte width of the bundled signals. Compare against the Compendium Part A recommended payload per connection.
6. Validate cycle time: Run a CFC compile and load the OB1 / OB35 trace. Verify that the connection-related OB segments do not exceed the configured cycle time.

A worked example shape:

Signal groups: 240 BOOL, 80 REAL, 30 INT, 5 DWORD
AS partners: 4
BOOL grouped 16 per packed byte → 15 packed bytes
REAL 80 × 4 bytes = 320 bytes
INT  30 × 2 bytes = 60 bytes
DWORD 5 × 4 bytes = 20 bytes
Total payload per connection ≈ 415 bytes

Compare the 415 bytes against the recommended payload per AS-AS connection from Compendium Part A. If the value is exceeded, split the signal set across multiple AS-AS connections to the same partner. The CFC compiler can be forced to split by assigning distinct connection groups in the CFC chart properties — this is the standard technique when a single AS-AS bundle exceeds the recommended payload.

Reserve a safety margin of 20 % when sizing for future expansions:

Required_payload_with_margin = Payload × 1.20
Required_connections = ceil(Required_payload_with_margin / Compendium_max_payload_per_connection)

Headroom against the connection ceiling is calculated as:

Available = Total_S7_connections
          - OS_connections
          - ES_connections
          - Subordinate_routes
          - Required_AS_AS_connections

If Available < 0, the AS-AS design must be reduced, additional AS stations must be added, or the OS / ES route count must be trimmed. If Available < 5, the design is borderline and any later scope addition will force a re-spin of the connection table.

Cemat-Specific Deployment Considerations

PCS 7 Cemat is the cement and minerals library that runs on top of PCS 7 V8.x. Cemat applications are characterized by:

• High count of drive blocks (MOT_SPEED, MOT_REV) for conveyors, crushers, mills.
• Many interlock signals (DINT, REAL, BOOL) exchanged between AS that represent physical sections of the plant.
• Long chain of AS stations following the process flow (raw material → crusher → preheater → kiln → cooler → cement mill).
• Heavy use of AS-wide interconnections for section-ready, section-running, and section-fault handshakes.
• Extensive use of route control blocks (CEM_ROUTE) that pass start / destination AS identifiers across the plant bus.

A typical Cemat AS-AS topology has 4 – 8 AS stations arranged in a chain, each exchanging 200 – 400 AS-wide interconnections with its upstream and downstream neighbours. The combined payload per AS easily reaches 1 – 2 KB per partner if not carefully grouped.

Engineers must apply the following Cemat-specific best practices:

• Use drive blocks rather than raw S7 I/O for cross-AS handshakes; the block interfaces are already tuned for AS-wide interconnections.
• Avoid passing entire structures across AS boundaries; break them into named tags before bundling.
• Use CH_CEM library status words rather than individual bits where possible.
• Configure OB35 cycle time to at least 100 ms; faster cycles amplify AS-AS payload pressure.
• Assign explicit connection groups in the CFC chart so the compiler does not bundle unrelated signals into a single AS-AS connection.

Cemat-specific block categories that drive cross-AS traffic:

For very large Cemat plants (≥ 8 AS), consider the PCS 7 multi-project concept: split the project into multiple MP servers, each with its own OS network, and use plant-bus routing for cross-MP communication rather than stuffing all AS into a single engineering project.

Verification, Diagnostics, and Monitoring

After configuration, verify the AS-AS sizing with the following checks:

1. NetPro connection list: Open the PCS 7 project in NetPro, select the CPU 410-5H, and review the Connections table. The Status column shows established, not established, or fault. All AS-AS connections should show established in both H-subsystems.
2. CPU diagnostic buffer: Open the online diagnostic buffer of each CPU 410-5H. Filter on connection events. Recurring connection abort / re-establish entries indicate overload.
3. Connection table (online): From STEP 7, right-click the CPU, choose PLC → Monitor/Modify, and open the connection table. The Active column confirms each AS-AS connection is using both halves of the H-system.
4. CFC compilation report: The CFC compiler reports the count of AS-wide interconnections per AS and the number of AS-AS connections it generated. Cross-check against the engineer's inventory.
5. OB1 / OB35 trace: Use the trace function to record the run time of OB1 and OB35 over a 10-minute window. Look for peaks above the configured cycle time. AS-AS communication-related OBs typically dominate the cycle time when overloaded.
6. Connection diagnostics SFB: Use SFB 52 / SFB 53 / SFB 54 in the user program to read connection status at runtime and surface degradation to the OS faceplate.

Diagnostic event categories frequently seen on AS-AS faults (firmware V8.2):

For long-term monitoring, configure the PCS 7 OS to display the diagnostic status of each AS-AS connection as a custom faceplate. Cemat users can attach the standard DIAG block to the connection status word. The OS picture should show a single traffic-light indicator per AS-AS connection: green for active, yellow for fault-tolerant degraded, red for both halves lost.

Firmware Migration Considerations (V8.2 to V9.x)

Although the original sizing question targets PCS 7 V8.2 firmware, engineers upgrading the CPU 410-5H to V9.x must re-verify the AS-AS limits because the connection ceilings and the Compendium Part A recommendations change between major firmware revisions. The V9.1 reference PDF Process automation with the SIMATIC PCS 7 CPU 410-5H controller lists the V9.1-specific enhancements, but the binding connection counts must be read from the V9.x product page.

The upgrade workflow is:

1. Read the V9.x product manual for the new connection ceilings.
2. Compare against the V8.2 values.
3. If V9.x is more permissive, no project changes are needed.
4. If V9.x is more restrictive, re-bundle the AS-wide interconnections to fit the new ceiling.
5. Perform a full OB1 / OB35 trace validation after the upgrade.
6. Validate that all fault-tolerant S7 connection IDs are still unique after the firmware update.

Firmware V9.x also changes the default fault-tolerant connection behaviour for migrated projects. Always regenerate the S7 connection IDs after a major firmware change; the migrated IDs from V8.2 may collide with new V9.x internal resources and cause intermittent drops during switchover. Engineering teams should keep a backup of the V8.2 NetPro connection list so the rollback path is verifiable.

Frequently Asked Questions