Resolving Inactive @CPU_RT Diagnostic Blocks in S7-400H Controllers - SFC78 Support and Firmware Requirements

When the automatically generated @CPU_RT diagnostic block in SIMATIC PCS 7 displays static zero values and block output SFC78_EX = 0 on an S7-400 controller (such as a redundant CPU 417-4H), the failure indicates that the CPU's internal firmware does not support System Function SFC 78 (OB_RT). The @CPU_RT block relies fundamentally on SFC 78 to query the operating system kernel for net runtime consumption, cyclic OB execution durations, and communication load. Because system functions reside in the controller's permanent firmware ROM, SFC 78 cannot be downloaded as an external block. Resolving the issue requires upgrading the physical CPU firmware to a version supporting SFC 78 (e.g., CPU 417-4H Firmware V4.5 or later) or migrating to a modern AS 410-5H controller where SFC 78 is natively supported.

The Diagnostic Function of @CPU_RT and System Function SFC 78 (OB_RT)

In SIMATIC PCS 7 Maintenance Station and Asset Management architectures, monitoring the performance and computational loading of Automation System CPUs is essential for plant reliability. When compiling CFC charts, the Driver Generator automatically instantiates the @CPU_RT block within a dedicated @-prefixed runtime group in OB100 and OB1.

The @CPU_RT block monitors CPU net cycle time, peak runtime, execution duration of individual organization blocks (OB32, OB35, OB80), and communication buffer consumption. It collects this telemetry by invoking SFC 78 (OB_RT). SFC 78 directly queries the hardware executive on the CPU ASIC. If the underlying controller hardware or firmware lacks the SFC 78 instruction in its operating system table, @CPU_RT cannot retrieve data, setting its boolean status output SFC78_EX = 0 (SFC 78 Not Existing) and leaving all runtime metrics at zero.

CPU Firmware Prerequisites and the Significance of SFC78_EX

Support for SFC 78 was introduced progressively across S7-400 CPU generations:

Controller Family / ModelHardware MLFBMinimum Firmware for SFC 78SFC78_EX StatusSIMATIC AS 410-5H6ES7410-5HX08-0AB0All Firmware Versions (V8.0+)1 (Fully Supported)CPU 417-4H (Modern)6ES7417-4HT14-0AB0Firmware V4.5 or higher1 (Fully Supported)CPU 417-4H (Legacy)6ES7417-4HL01-0AB0Maximum Firmware V3.10 (Unsupported)CPU 416-3 / 416-26ES7416-3XR05-0AB0Firmware V5.0 or higher1 (Supported)CPU 414-4H (Legacy)6ES7414-4HJ00-0AB0Maximum Firmware V3.10 (Unsupported)

On older legacy S7-400H systems running Firmware V3.1, SFC 78 does not exist in the CPU microcode. Because system functions are hardcoded into the CPU firmware ROM and cannot be downloaded into load memory, user programs cannot inject SFC 78 artificially.

Evaluating Controller Firmware and Modernization Options

When encountering SFC78_EX = 0, engineering teams should evaluate their options based on the physical MLFB:

Check Firmware Upgrade Eligibility: In SIMATIC Manager, select the CPU and press Ctrl+D (Module Information). Inspect the hardware order number (MLFB). If the CPU is a 6ES7417-4HL04-0AB0 or 6ES7417-4HT14-0AB0, download the latest operating system firmware update (V4.5.x) from Siemens Industry Online Support and flash the CPU memory cards.
Hardware Modernization for Discontinued Controllers: If the CPU is an older revision (6ES7417-4HL01-0AB0), it cannot be upgraded to firmware V4.5 due to microprocessor hardware limitations. The controller must be upgraded to the modern AS 410-5H process automation controller.
Alternative Diagnostic Strategy: On un-upgradable legacy controllers, the @CPU_RT block cannot provide telemetry. Engineers must monitor CPU performance manually via the STEP 7 Module Information dialog (Performance data and Cycle time tabs).

Verification

Verify @CPU_RT functionality and SFC 78 presence using these diagnostic steps:

Online Block Inspection: Open the CFC chart containing @CPU_RT in online test mode. Inspect output SFC78_EX. Confirm that SFC78_EX = 1.
Telemetry Validation: Observe block outputs NETTO_R (Net runtime percentage) and OB35_R (OB35 execution time). Confirm that values update dynamically rather than remaining frozen at 0.0.
Maintenance Station Visualization: In WinCC Runtime, open the AS Diagnostic Faceplate on the Maintenance Station. Verify that the CPU bar graphs for CPU load and cyclic task duration render live data.

Limitations and Operational Precautions

Observe the following precautions when working with CPU runtime diagnostics:

Overhead in High-Frequency Tasks: Do not call @CPU_RT within ultra-fast cyclic tasks (such as OB32 at 20ms); the block should execute in OB1 or OB36 (500ms) to avoid adding unnecessary monitoring overhead.
Redundant H-System Flashing: When updating firmware on redundant S7-400H CPUs to enable SFC 78, execute the firmware update in redundant mode following official Siemens H-system non-stop firmware upgrade guidelines to maintain continuous operation.
Commercial Procurement and Licensing Disclaimer: Upgrading legacy CPU 417-4H hardware to modern AS 410-5H controllers requires proper hardware procurement schedules, System Expansion Cards (SEC), and migration planning. Confirm all MLFBs and Process Object capacity with an authorized Siemens distributor.

Engineer FAQ

Can I copy SFC 78 from a CPU 410 project into my CPU 417 block folder?

No. System Functions (SFCs) are internal firmware interfaces etched into the CPU operating system. They cannot be extracted, copied, or downloaded across different CPU architectures.

Does an inactive @CPU_RT block impact plant control logic?

No. The @CPU_RT block is purely diagnostic. If SFC78_EX = 0, technological control loops, motors, and valves continue running normally; only CPU asset management telemetry is unavailable.

Is @CPU_RT supported on S7-PLCSIM?

S7-PLCSIM V5.4 has limited emulation of SFC 78. In many PLCSIM versions, @CPU_RT will report static zero values because virtual simulation does not accurately model physical hardware CPU cycle times.