Resolving PCS 7 AS Download Error 'Task Not Available in Online CPU (OB101)'

The AS download error 'Task not available in online CPU (OB101)' accompanied by 'No text in resourceDLL S7comfb.dll' on a redundant S7-400H controller occurs because a CFC block or runtime group was erroneously assigned to Organization Block 101. S7-400H fault-tolerant CPUs do not support hot restart (OB101) in redundant operating mode; they support only warm restart (OB100). When CFC attempts to link tasks to the non-existent OB101, the CPU firmware rejects the download. To resolve the failure, open the CFC Run Sequence (Ctrl+F5), locate all blocks or runtime groups placed under OB101, move them to OB100 or OB35, compile changes, and download.

S7-400H Controller Operating System Tasks: Why OB101 Is Unsupported

In SIMATIC S7-400 automation stations, the CPU operating system organizes code execution into prioritized Organization Blocks (OBs). For system startup, the architecture defines three restart mechanisms:

OB100 (Warm Restart / Neustart): Restores non-retentive memory to initial states while preserving retentive memory. This is the standard and mandatory restart mode for all process automation controllers.
OB101 (Hot Restart / Wiederanlauf): Resumes execution from the exact interruption point without resetting timers, counters, or non-retentive data. Supported only on specific standard single CPUs with backup battery buffering.
OB102 (Cold Restart / Kaltstart): Completely clears all retentive and non-retentive work memory to default initial values.

In fault-tolerant redundant controllers (such as S7-400H and CPU 410-5H), maintaining redundancy synchronization requires deterministic memory states upon startup. Consequently, Siemens operating system firmware for S7-400H completely omits OB101. Hot restart is architecturally impossible in a redundant H-system.

How OB101 Inadvertently Enters CFC Run Sequences

If OB101 is not supported, how does it end up in a PCS 7 project?

Importing Third-Party or Legacy Blocks: When migrating blocks from classic discrete STEP 7 projects or third-party libraries, blocks authored in SCL or STL often carry task insertion attributes: { S7_task := 'OB101' }.
Accidental Manual Creation in Run Sequence: An engineer opening the CFC Run Sequence editor (Options -> Edit Run Sequence) may accidentally insert a new runtime group under the OB101 task node.
CFC Compiler Behavior: The CFC compiler generates execution calls for whatever tasks are populated in the run sequence. When downloading to the online S7-400H CPU, the loader queries the CPU's task descriptor table. Because OB101 does not exist in firmware, the CPU rejects the transaction with the obscure error:

'Message 0x0 No text in resourceDLL S7comfb.dll. Task not available in online CPU (OB101)'

Step-by-Step Procedure to Remove OB101 and Restore AS Downloads

Execute the following steps to clean the CFC run sequence and restore delta download capability:

Open CFC Run Sequence:
- Open any CFC chart within the affected AS program.
- Select Options -> Edit Run Sequence (or press shortcut Ctrl+F5).
Locate Task OB101:
- In the run sequence tree, scroll through the task list (OB1, OB30 to OB38, OB100, etc.).
- Locate the branch labeled OB101.
- Expand OB101 to reveal any runtime groups or individual blocks residing within it.
Relocate Blocks and Groups:
- Drag all runtime groups or blocks out of OB101 and move them into OB100 (if startup logic) or the appropriate cyclic interrupt task (e.g. OB35 for 100ms cyclic control).
- Once emptied, right-click the empty runtime group under OB101 and select Delete.
- Ensure the OB101 task node is completely empty (no groups, no blocks).
Compile CFC Changes:
- In CFC, select Chart -> Compile -> Charts as Program....
- Select Scope: Changes only and compile.
Execute AS Delta Download:
- Select PLC -> Download -> Scope: Changes only.
- The download manager bypasses OB101, and the download completes successfully in RUN.

Organization Block Support Matrix: Standard S7-400 vs Fault-Tolerant H-CPU

The following table highlights startup and cyclic OB availability between standard and fault-tolerant controllers:

Organization BlockOperational PurposeStandard S7-400 CPU (e.g. 416)Fault-Tolerant H-CPU (e.g. 417-5H / 410-5H)Permitted in PCS 7 CFC?OB100Warm Restart (Neustart)SupportedSupported (Mandatory)Yes (Primary startup task)OB101Hot Restart (Wiederanlauf)Supported (with battery)NOT SUPPORTED (Firmware absent)FORBIDDEN (Causes download error)OB102Cold Restart (Kaltstart)SupportedNot used in PCS 7NoOB70 / OB72Redundancy error evaluationNot ApplicableSupported (H-System core)Yes (Automatically generated)OB80 to OB88Hardware / Cycle fault OBsSupportedSupportedYes (System generated)

[!NOTE]
Commercial Accuracy Notice: System Organization Blocks are integral firmware features of SIMATIC S7-400H controllers > (6ES7417-5HT06-0AB0) and CPU 410-5H (6ES7410-5HX08-0AB0). Reorganizing runtime tasks inside CFC does not alter Process > Object counts or consume engineering licenses.

Verification

Confirm that the download error is resolved and startup execution is verified:

Successful Delta Download Log: Confirm that the CFC download log displays: 'Download changes completed with 0 errors' without task availability rejections.
Run Sequence Verification: Open Chart Reference Data (Options -> Chart Reference Data -> Run Sequence). Confirm that OB101 is completely omitted from the active execution table.
Online Diagnostic Buffer Check: Open Target System -> Diagnostic Buffer. Verify that no OB-missing or task fault events are logged.

Limitations and Operational Precautions

Follow these precautions when developing or migrating blocks for PCS 7:

Audit SCL S7_task Attributes: When authoring SCL blocks intended for S7-400H controllers, never specify S7_task := 'OB101'. Always target OB100 for initialization routines.
Do Not Delete System OBs: In the SIMATIC Manager block folder, never manually delete standard fault OBs (such as OB70, OB72, OB82, OB85, OB86), as their absence forces CPU STOP during fieldbus transients.
Consistent Task reduction: When configuring cyclic OBs (OB30 to OB38), ensure scan phase reductions do not exceed controller execution limits.

Engineer FAQ

Why did this download error only occur on one PLC in my multiproject?

Only that specific PLC's chart folder contained a block or runtime group assigned to OB101. The other two PLCs were correctly configured without OB101 in their run sequences.

What does 'No text in resourceDLL S7comfb.dll' mean?

It is a generic message indicating that the error code returned by the online CPU (code for 'task non-existent') lacks a translated human-readable string in that specific communication library DLL.

Can I download this change in RUN?

Yes. Moving blocks out of OB101 into OB100 or cyclic OBs can be compiled and downloaded as changes in RUN mode without stopping the process.

Why doesn't the S7-400H CPU support OB101?

Hot restart (OB101) resumes execution without clearing temporary buffers. In redundant fault-tolerant systems, both CPUs must initialize from identical, deterministic memory states upon restart to achieve redundancy lock.