Resolving S7-300 D280 Error Compiling Block During Download

Problem Overview

Engineers migrating or commissioning STEP 7 V5.x programs onto an S7-300 CPU 317-2 PN/DP (6ES7317-2EK14-0AB0) or an ET 200S IM151-8 PN/DP CPU (6ES7151-8AB01-0AB0) frequently encounter the diagnostic string:

(D280) Error compiling block in S7-300 CPU — Unable to copy function block / organization block / data block

The outer STEP 7 dialog reports (294:6) Unable to copy block; the inner event (D280) is the specific Siemens message number that explains why the block could not be transferred. When the user opens the dialog Details…, the line "(D280) Error compiling block in S7-300 CPU" is rendered against the affected block, for example FB36, OB1, DB10, or any user-defined block. With 25+ blocks refusing to download at the same time, the fault is almost always a global resource violation — not a per-block coding error.

The fix in the field is almost always straightforward: bring all timer, counter, and block-number references inside the limits allowed by the target CPU. The remainder of this reference walks through identification, root-cause analysis, repair, and verification.

Affected Hardware and Firmware

The D280 event is reported across the entire S7-300 family when the project contains constructs the target CPU cannot accommodate. The dialog text includes the literal string "in S7-300 CPU" because S7-400 CPUs have different (typically higher) limits and surface the same fault under a different event code.

Root Cause Analysis

The most common D280 trigger on the CPU 317-2EK14-0AB0 and IM151-8 is a timer or counter operand that exceeds the CPU's defined range. The compiled STL/LAD/FBD block contains symbolic references such as T1000, T1001, T1002 which are valid on the source CPU (or on a migrated S7-400 project) but are illegal on the S7-300 target because timer numbers are reserved only from T0 to T511.

STEP 7 enforces this at compile time, not at download time, which is why the same STEP 7 environment reports the failure on every block that contains the offending operand. The compiler walks all block calls (FB, FC, OB, instance DB) within the compiled blocks; the moment it encounters the illegal timer, every parent block that references it becomes un-compilable.

Per the official TIA Portal / STEP 7 documentation, "All the block calls within the compiled blocks are checked. In case of changes to the interface of called blocks, errors will be shown in the 'Compilation' tab". The same logic applies in STEP 7 V5.x when blocks are downloaded to the device.

Other D280 Triggers (Less Common, but Documented)

• Block-number out of range: e.g. FB6000 on a CPU whose max FB is 2048.
• Block larger than work memory: a single block bigger than the CPU's max block size (typically 64 KB for S7-300).
• Block contains operand errors: a real or symbolic address not present in the project (e.g. DB5000 when only 500 DBs are configured).
• Block contains syntax errors: introduced by manual STL edits or by an external tool.
• Block contains DB-specific errors: length mismatch between an FB interface and its instance DB.
• Interface description of FC/FB incorrect: parameter types or IN/OUT/STAT declarations that the target CPU firmware does not support.

S7-300 Resource Limits Reference

When migrating from an S7-400 to an S7-300, or between S7-300 CPU classes, the engineer must realign every absolute operand to the target's maximum. The table below summarizes the S7 timer (T) ranges that are most often mis-sized in field reports:

The S7 counter (Z) ranges follow an identical pattern. So a project that was originally written for an S7-400 (T0-T2047) and is being moved to a CPU 317-2 (T0-T511) will reject every block that uses T512 through T2047.

Diagnostic Procedure (STEP 7 V5.x)

1. Open the failed download dialog and click Details…. Make a note of every block number listed against the D280 string — this is your suspect list.
2. Cross-reference timer and counter usage: in SIMATIC Manager select the S7 program, then Options → Cross-references (or press Ctrl+Alt+F7). Filter the operands to Timer and Counter and sort by Address. Look for any Tnnn or Znnn with nnn above the target CPU's max (512 for CPU 317-2EK14).
3. Check the resource allocation: in HW Config, double-click the CPU row and inspect the Diagnostics / Resources tab (firmware-dependent). The values shown here are authoritative; they are read from the CPU's system data.
4. Verify the target CPU in the project: PLC → Module Information on an online connection, or HW Config → CPU Properties → Memory tab when offline. The Number of timers / counters rows are the field-of-truth limits.
5. Search for the literal timer strings: open a Find/Replace in the source blocks (Ctrl+F) for T10, T11, T12, T13 — if your T-numbering is sequential from T1000, all four will match and confirm the pattern.

Step-by-Step Resolution

The most economical fix is to renumber timers/counters into the allowed range. In a small utility block the change can take minutes; in a 25-block program it is a search-and-replace operation.

1. Create a new timer/counter map: identify the highest actually-used Tnnn in the project. If only 32 timers are referenced, re-base them to T0…T31. The S7-300 reserves timer word patterns; all timer memory is available, so no "reserved range" collision occurs.
2. Use Edit → Find and Replace in each offending block, or in the S7 program sources. Replace, for example, T1000 with T0, T1001 with T1, and so on. Keep an offline map in a notepad for traceability.
3. If the timer is named via a symbol table entry, rename only the symbol; the underlying T operand is what the CPU cares about.
4. Recompile the affected blocks: Program → Compile All in SIMATIC Manager. Look for any Errors in the output window — they should now be empty.
5. Check block consistency: Program → Check Block Consistency (or Check Block Consistency Group if you have a coordinated …_DI source). The output should report No errors.
6. Download: PLC → Download to the target CPU. If D280 persists, jump to the Memory Diagnostics section below.

Memory Diagnostics (If Renumbering Does Not Fix It)

If the timer/counter audit comes back clean but D280 remains, suspect a work- or load-memory exhaustion. On a CPU 317-2EK14-0AB0, the work memory for code and data is 1 MB. Each FB/FC/DB consumes memory proportional to its compiled size, the instance data, and the local-data stack.

1. PLC → Module Information → Memory: this live tab shows Used load memory, Free load memory, Used work memory (code), Free work memory (code), Used work memory (data), and Free work memory (data). If Free approaches zero on any row, the next download will be rejected.
2. Delete obsolete blocks in the offline S7 program: Program → Clean Up, or right-click → Delete. Re-download only what is required.
3. Compress memory: on the online CPU, PLC → Online → Compress Memory. This consolidates fragmented load memory and frequently resolves "D2xx" download failures caused by a non-contiguous free block.
4. Verify the MMC / SIMATIC MC: on S7-300 CPUs, the load memory is on the SIMATIC Micro Memory Card. If the card is full or has worn out, a Memory Card error will appear in the diagnostic buffer; clear it by re-inserting or replacing the card.
5. Check max block size: a single compiled block cannot exceed the CPU's max block size. For S7-300 this is 64 KB; anything larger is rejected at download with a D280 family message. The PC tool S7 Information Tool reports block sizes via right-click → Object Properties → Size.

Block Consistency Check Procedure

STEP 7's block-consistency check is the most efficient way to surface all D280 candidates in one pass:

1. In SIMATIC Manager, right-click the Blocks folder of the S7 program and choose Check Block Consistency.
2. The tool reports every block whose interface does not match its calls, and every block that contains operand errors. Click Compile to auto-fix where possible.
3. Open the Compilation tab at the bottom of the editor (in TIA Portal: Project tree → Program blocks → Compile). Any remaining D280 roots will be listed by address or block number.

Related D2xx Error Codes

The D2xx family is the Siemens numbering scheme for block transfer / compile / consistency events. The most relevant cousins of D280 are:

The complete D2xx list is published in the Siemens support PDF "Message: Unable to copy function block FB36" (entry 7416913) on the Siemens Industry Online Support portal.

Migration Considerations (S7-400 → S7-300, or between S7-300 CPUs)

When a project is moved from a higher-class CPU to a lower-class CPU — for example from an S7-400 down to an S7-300, or from a CPU 319-3 down to a CPU 317-2 — the engineer must validate every absolute operand on the target:

• Timers: T0-T2047 → T0-T511 (or T0-T255 for IM151-8).
• Counters: Z0-Z2047 → Z0-Z511 (or Z0-Z255 for IM151-8).
• Block numbers: S7-400 supports up to 6000 FBs / 6000 FCs / 8000 DBs; S7-300 CPU 317-2EK14 supports up to 2048 FBs, 2048 FCs, 2048 DBs. Anything outside the range will trip D280.
• Local data stack: S7-300 default is 1024 bytes per priority class. If your FBs use heavy multi-instance declarations, the local data stack may overflow; rebalance using CPU Properties → Local Data in HW Config.

A pre-migration checklist that walks the project for out-of-range operands prevents almost every D280 event. A practical tool for this audit is the S7 project Reference Data → Program Structure view, sorted by address.

Verification

After the source has been corrected and re-compiled, confirm success with the following checks:

1. Re-run Check Block Consistency — must report No errors.
2. Re-run Compile All — output window must be clean.
3. Download the program — the dialog should close without (294:6) or D280 messages.
4. Read back the diagnostic buffer: PLC → Diagnostics → Diagnostic Buffer. The new entries should be operational, not block-transfer errors.
5. Run a Cold Restart on the CPU and observe the RUN LED: it must stay green; a flashing red indicates a residual block-consistency issue.
6. Verify timers online: with the program running, open Monitor / Modify on a corrected timer, for example T0, and confirm the time value and status running / expired behave as expected.

Best-Practice Recommendations

• Prefer IEC timers (SFB3/SFB4/SFB5 in the S7 system library, or S7-300/400 standard library TP / TON / TOF) for new code. They use instance DB addressing, which scales with the project and is not bound to fixed Tnnn numbers. S7-300 CPU firmware V3.x and above supports them without reservation.
• Maintain a resource map in the project documentation: list all timer and counter ranges in use per CPU. The map is a one-page defense against D280 during future migrations.
• Configure IEC check in STEP 7 (Options → Settings → LAD/FBD/STL Editor → Operand Fields) so out-of-range operands are flagged at the time of writing, not at download.
• When commissioning a new CPU class, run a dry compile in SIMATIC Manager before going online. The compiler will surface most D280 conditions without consuming CPU resources.