SINUMERIK 840D PLC OB1: Resolving Red CALL/END CALL Errors

1. Problem Definition: Red CALL/END CALL in OB1

When a SINUMERIK 840D / 840D sl PLC project backup is opened in SIMATIC Manager (Step 7 V5.x), operators frequently encounter networks in OB1 where the CALL and corresponding END CALL (or BEC) lines are rendered in red text instead of the normal black. The diagnostic buffer of the running PLC reports no fault, the machine continues to produce, yet the offline project cannot be compiled and any attempt to download the project as-is is expected to drive the integrated PLC into STOP mode.

The reported networks in the source case were: 1, 4, 5, 6, 7, 8, 11, 12, 14, 18, 25, 26, 28, 30 — twelve networks in a single OB1, all showing the same red CALL marker without a resolvable block operand after the instruction.

2. SINUMERIK 840D PLC Architecture Overview

The SINUMERIK 840D / 840D sl numerical control embeds a SIMATIC S7-300-class PLC. The standard configuration uses:

The PLC runs an S7-300-compatible instruction set with the cyclic organization block OB1 as the master scheduler. The PLC basic program supplied by Siemens consists of a fixed set of FCs, DBs and OBs that handle the NCK↔PLC interface, the MCP (Machine Control Panel), the HMI signals, axis/spindle handshakes, and the safety-integrated logic where fitted. The PLC basic program blocks are part of the SINUMERIK 840D sl toolbox and must be present in the offline project for compilation to succeed.

The official reference for PLC block architecture is the SINUMERIK 840D sl PLC Function Manual, which lists every standard FC/FB, its parameters, and the calling convention used by the basic program.

3. OB1 Structure and CALL/END CALL Mechanics

In the SINUMERIK 840D, OB1 is reserved for the user program. The NCK-side PLC basic program is normally executed by separate OBs (OB40 for hardware interrupts, OB100 for restart, OB82 for diagnostic interrupt, OB86 for rack failure, OB121/OB122 for error handling). The user OB1 therefore contains only user logic — typically a sequence of CALL FC xxx / CALL FB xxx, DB yyy statements that invoke function blocks implementing mode-group control, axis enabling, tool changer, coolant, lubrication, etc.

Two call syntaxes exist in Step 7:

The red rendering is the Step 7 editor's signal that the CALL instruction has no resolvable target. Common reasons:

1. The called FC/FB number is not present in the offline project (block was stripped from the backup).
2. The symbol referenced by a symbolic call no longer exists in the symbol table.
3. The source was originally written in KOP (ladder) or FUP (function block diagram), exported to STL, and the operand token was lost in the conversion.
4. The basic PLC program library (toolbox) is not installed; standard FCs like FC19 (MCP interface) cannot be resolved and CALL lines render red.

4. Why the Machine Continues Running with a "Red" Backup

The paradox — red blocks in the backup, no fault in the live PLC — is explained by the distinction between the offline project and the online user memory:

• The PLC on the machine holds the last successfully compiled and downloaded program in its load memory. That program has all CALL operands intact and executes correctly.
• The backup archive opened on the engineering laptop may be missing optional toolbox blocks, may have been saved with partial blocks, or may have been round-tripped through a third-party tool that stripped the operands after the KOP/FUP→STL translation step.
• As long as the offline project is not downloaded, the online program is untouched.

This is why the diagnostic buffer shows no error, why MCP/axes/HMI respond correctly, and yet the engineering display shows a dozen networks in red.

5. Root Cause Classification

Red CALL/END CALL lines in OB1 fall into four diagnostic classes:

The source case study aligns with Class D — the discussion explicitly notes that the segment between CALL and END CALL is the residual STL generated when Step 7 Manager re-translates a KOP/FUP source.

6. Diagnostic Procedure (Offline + Online)

Follow this procedure before editing any block. It is non-destructive and does not place the PLC at risk.

1. Open the project read-only. In SIMATIC Manager, right-click the project, choose File > Open as read-only, or use Options > Customize > View > Offline/Online to inspect both views side-by-side.
2. Establish an online connection. With Accessible Nodes or PLC > Display Accessible Nodes, locate the CPU (default address 2 on MPI/PROFIBUS, or the configured Ethernet IP). Use PLC > Connect to Target System.
3. Open OB1 in online view. Double-click the online OB1 to display the block currently executing on the CPU. Read each network to record the actual CALL FCxxx / CALL FBxxx, DByyy operand.
4. Compare online vs. offline. Use Options > Compare Blocks (online/offline). The Step 7 comparator highlights mismatches. Red CALL lines in offline blocks that are identical to the online block confirm a display/serialization issue rather than a logic change.
5. Check the CPU diagnostic buffer. PLC > Diagnostics/Settings > Diagnostic Buffer. Look for entries with event ID 0x4302 (STOP due to programming error), 0x4570 (OB not found), or 0x4xxx series I/O faults. Absence of any of these confirms the online program is healthy.
6. Inspect the Symbol Table. If CALL lines reference names like "MCP_IFM", open S7 Program > Symbols and verify each name maps to a valid FC/FB number.
7. Verify the toolbox library. In File > Open > Libraries, open the SINUMERIK 840D sl toolbox library. If absent, this is the primary cause of red standard FCs.

7. Resolution Path by Class

7.1 Class A — Toolbox Not Installed

Install the SINUMERIK 840D sl toolbox appropriate to the NCU firmware version (e.g., toolbox V4.95 for SW 4.95, V5.3 for SW 5.3). Insert the basic program into the offline project:

1. Open the toolbox library in SIMATIC Manager.
2. Drag the Basic Program container into the S7 Program of the offline project.
3. Recompile all blocks (Program > Compile All, or right-click the Blocks container > Compile).
4. Confirm CALL lines in OB1 now render black.

The toolbox content and block-level API are documented in the SINUMERIK 840D sl PLC Function Manual (Chapter 2, PLC basic program).

7.2 Class B — Single User FC/FB Missing

1. Locate the block in the online CPU: right-click Blocks (online) > Download to PG for the suspected FC/FB.
2. Insert the downloaded block into the offline S7 Program > Blocks.
3. If the block intentionally does not exist on the machine, edit OB1 to remove the CALL pair entirely.

7.3 Class C — Symbolic Resolution Failure

1. Open the Symbol Table.
2. Either add the missing symbol with the correct absolute address (e.g., FC 19) or convert the symbolic CALL to an absolute CALL by deleting the quoted name and entering the numeric operand.

7.4 Class D — KOP/FUP→STL Conversion Loss

This matches the source case study:

1. With online OB1 open, scroll to the network number(s) flagged in red offline.
2. For each affected network, copy the resolved CALL FC xxx operand from the online view.
3. Switch to offline view, open OB1 in STL, and edit the network: replace the empty operand after CALL with the recorded FC/FB number. Re-key END CALL if Step 7 has stripped it.
4. Save the block. Step 7 may prompt "Block is inconsistent" — confirm Yes; the next compile pass regenerates the block interface.
5. Run Program > Compile All. All CALL lines should now render black.

8. Safe Download Procedure (Avoiding PLC STOP)

After the offline project compiles cleanly, follow the documented Siemens cold/warm download path:

1. Back up the current online project first. PLC > Upload Station to PG. Save to a dated folder. This is the rollback path if the new program fails.
2. Place the machine in a safe state. NCK reset ("Reset" key on MCP), all drives disabled, axes at safe positions, doors closed. Follow the site lockout-tagout (LOTO) procedure.
3. Switch the CPU to STOP. Use the MCP selector or PLC > Operating Mode > STOP. Note the STOP transition in the diagnostic buffer (event ID 0x4301) for traceability.
4. Delete the user program blocks. PLC > Clear/Reset. This is required when block interfaces (FB signatures, DB structures) change. If only CALL operands were edited and FB/DB structures are unchanged, this step may be skipped.
5. Download the corrected blocks. Select the Blocks container > PLC > Download. Confirm only the intended blocks are selected.
6. Switch to RUN. PLC > Operating Mode > RUN-P. Watch the diagnostic buffer for event ID 0x4302 (which would indicate the download failed) or normal restart event 0x4300 series.
7. Verify online ↔ offline parity. Options > Compare Blocks again. All blocks should report identical.
8. Smoke-test the machine. Jog axes, run a dry program (e.g., M00 block with no motion), confirm MCP, HMI, and tool changer respond.

9. Toolbox and Software Requirements

For the SINUMERIK 840D sl NCU line, the PLC programming environment requires:

The toolbox is not optional for the SINUMERIK 840D PLC. Without it, the standard FCs and the symbol table entries for the basic program are missing, and any reference to them in the offline project — including CALL FC 19, CALL FB 4, DB 7, etc. — renders red. The user will not see "errors" in the PLC because the actual online program is unaffected, but compilation and any new download will fail.

The toolbox version must match the NCU software version. Mismatched toolbox and NCU SW combinations can produce SF (System Fault) on the PLC after download due to interface mismatches in DB structures used by the basic program.

10. Verification and Commissioning Checks

After recompiling and downloading, run the following verification matrix:

11. Edge Cases and Field-Proven Caveats

From documented Siemens support practice and field commissioning notes:

• Backup obtained from a different NCU SW line. A project archived from a 840D sl SW 4.5 system opened on a SW 5.3 environment will show CALL lines red for blocks whose interface changed. The toolbox provides version-specific FC/FB variants — install the matching toolbox and replace the affected FC/FB pair.
• FB instance DB misalignment. If an FB in the offline project has a different interface signature than the online FB, the corresponding DB instance shows red. Delete the inconsistent DB and let the next compile pass regenerate it, or use PLC > Download with the "Include user blocks" option to force a fresh instance DB.
• Multiple projects in same S7 program. If the S7 program contains both user blocks and basic-program blocks, ensure the basic program is referenced (not copied) — copied blocks do not auto-update with NCU firmware changes.
• MPI vs. PROFIBUS vs. Ethernet routing. Online diagnosis of OB1 over Ethernet to a 840D NCU requires the PG/PC interface to be set to TCP/IP with the NCU's IP address (default 192.168.214.1) and the access list of the NCU allowing the engineering station. If "Go online" fails, check Set PG/PC Interface in the Control Panel and confirm the route via the PLC basic program's FB 9 (if used).

12. Preventive Best Practices

1. Archive with the toolbox bundled. When taking a project backup, include the toolbox library as a sibling in the archive so that any restore environment has the reference blocks.
2. Document NCU SW / toolbox version pairs. Tag each archive with NCU software version (e.g., "SW 4.95 SP5") and toolbox version to avoid mismatched restores.
3. Use the upload-from-station as the canonical source. After commissioning, perform PLC > Upload Station to PG once and compare against the engineering project. Resolve any drift before the next backup cycle.
4. Prefer absolute calls for the basic program. Symbolic CALLs to FC 19, FC 21, etc. can be lost across toolbox upgrades; absolute calls survive re-installation of the toolbox library.
5. Always compile-all before download. Step 7's per-block compile can hide cross-block inconsistencies (e.g., interface changes in an FB not propagating to all instance DBs). Program > Compile All catches these.
6. Use S7-PLCSIM for new logic. When adding a new FC to OB1, simulate in S7-PLCSIM with a stubbed FC 19 before downloading to the live system.

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