SINUMERIK 840C Error 43 PLC-CPU Not Ready: Restoring Missing S5 PLC Blocks

The SINUMERIK 840C controller family, used extensively on late-1980s through mid-1990s Index G300 lathes, turning centers, and machining cells, depends on a SIMATIC S5 PLC subsystem for all discrete I/O, axis enables, spindle logic, tool changer sequencing, hydraulic/coolant interlocks, and operator-panel handshake. When the machine comes up with alarm 43 "PLC-CPU not ready for operation" and the PLC service page reports cause 0005 / ZInfo 0000 0000 0000 0000, the controller is signalling that the S5 CPU has either stopped, is in cold-restart, or is rejecting the cyclic program because a block referenced by another block is not present in the loaded image. The most common trigger after a power-supply replacement is a corrupt or incomplete PLC backup on the machine hard disk, where organisation blocks, program blocks, or sequence blocks are missing from the EPROM/Flash image even though the NCK archive was restored successfully.

This technical reference walks through the diagnostic steps, block-level analysis, and recovery procedure that have proven reliable on 840C retrofit work. It assumes the engineer has STEP 5 V7.x installed on a PG with a CP (e.g., CP5611, MPI/AS511 converter), access to the original commissioning archive, and the ability to stop and restart the machine safely.

1. Problem Overview

2. SINUMERIK 840C Architecture and the PLC Subsystem

The 840C is built around two cooperating processors. The NCK (numerical control kernel) handles interpolation, axis position control, and operator interface. The PLC-CPU is a discrete SIMATIC S5-135U or S5-150U module (depending on the machine's option list) installed in the same rack as the NCK. Communication between the two is via a dual-port RAM (DPR) and a defined mailbox of interface DBs.

The PLC program is stored on a Flash card (or EPROM module) plugged into the S5 CPU. When the CPU boots it copies blocks from the card into internal RAM, performs a cold-restart sequence, and signals readiness to the NCK via the mailbox. If the cold-restart fails — typically because an OB, PB, SB, or referenced FB/FX/DB is missing — the CPU enters the STOP state with a diagnostic buffer entry that the 840C interprets as cause 0005.

When the PLC power supply was replaced, the new unit started clean, but the Flash card image had been either overwritten by a partial backup reload, or had never contained the complete original program. A general reset clears work flags, counters, timers, and process images but does not reload program blocks from disk — that operation requires a deliberate PLC menu restore in the 840C service screen or an explicit Flash card write through STEP 5.

3. Decoding Alarm 43 — "PLC-CPU not Ready for Operation"

Alarm 43 is generated by the NCK at power-on and whenever the PLC readiness handshake bit transitions low. The NCK monitors a dedicated flag in the DPR mailbox (typically DB10, byte 0 — bit conventions vary by SW version) and declares the PLC not ready if the bit is not asserted within the configured PLC startup time-out (default ~6 seconds on 840C; can be stretched to 20 s on heavily loaded machines).

Cause 0005 with ZInfo 0000 0000 0000 0000 narrows the problem to a missing block. The ZInfo normally contains the block number and block type when the CPU captures the failure; a value of zero indicates that the CPU either failed before populating the diagnostic buffer (incomplete image) or the buffer itself was cleared by the reset sequence. Either way the recovery path is the same: confirm exactly which blocks are loaded and which blocks the program tries to call.

4. Root Cause Analysis — Why the Error Persists After Reset

Resetting the NCK and PLC and reloading the backup from the machine hard disk is the textbook first step, but on a 840C installation it can mask the actual problem for three reasons:

1. The NCK backup and the PLC backup are separate archives. The 840C stores the NCK archive under a different path than the S5 program. Reloading the NCK restores axes, drives, lead-screws, and tool tables — which is why the machine appears correctly configured (5 axes, drives recognised) even with a totally missing PLC program.
2. The PLC backup on the machine hard disk may itself be incomplete. 840C service technicians frequently make a single backup after a software option change and never re-archive. If the file has bit-rot, been partially overwritten, or was made before a late-stage modification, the PLC image returned to the Flash card is missing blocks.
3. The general reset procedure overwrites the PLC runtime data but does not erase the Flash card. If the Flash card was never re-written with the recovered program, the CPU boots from the (still corrupted) image on the card.

5. Diagnostic Procedure with STEP 5 V7.3

Connect a programming device with STEP 5 V7.3 to the S5 CPU via the MPI/AS511 port. The PLC-CPU on a 840C is online as long as the CPU is not in hard STOP; if the CPU is in STOP, you can still read the diagnostic buffer and the block directory.

5.1 — Read the PLC Diagnostic Buffer

From the STEP 5 online editor, open PLC > Diagnostic Buffer (or equivalent in V7.3). Look for entries with stop cause, stack content, and block reference. Common messages related to cause 0005 include:

• OB not loaded — OB1 or OB21/OB22 missing
• PB not loaded — referenced PBx absent
• FB not loaded — referenced FB/FX absent
• SB not loaded — step sequence block missing
• DB not loaded — called DB not generated

5.2 — Compare Block Directory vs. Program Cross-Reference

Open the online block directory (PLC > Block Directory or Project > Blocks) and capture the full list of blocks present in the CPU. Then open the offline project (the archive extracted from the machine) and compare. The mismatch is your smoking gun.

The report that only FBs, FXs, and DBs are visible is a strong indicator that the OB and PB/SB layer is missing. Without OB1 the CPU never enters the cyclic program, so the PLC service flags "not ready" even though all the function blocks were loaded.

5.3 — Capture the Block Cross-Reference

In STEP 5 V7.3 use Editor > Cross-Reference List or PLC > Reference Data to generate a list of every CALL/JU from each loaded block. Blocks that are called but absent in the online image are the missing targets. Pay special attention to:

• CALL FB 100 from OB1 — but FB 100 is not present
• CALL PB 30 from OB1 — but PB 30 not loaded
• CALL SB 5 from PB 12 — but SB 5 not loaded

6. S5 Block Structure Reference

Understanding what each block type is for helps identify the gap when only some block types survive a backup.

Note: KW = Kilo-Word (1 KW = 16-bit S5 word). DW = Data Word. The above sizes are typical; actual sizes vary with program complexity.

7. Recovery Procedure — Step by Step

7.1 — Prerequisites

• STEP 5 V7.3 installed on a PG with serial/MPI interface (CP5611 or AS511 converter)
• Original commissioning archive on PG, USB stick, or CD-ROM (the complete S5 program, not just the NCK archive)
• Documentation set for the machine: parts list, PLC program listing, IO assignment sheet
• Backup of the current (non-working) PLC image for forensic purposes
• Access to the Flash card or EPROM module on the S5 CPU

7.2 — Recover the Missing Blocks

1. Connect STEP 5 to the S5 CPU. Read the current block directory to confirm what is loaded.
2. Open the offline project from the original commissioning archive.
3. Compare offline vs. online block lists. Note every block missing online.
4. Generate the cross-reference list to verify the missing blocks are indeed called.
5. In the offline project, select all blocks. Force PLC > Download with the option Replace blocks and Include all block types.
6. Confirm the online block directory now matches the offline image. Pay particular attention to OB1 and OB21/OB22.

7.3 — Reset the PLC

After the block image is corrected, perform a PLC overall reset from the 840C service menu:

Menu → Commissioning → PLC → Overall Reset → Yes

or via STEP 5:

PLC → CPU Functions → Overall Reset

This clears work flags, counters, timers, and process images but leaves the loaded program intact. The PLC should transition to RUN within a few seconds.

7.4 — Reload the Flash Card (if backup was on Flash)

If the S5 CPU boots from a Flash card and the Flash card itself is corrupt, the blocks must be written back to Flash using a S5 programmer (PG with EPROM/Flash programming adapter) or by booting from RAM long enough to perform a Flash update:

1. Place CPU in STOP.
2. Insert an empty (or formatted) Flash card in the programming socket.
3. From STEP 5 select PLC → EPROM → Write Flash.
4. Mark all blocks to be written.
5. Confirm write and verify each block after the write completes.
6. Power-cycle the PLC-CPU and verify it boots cleanly from Flash.

8. Hardware Verification After Power Supply Replacement

The replacement of the power supply module is the original trigger and should not be overlooked. After replacing an S5 power supply, verify the following before trusting any software fix:

A marginal PSU can produce intermittent "block not loaded" symptoms because the Flash card read returns garbage; replacing the PSU alone may not cure the problem if the card has been corrupted.

9. Commissioning and Final Verification

Once the PLC program is restored and the CPU is in RUN, validate the system from bottom up:

1. PLC service page: Confirm PLC-CPU ready = TRUE, cause code = 0000, ZInfo = 0000 0000 0000 0000. Alarm 43 should clear.
2. NCK reference: Drive the machine back to reference in each axis (REF mode) and confirm all axes home without alarm.
3. I/O handshake: From the 840C diagnostic page, force individual outputs and confirm the corresponding LEDs/contacts. Cross-check against the IO assignment sheet.
4. Mode switching: Verify JOG, MDI, AUTO, and TEACH-IN modes are recognised by the NCK and that the PLC enables the corresponding functional groups.
5. Tool changer dry-run: In MDI, call T1 M6 and observe the tool changer sequence. Verify PLC drives chuck, arm, clamp, and unclamp in the correct order with the expected interlocks.
6. Spindle test: Spindle start/stop, direction reversal, and orient. Confirm spindle actual speed matches commanded within tolerance.
7. Coolant, hydraulics, lubrication: Verify each auxiliary with the relevant NCK M-function.
8. Safety chain: Test emergency stop, guard interlock, and axis drive enable. The PLC must drop the drive enable and the NCK must drop axis enables within the configured reaction time.

For a 840C Index G300 lathe, a typical full commissioning dry-cycle (axes referenced, tool change, spindle cycle, tailstock advance/retract, chuck open/close) takes 60–90 minutes once the PLC is ready.

10. Preventive Measures and Documentation

• Two independent archives. Maintain the S5 program and the NCK archive on at least two separate media (machine hard disk + external USB stick + DVD). Verify checksum or file size after each save.
• Date-stamp and version label. Embed a comment block in OB1 with the archive date, machine serial number, and SW version. This makes cross-checks fast.
• Annual battery check. The S5 CPU backup battery should be replaced every 2–3 years. Mark the replacement date on the cabinet door.
• Document hardware swaps. Whenever the PSU, battery, or Flash card is replaced, save a fresh archive immediately afterwards and verify boot.
• Photograph the Flash card label. Keep a record of the Flash card P/N and firmware version alongside the S5 program listing.

11. Related Siemens Documentation

• SINUMERIK 840C Commissioning Manual (6FC5197-2AA20-0BP0)
• SINUMERIK 840C Operator's Guide
• SIMATIC S5-135U/155U CPU Manual
• STEP 5 V7.x Programming Manual
• Siemens Technical Forum — Reset CPU PLC for 840C (public thread)

12. Frequently Asked Questions