Siemens 21600: Siemens SINUMERIK 840D Error Code 21600

Symptoms

The CNC system displays alarm 21600, indicating the PLC has not started or has stopped running. The machine cannot execute any PLC-controlled functions such as door interlock, coolant, or spindle start.

Possible Causes

PLC program crashed or entered a fatal error state
PLC hardware module on the CCU/NCU failed
PLC program memory corrupted due to battery depletion
MPI/PROFIBUS communication between NCK and PLC interrupted
PLC startup timeout exceeded due to excessive block count or circular references

Troubleshooting Steps

Step 1: Check the PLC status LED on the CCU/NCU module
Green RUN LED is on; SF LED is off
Step 2: If SF (System Fault) LED is red, check PLC diagnostic buffer via Step 7 or SCP tool
Diagnostic buffer shows the specific PLC fault cause
Step 3: Verify PLC battery voltage if the SF LED indicates memory fault
Battery voltage above 2.7V
Step 4: Reload the PLC program from a known-good backup if memory is corrupted
PLC program loads and enters RUN state
Step 5: If PLC hardware is suspected faulty, replace the CCU/NCU module
New module starts PLC successfully after program reload

How to Reset

To reset: 1) Fix the PLC fault cause (reload program, replace battery, fix communication). 2) Power-cycle the control. 3) If alarm persists, contact Siemens service. Caution: PLC faults may indicate deeper hardware issues.

Common Replacement Parts

6ES7951-1AK00-0AA0: SIMATIC S7 PLC battery (Use genuine Siemens battery for reliable PLC memory retention)

Frequently Asked Questions

Q: Why does the PLC stop on a SINUMERIK 840D?
The PLC can stop due to a program crash, hardware fault, corrupted memory, or communication failure with the NCK. The diagnostic buffer in Step 7 will show the exact stop reason.
Q: Can I run the CNC without the PLC?
No. The PLC controls all safety interlocks, auxiliary functions, and spindle logic. The machine cannot operate safely without a running PLC.

Siemens SINUMERIK 840D Error Code 21600