Troubleshooting Program Wiping and Memory Loss on CPU 410-5H After Power Cycling

In SIMATIC PCS 7, if an AS410 single station (CPU 410-5H 6ES7410-5HX08-0AB0) completely loses its program and requests a memory reset (MRES) after a power cycle, the failure is caused by an unseated, defective, or missing System Expansion Card (SEC), a power supply brownout during voltage ramp-up, or reliance on un-depassivated backup batteries. Unlike classic S7-400 CPUs which require 3.6 V lithium batteries (6ES7971-0BA00) to retain volatile RAM, the CPU 410-5H utilizes integrated non-volatile FeRAM load memory. However, if the SEC card is unseated or power rail timing is compromised, the CPU halts startup and clears execution RAM.

Memory Architecture: Classic S7-400 RAM vs. CPU 410-5H FeRAM

A fundamental engineering distinction exists between classic SIMATIC S7-400 CPUs (e.g. CPU 414-3, CPU 416-2, CPU 417-4) and the modern process automation controller CPU 410-5H (6ES7410-5HX08-0AB0):
* Classic S7-400 Architecture:
- The internal load memory and work memory are volatile CMOS RAM.
- When line power (120/230 V AC or 24 V DC) is removed, program retention depends entirely on the 3.6 V Lithium Backup Batteries (6ES7971-0BA00) installed in the power supply module (e.g. PS 407 6ES7407-0KA02-0AA0 / 6ES7407-0RA02-0AA0).
- If the batteries are missing, depleted, or defective, power cycling completely wipes the controller's RAM, leaving the CPU in an empty, unprogrammed state.
* CPU 410-5H Architecture:
- Engineered specifically for continuous process automation without battery maintenance.
- It features integrated non-volatile memory (FeRAM / NVRAM).
- The downloaded program is stored in persistent non-volatile memory; backup batteries in the power supply are NOT required for program retention on CPU 410-5H!

The Critical Role of the System Expansion Card (SEC)

If the CPU 410-5H does not rely on batteries to retain its program, why would a brand-new CPU 410-5H wipe its program upon power-on?

The primary culprit is the System Expansion Card (SEC):
* Every CPU 410-5H requires a small physical hardware card inserted into its rear slot—the System Expansion Card (e.g. 6ES7653-2BA00-0XB0 for 100 POs up to 6ES7653-2CC00-0XB0 for 2600+ POs).
* The SEC card defines the licensed Process Object (PO) capacity of the controller.
* Startup Validation: During power-up, the CPU 410 bootloader reads the cryptographic signature on the SEC card. If the SEC card is loose, unseated, oxidized, or damaged, the CPU cannot validate its operational license. As a safety protection mechanism, the CPU halts startup, declares an unrecoverable hardware exception, requests a memory reset (MRES), and purges execution RAM.

Lithium Battery Passivation in S7-400 Power Supplies

In configurations utilizing classic S7-400 CPUs or where power supply battery monitoring is active:
* Lithium thionyl chloride ($Li-SOCl_2$) batteries (6ES7971-0BA00) naturally form an internal chemical passivation layer during storage to prevent self-discharge.
* If a new battery is inserted directly from packaging into a PS 407 power supply without de-passivation:
- The battery exhibits high internal resistance.
- When the power supply is switched off, the CPU suddenly attempts to draw standby current from the battery.
- The battery terminal voltage instantly collapses under load (voltage dip below 2.7 V).
- The CPU detects loss of backplane buffer voltage and purges RAM, even though the battery tests 'healthy' under zero load!
* De-Passivation Procedure: Momentarily short-circuiting the battery with a 10 Ω resistor (or brief contact for 2–3 seconds) draws sufficient current to dissolve the passivation layer, restoring low internal impedance.

S7-400 vs. CPU 410-5H Memory Retention Comparison Matrix

The following matrix compares memory retention mechanics across controller generations:

Feature / MetricClassic S7-400 (CPU 414 / 416 / 417)SIMATIC PCS 7 CPU 410-5HLoad Memory TechnologyVolatile RAM (or External Flash Card)Integrated Non-Volatile FeRAMBackup Battery RequirementMandatory (6ES7971-0BA00)Zero Batteries RequiredPower Supply Battery SlotMust contain 1 or 2 active batteriesCan remain completely emptyBehavior on Battery FailureProgram completely wiped on power-offProgram 100% retainedSystem Expansion Card (SEC)NoneMandatory Hardware License KeyPrimary Cause of Memory LossDepleted or passivated backup batteryUnseated SEC or Power Supply Brownout

Step-by-Step Diagnostic and Remediation Procedure

Follow this procedure to resolve memory loss on power-up:
1. Inspect and Reseat the System Expansion Card (SEC):
- Power down the rack and disconnect the AC/DC power supply.
- Remove the CPU 410-5H from the rack.
- Locate the SEC card on the rear of the CPU.
- Remove the SEC card, inspect contacts for bent pins or dust, and firmly reinsert it until it clicks securely into place.
- Reinstall the CPU into the subrack (UR2 / UR1).
2. Verify Power Supply Voltage and Ramp-Up Speed:
- Check the power supply (e.g. PS 407 10A / 20A): Verify input voltage is stable (e.g. 230 V AC $\pm 10\%$).
- A slow or sagging voltage ramp-up (> 100 ms) during power-on can trigger an internal CPU hardware watchdog reset before the bootloader completes.
- Ensure the power supply backplane connectors make firm contact with the rack rail.
3. Inspect CPU Diagnostic Buffer in STEP 7:
- Power on the rack. If the CPU flashes STOP or requests MRES, connect an engineering laptop via Industrial Ethernet.
- Open SIMATIC Manager -> PLC -> Diagnostic Buffer.
- Inspect event messages:
- Event 16#4550: System Expansion Card missing or invalid.
- Event 16#4300: Memory reset requested by operating system.
4. Execute Clean MRES and Re-Download:
- Hold the CPU operating mode switch in the MRES position for 3 seconds until the STOP LED flashes slowly.
- Release the switch, then immediately turn it to MRES again. The STOP LED flashes rapidly, indicating a clean memory formatting.
- Open SIMATIC Manager: Download hardware configuration and the complete CFC program.
5. Execute a Controlled Power-Cycle Test:
- Switch off the main power supply breaker for 30 seconds.
- Restore power. Confirm the CPU 410-5H boots smoothly into RUN mode with the complete program intact.

Verification

Verify persistent program retention and startup stability:
1. Observe the front panel of the CPU 410-5H upon power-on:
- The STOP LED illuminates temporarily during boot, followed by the solid green RUN LED.
- Confirm that the EXTF / INTF / SF fault LEDs remain dark.
2. In STEP 7 HW Config online: Verify the module information reports CPU 410-5H available and OK.
3. Open a CFC chart: Verify live signal monitoring resumes without requiring a re-download.
4. Open the CPU Diagnostic Buffer: Confirm that the startup event records: Startup without memory loss (Warm restart).

Limitations and Operational Precautions

Never remove or insert the System Expansion Card (SEC) while power is applied to the CPU 410-5H; doing so will permanently destroy the cryptographic memory chip on the SEC.
If backup batteries are used in classic S7-400 power supplies, replace them every 2 to 3 years regardless of fault LED status during scheduled plant turnarounds.
Hardware procurement note: The System Expansion Card (SEC) is an orderable hardware item tied to PO capacity (e.g. 6ES7653-2BA00-0XB0); validate order numbers with an authorized Siemens distributor.

Engineer FAQ

Why did the power supply BAF (Battery Fault) LED turn on when I removed the batteries from CPU 410-5H?

The PS 407 power supply has a battery monitoring switch (1 BATT / 2 BATT / OFF). If the switch is set to 1 or 2 batteries but the battery compartments are empty, the power supply asserts the BAF fault LED. On CPU 410-5H, set the battery switch to OFF to silence the alarm.

Can I use standard consumer 3.6V lithium batteries in an S7-400 power supply?

No. Standard AA consumer batteries are 1.5 V alkaline cells. S7-400 power supplies strictly require specialized 3.6 V Lithium Thionyl Chloride AA cells (6ES7971-0BA00).

Does CPU 410-5H support an external Flash memory card?

No. CPU 410-5H does not have a slot for external S7-400 memory cards. All program and configuration storage is managed internally by integrated FeRAM and the SEC card.