Resolving S5-150U Cold Restart Failure After Battery Replacement

Resolving SIMATIC S5-150U Cold Restart Failure After Battery Replacement

The SIMATIC S5-150U PLC family, often paired with the SINUMERIK 850 CNC family in retrofitted milling machines, exhibits a well-known recovery sequence when its lithium backup battery has discharged. When the 3.6 V buffer cell is allowed to drop below the supervision threshold, the CPU drops to STOP and demands a Cold Restart (Kaltstart) before program execution is permitted. In the field, the LED pattern that most often halts commissioning is red STOP LED steady + yellow "Cold Restart Required" LED steady, with the mode selector returning to STOP every time it is flipped to RUN. This article consolidates the manual restart sequence, the AG mode selector topology, the meaning of the Interrupt Stack (ISTACK) marker, and the most common user-program defect (illegal access to DB0) that prevents the S5-150U from leaving the Cold Restart state.

1. System Context: S5-150U in a SINUMERIK 850M.74-D Milling Machine

The reported cabinet combines a SINUMERIK 850M.74-D CNC controller with a SIMATIC S5-150U programmable controller. The PLC acts as the I/O coupling layer between the CNC and the machine (axes, spindle drives, tool changer, hydraulics, guarding). Loss of the PLC therefore stops the mill regardless of whether the NC channel itself is healthy.

1.1 Reported Module Population

2. Battery Backup Subsystem and Supervision

The CPU 150U monitors the lithium backup battery that retains RAM and the timer/counter/marker area across power-down events. The reference cell is a 3.6 V lithium thionyl chloride (Li-SOCl₂) type, typically 3.6 V / 1.7 Ah in original Siemens assemblies, with continuous load in the low µA range.

2.1 Battery-Failure Detection Thresholds

Measured open-circuit voltage of 3.6 V does not guarantee usable capacity. Internal impedance rises at end-of-life; verify with a load test (e.g., 1 mA for 10 s) before trusting the cell.

2.2 What a Cold Restart Demands

Cold Restart (Kaltstart, sometimes translated as "Overall Reset + Restart") on the S5-150U requires:

1. Retention (or re-load from EPROM/Flash) of the user program.
2. Re-initialization of all DBs (DB1..DB255) to the values declared in the source.
3. Reset of timers, counters, flags, and process image.
4. Explicit handshake from the operator — pressing the Warm Restart button on the CPU front while moving the selector to RUN, or issuing the equivalent through STEP 5 over the PG interface.

3. LED Pattern Decoding on the CPU 150U Front Panel

The combination reported in the field (red STOP + yellow Cold Restart) means the CPU is alive, the battery is now recognized as good (or at least the BATF flag has been re-evaluated), but the user program has not been acknowledged. The CPU will not enter RUN on its own — it requires an explicit restart handshake.

4. Root Cause Analysis: Why the Manual Restart Fails

The user reports having performed the canonical S5-150U restart sequence — flipping RUN/STOP while holding the Warm Restart button — without success. There are four families of root cause to investigate, in order of likelihood on a retired CNC:

4.1 Defect 1 — Illegal Access to DB0

DB0 in SIMATIC S5 is the system data block; it is not a user DB. Operations such as DB0, A DB0, DO DW0, or any C DB0 / JU FB/OB... DX0 that resolves onto DB0 are illegal at runtime and raise a stack fault during Cold Restart initialization, before RUN is reached. The CPU then latches into STOP with Cold Restart required.

4.2 Defect 2 — Startup OB Signature Mismatch

After a battery-failure cold restart the S5-150U expects an Overall Reset (Gesamt-RESET) before the first RUN transition. If the EPROM/Flash carries an OB21 (Warm Restart) but the CPU is currently demanding Cold Restart, OR the EPROM does not contain a valid OB20 (Cold Restart) signature, the restart aborts.

4.3 Defect 3 — PG Online Conflict

If STEP 5 / S5-DOS is online on the PG interface (AS511 / TTY 20 mA current loop, 9600 baud, odd parity) and the PG is parked in "STOP" or "Force" mode, it can hold the CPU in STOP even while the operator toggles the selector. Disconnect the PG cable, or switch the PG to Online / Run.

4.4 Defect 4 — Battery Sense Resistor / Holder Wiring

3.6 V measured across the battery terminals with the CPU unpowered is not the value the CPU sees. The sense leads in the S5-150U battery holder include a polarity detector and a PTC fuse. A reversed or oxidized holder reads 3.6 V at the cell, but the CPU still sees a BATF condition and refuses to leave STOP.

5. Step-by-Step Recovery Procedure

Perform the steps below in order. Do not skip from Cold Restart to RUN before verifying the ISTACK.

5.1 Prerequisites

• STEP 5 software ≥ V6.6 with S5-150U support, on a PG720/PG740/PG760 (or modern PC with S5-USB adapter).
• AS511 cable (Siemens 6ES5734-2BD20 or compatible, 20 mA TTY).
• Verified battery holder, freshly-loaded 3.6 V Li-SOCl₂ cell.
• Printed EPROM/PCB listings (LDA, FB, OB, DX, DB, PB, SB symbols).
• ESD-safe workspace; the S5-150U boards are sensitive to static discharge.

5.2 Step 1 — Mechanical and Battery Verification

1. Power down the cabinet. Open the battery holder, confirm cell orientation (+ toward holder clip), clean contacts with isopropyl alcohol, reinstall.
2. With mains ON, measure battery voltage at the CPU sense test point (TP on CPU 150U board, typically labelled +BAT/−BAT). It must read between 3.2 V and 3.6 V under load.
3. Confirm the BATF LED on the CPU is OFF.

5.3 Step 2 — Connect PG, Read ISTACK and USTACK

1. Connect the AS511 cable between PG and CPU.
2. Start STEP 5, switch to online, choose Test → Stack Display (Test / Stack) for the CPU 150U.
3. Read the ISTACK (Interrupt Stack) and the USTACK (User Stack). The "marked line" indicator points at the very instruction that aborted the restart — usually a DO, DI, C DB, A DB or JU FB...DX0 on DB0.
4. Take a screenshot of the ISTACK for the record.

5.4 Step 3 — Eliminate DB0 References

1. Open the STEP 5 program (or cross-reference list via Ctrl+F8 in the LAD/FBD/STL editor).
2. Search for every occurrence of DB0 in OB20, OB21, OB22, OB1, and all FBs/FCs.
3. Replace the offending calls with a valid user DB (typically DB1..DB255). A common migration error is leftover diagnostics code from a 135U port that assumed DB0 availability — it does not exist on S5.
4. If the call is inside an FB that pre-dates the cold-restart, comment it out with *** for the immediate recovery, but plan a permanent fix.

5.5 Step 4 — Perform Overall Reset

1. Place the mode selector on STOP.
2. From STEP 5, select PLC → Overall Reset (Gesamt-RESET). Confirm on the dialog.
3. The CPU clears RAM, all DBs, all flags, all timers/counters. The Cold Restart LED extinguishes once reset completes.

5.6 Step 5 — Cold Restart Handshake

1. With the mode selector still on STOP, press and hold the Warm Restart button on the CPU front.
2. Flip the selector from STOP to RUN.
3. Keep the Warm Restart button held for ~3 seconds. The Cold Restart LED must extinguish; the RUN LED should come on.
4. If only a Warm Restart is desired, use the Warm Restart button. If a true Cold Restart is required, hold the button for >5 s (some firmware revisions require a separate Cold Restart button sequence — confirm with the cabinet manual).

5.7 Step 6 — PG-Assisted Restart (If Manual Button Does Not Latch)

1. From STEP 5, online, choose PLC → Cold Restart or Warm Restart.
2. The PG sends the appropriate command over AS511, and the CPU executes the restart without needing the front-panel button.
3. Observe the LEDs; RUN should come on within ~2 s.

6. Verification

After RUN is reached, perform the following verification before returning the cabinet to production:

7. Field-Proven Caveats and Edge Cases

7.1 Running Without the Battery to Force Cold Restart

A second suggestion raised in the discussion was to set the PLC in RUN without the battery. This is not recommended. Removing the battery with mains ON does not help and risks the CPU dropping into an undefined state. With mains OFF, removing the battery simply clears RAM, which is the same effect as the Overall Reset and STEP 5 should always be preferred for a documented, repeatable cold restart.

7.2 SINUMERIK 850M.74-D Coupling

The SINUMERIK 850M series exchanges I/O and status words with the PLC through a fixed handshake, typically:

• DB1000 — NC commands to PLC.
• DB1100 — NC signals to PLC.
• DB1200 — PLC commands to NC.
• DB1300 — PLC signals to NC.

If any of these DBs is missing in the Cold Restart initialization, the NC will hold the machine in Not Ready. After RUN, confirm in the SINUMERIK operator panel that the PLC channel reports Ready and the field coupling flags toggle on a deliberate M-function.

7.3 AS511 Communication Issues

The AS511 interface uses a 20 mA current loop at 9600 baud, odd parity, 8 data bits, 1 stop bit. Cable length is limited to ~1000 m but is very sensitive to ground loops on a mill. If the PG cannot connect:

• Confirm the current-loop supply (15 V nominal) by measuring the in-line adapter.
• Confirm correct polarity of the active receiver pair.
• Try a different PG; some older PGs require a driver disk for S5-150U.

7.4 EPROM vs RAM Strategy

For long-term reliability, transfer the corrected user program (without DB0 references) from RAM to EPROM submodule (6ES5511-5AA12 or compatible) using an EPROM programmer (e.g., PG675, PG685, or third-party). Cold Restart after a battery event then loads automatically from EPROM and avoids the ISTACK-fault loop entirely.

8. Preventive Maintenance Checklist

1. Replace the 3.6 V lithium cell every 3 years, regardless of measured voltage.
2. Record the battery lot and install date on the cabinet door.
3. Keep a printed copy of the ISTACK output for the current program; it halves the mean-time-to-repair on the next battery event.
4. Maintain a backup of the EPROM contents on a modern flash drive via an EPOM reader.
5. Cross-reference any PG-side diagnostics program for DB0 calls before deploying.

9. Troubleshooting Matrix

10. Quick Reference — Front-Panel Restart Sequences

11. Standards and Reference Material

• SIMATIC S5-150U System Manual (Siemens, 6ES5 998-0SH21 or successor editions).
• STEP 5 Programming Manual (Siemens, 6ES5 998-0SPx).
• SINUMERIK 850M Commissioning Manual (Siemens, 6FC5 198-0ABxx family).
• Siemens S5-150U Service Bulletin on Lithium Battery Handling.
• SIMATIC S5-150U Module Catalog — CPU 150U (6ES5 756-0AAxx) variant list.