Schneider Modicon M340 backup battery
Part: TSX MBP 100
Battery part cited for BAT LED and program-retention faults.
$10-$35
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Schneider Modicon M340 PLC Error Codes
The Schneider Electric Modicon M340 is a mid-range PLC platform using the Unity Pro (now EcoStruxure Control Expert) programming environment. Faults are indicated through front-panel LEDs, Unity Pro diagnostic screen, and system words accessible in the ladder/FB program.
M340 CPU LED Indicators
| LED | State | Meaning |
|---|---|---|
| RUN | Green solid | Application running normally |
| RUN | Green blinking | Standby or stopped |
| ERR | Red solid | Fatal error — CPU stopped |
| ERR | Red blinking | Non-fatal error (application continues) |
| I/O | Yellow solid | I/O module fault |
| I/O | Yellow blinking | I/O communication intermittent |
| ETH | Green solid | Ethernet link active |
| ETH | Green blinking | Ethernet activity |
| BAT | Red | Battery low (backup battery) |
| MEM | Yellow | Memory card issue |
Common M340 Error Codes (via Unity Pro / Control Expert)
CPU Faults
| Fault Code | Description | Action |
|---|---|---|
| 1 | Internal CPU fault | Power cycle, check firmware version |
| 2 | Memory fault | Check application download, replace SDRAM |
| 3 | Configuration error | Re-download application from Control Expert |
| 4 | I/O configuration mismatch | Verify rack hardware matches project config |
| 5 | Communication module fault | Check module seating, replace if needed |
| 10 | Watchdog timeout | Program scan time exceeded — optimize code |
| 11 | Task overflow | Mast/fast task overrun — reduce scan load |
| 20 | Power supply fault | Check 24VDC supply to CPU bus |
I/O Module Faults
| Fault | Cause | Fix |
|---|---|---|
| Module not present | Module removed or loose | Reseat module, check card guides |
| Module configuration error | Wrong module type in slot | Update hardware configuration in project |
| Channel fault | Individual I/O channel failure | Test wiring on channel; replace module if channel is burned |
| Bus communication lost | Backplane fault or module hardware failure | Replace module; check backplane ribbon |
Ethernet/Communication Faults
| Fault | Meaning |
|---|---|
| IP conflict | Two devices with same IP on network |
| No heartbeat from master | EtherNet/IP or Modbus TCP scanner not polling |
| Time-out on remote I/O | Distributed Quantum or X80 remote I/O not responding |
Need Replacement Parts?
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Accessing Faults in Control Expert (Unity Pro)
Online fault display:
- Connect to the PLC with Control Expert (online mode)
- Open Diagnostic Viewer (View → Diagnostic Viewer)
- Review the fault table — faults listed by type, module, and timestamp
System words:
%SW125— Mast task watchdog status%SW116— Last application error code%SW117— Last I/O error code%SW124— Configuration error code
Watchdog / Task Overflow (Code 10/11)
Task overflow is common after large program changes. The M340 Mast task has a watchdog timeout (default 250 ms). If the scan takes longer, the CPU stops and ERR LED flashes.
Fix options:
- Increase watchdog timeout in project settings (CPU properties → Tasks → Mast → Watchdog)
- Move non-critical logic to Event or Fast tasks
- Optimize loops — avoid large FOR loops inside Mast task
- Check for runaway indirect addressing
Battery Replacement (BAT LED)
The M340 CPU uses a CR2032 or similar lithium cell to retain program data during power loss. When BAT LED illuminates:
- Download the application from Control Expert as backup
- Power the PLC on
- Replace battery while powered (hot-swap within 1 minute on most models)
- M340 battery: Schneider P/N TSX MBP 100 or equivalent CR2032 3V lithium
Firmware Updates
M340 CPUs run embedded firmware. Firmware mismatch between CPU and I/O modules can cause intermittent faults. Use Control Expert → Tools → Firmware Update to check and update.
Emergency Recovery
If the CPU won’t start (ERR solid, no RUN):
- Clear memory: hold CLEAR button (if present) during power cycle
- Transfer application from Control Expert in PROGRAM mode first, then switch to RUN
- If SDRAM failure is suspected, contact Schneider Electric technical support — memory cannot be field-replaced on most M340 CPUs