A MicroLogix 1400 loses internal-coil states, variable values, and output states after a power failure, even though the program uses retentive coils. The same program retains data correctly in another controller, while the affected controller's battery is reported to be in good condition. This comparison points toward a controller-specific startup, memory, power, or installation condition rather than ladder logic alone.
Separate the Observed Data-Loss Symptoms
Do not treat every zeroed value as one failure. Record which retentive internal coils and variables lose their values, then evaluate physical outputs separately. The evidence confirms that all three categories become zero, but it does not establish that they share the same retention behavior.
| Observation | Engineering implication |
|---|---|
| Retentive coils and variables become zero | Investigate startup restoration and controller-specific memory conditions. |
| Outputs become zero | Determine whether ladder execution writes zero after startup rather than assuming a memory failure. |
| The same program works in another PLC | Compare controller status, startup configuration, hardware, and installation conditions. |
| Battery is reported good | Keep the battery finding documented, but verify the affected controller rather than relying only on its reported condition. |
Check Startup Memory Restoration
Inspect status bit S:5/8, identified in the evidence as “Memory Module Loaded on Boot.” A startup memory-module load is a plausible explanation for restored or initialized values overriding the retained runtime state. The evidence does not confirm that this bit is set, so treat it as a diagnostic discriminator rather than the established root cause.
If the affected and working controllers differ at S:5/8, review their memory-module and boot-loading configuration. Determine whether the affected unit loads a stored image during startup and whether that image contains zero values.
Compare the Working and Failing Controllers
- Reproduce the failure under controlled conditions and record the affected coils, variables, and outputs immediately before power loss and after restart.
- Connect with RSLogix and check for reported errors or controller status indications.
- Inspect
S:5/8after startup and compare it with the working controller. - Compare memory-module and startup-loading settings between the two controllers.
- Verify the battery condition on the affected controller under the manufacturer's prescribed test method.
- Inspect grounding and bonding differences between installations. Grounding was raised only as a possible contributor, so do not assign it as the cause without measurement or a repeatable correlation.
Verify the Corrective Action
After correcting any confirmed configuration, memory-module, battery, or installation discrepancy, assign known states to selected retentive coils and variables, cycle power, and compare the restored values. Repeat the test on the working controller using the same procedure. Confirm separately whether outputs become zero because stored data changed or because ladder logic writes them during startup.
FAQ
Why does my MicroLogix 1400 lose retentive coil values after a power failure?
Check whether startup memory restoration is replacing runtime values. Inspect S:5/8 and compare the memory-module boot configuration with a controller that retains the same program correctly.
What does S:5/8 indicate on a MicroLogix 1400?
The evidence identifies S:5/8 as “Memory Module Loaded on Boot.” Use its state to determine whether a boot-time memory-module load may be involved; the bit alone does not prove why individual values became zero.
Can grounding cause MicroLogix 1400 data loss?
Grounding is only a hypothesis in the available evidence. Compare grounding and bonding between the failing and working installations, and require measurements or repeatable test results before identifying it as the root cause.