Troubleshooting

PLC Loses Counters and Setpoints After Power Off: Retentive Memory Explained

Every Monday the shift totals and recipe setpoints are back to zero. Here is how retentive memory really works, and how to make values survive.

· 8 min read · By TryPLC Team

  • Retain
  • Memory
  • Structured Text
  • Power Cycle

The symptom

After a weekend shutdown or a power failure, the production counter, the run-hours total and the operator setpoints are all back to their initial values.

Three kinds of variable lifetime

IEC 61131-3 and the major vendors distinguish three lifetimes:

KindSurvives power cycleSurvives program download
Normal (non-retentive)NoNo
VAR RETAINYesUsually reset on a cold reset or full download
VAR PERSISTENT (CODESYS/TwinCAT)YesYes, in most download cases
In TIA Portal the equivalent is the Retain checkbox, set per tag in an optimized data block or for the whole block in a standard one. In Studio 5000, controller-scope tags keep their values through power loss, but a download that includes values overwrites them.

Step 1: check that the variable is declared retentive

VAR RETAIN
    ShiftCount   : DINT;
    RunHours     : REAL;
END_VAR

A frequent error is a counter that is declared retentive but lives inside a function block instance whose instance data is not. In TIA Portal, check the retain setting of the instance DB, or of the multi-instance inside the parent block.

Step 2: look for code that overwrites it at startup

Search the cyclic code and any startup OB for initialisation that writes ShiftCount := 0 unconditionally. Start-up logic should initialise only the values that are invalid (out of range) and leave the rest alone.

Step 3: retentive memory capacity and hardware

Retentive memory is limited, for example to a few hundred kilobytes on mid-range CPUs, and exceeding it gives a compile warning or error. Older PLCs keep retentive data in battery-backed RAM, so a dead battery loses everything at power off. Check the battery LED and replace the battery on schedule, with the PLC powered if the manual says so.

Step 4: downloads and resets

A reset to factory, a cold reset or a download with re-initialisation can clear retentive values by design. Before a major download, back up the live values (snapshot them to the project, or export the DB values) and restore them afterwards.

Where to keep values you cannot afford to lose

  • Keep recipes and calibration in files or a database on the HMI or the PLC memory card, as well as in retentive variables.
  • Use PERSISTENT where the platform supports it, for values that must survive downloads.
  • Write a sanity check that runs at startup. If a retained setpoint is out of range, load a safe default and raise an alarm, so nobody runs the machine on a silent default.

    • You can practise variable declarations in the Structured Text reference.

    Frequently asked questions

    What is the difference between RETAIN and PERSISTENT?

    RETAIN values survive a power cycle but are normally reset by a cold reset or full download. PERSISTENT values (CODESYS, TwinCAT) also survive most downloads.

    Why does my retentive counter still reset?

    Common causes are a non-retentive function block instance, startup code that writes the value unconditionally, a dead backup battery, or a download that re-initialises data.