Schneider Modicon Premium and Quantum LED Faults: What RUN, ERR and I/O Mean

The line is down and your only diagnosis is four small lamps on the front of the processor. On a Modicon Premium that is enough. Steady ERR is a processor or system fault. Flashing ERR is a different animal: application missing, invalid, wrong processor type, or a memory card battery gone flat. And ERR flashing with I/O is not the processor at all. That pair means a Bus X fault, out in the rack and the extension cabling. Steady against flashing is the diagnosis, and the two send you to opposite ends of the cabinet.

Below: the display block on TSX P57 processors, the LED row and keyswitch on Quantum 140 CPU controllers, and how to tell a dead processor from a dead rack supply. If you already know what failed, our Modicon Quantum and Premium spare parts guide covers ordering.

The Premium display block, read properly

Every TSX P57 carries the same four lamps, plus a FIP lamp on processors with an integrated FIPIO link. These states come from Schneider's documentation, not folklore.

Note what is missing. No battery lamp. No memory card lamp. A flat card battery reports itself as a flashing ERR, exactly like a missing application.

ERR and I/O flashing together is the rack, not the CPU

Memorise this one. Schneider documents a Bus X fault as ERR and I/O flashing simultaneously. Bus X is the backplane and extension path: the processor is alive and cannot see part of its own I/O.

People replace processors over this. A good TSX P57 comes out, a replacement goes in, the same two lamps flash, and now two parts are suspect. Check the extension cabling, the terminator on the last rack and the rack addressing, then reseat the modules. Fifteen years of thermal cycling in a dusty cabinet makes connector faults that look like a dead CPU.

On Premium, the program lives on the card

The second mistake is quieter. A Premium application can sit on a PCMCIA memory card in the front of the processor, its SRAM held alive by a battery on the card. Swap the processor, leave the card behind, and the replacement boots with nothing: RUN off, ERR flashing, and an engineer who blames the used processor.

So move the card with the processor, by the documented procedure rather than hot swapping, which Schneider covers in its FAQ on removing the Premium PCMCIA card. And before going near a card battery, upload. Not just the program: the data. Counters, retained values and recipe tables live in that SRAM and do not come back. If the battery is already flat when you arrive, you are recovering a machine, not diagnosing a fault.

Quantum: a longer LED row and a keyswitch that bites

Quantum 140 CPU controllers show more, and hide one trap. Per Schneider's hardware reference:

Ready lit with Run dark is a healthy processor that is not running: a stopped controller or a keyswitch position, not a failure.

Now the trap. On controllers such as the 140 CPU 434 12A the front keyswitch has three positions: Stop, Mem Prt and Start. In Mem Prt the controller refuses downloads. Less known is what it does to a power cycle. Only Start transfers the application from Flash into RAM at power-up, a cold restart. Stop and Mem Prt give a warm restart instead.

So a panel left in Mem Prt after commissioning has quietly changed how that machine recovers from an outage. Read the keyswitch before you read the logic. If Mem Prt stays lit with the switch at Start, the contacts have failed.

When the I/O lamp really means a module

Steady I/O with a healthy RUN means the configuration does not match reality: a failed module, a failed channel, or a rack that does not hold what the application expects. That last case catches people mid-repair. Premium modules carry a PV product version and an SV software version on the label, and a replacement of a different vintage can come up as a configuration fault. The TSX PBY 10 Profibus module we hold is labelled PV 04, SV 1.2: quote both when ordering. Motion and counting modules are fussiest, so photograph the label on a TSX CAY 33 before the old one goes in the box.

When it really is the processor

Steady ERR that survives a power cycle, a known good supply and a card you trust is a processor fault. The unhelpful truth: repair is rarely economic. Surface mount boards, custom silicon, no spares channel. A functional used unit is the fastest way back into production.

We hold Premium processors across the range, including the TSX P57 4823M, the TSX P57 3623M, the TSXP57303AMR and the TSX P57 203MR. Order by the exact reference on the old label. Levels are not interchangeable: an application built for one refuses to run on another. The rest sits in the PLC CPUs collection.

Quick answers

What does a flashing ERR LED mean on a TSX P57?

A flashing ERR on a Modicon Premium processor means the PLC is not configured, the application is missing or incompatible with that processor type, there is a software blocking fault, the memory card battery has failed, or there is a Bus X fault. It is not a dead processor.

What does it mean when ERR and I/O flash at the same time?

Simultaneous flashing of ERR and I/O on a Premium processor indicates a Bus X fault: it cannot see its own rack or extension bus. Check extension cabling, the terminator on the final rack, rack addressing and module seating first.

Can I replace a Premium processor without losing the program?

Only if you keep the memory card and hold an archive. The application may live on a PCMCIA card in the front of the processor, backed by a battery on the card. Move that card across, and upload program and data first.

What does the amber Mem Prt LED on a Quantum 140 CPU mean?

Mem Prt lit means memory is write protected because the protect switch is on, so the controller rejects downloads. Turn the front keyswitch from the middle Mem Prt position to Start and the lamp should go out. If it stays lit, the contacts have failed.

Does the RESET button on a Premium processor clear a fault?

The pencil-point RESET button activates a PLC cold restart, not a harmless way to clear a warning. A cold restart reinitialises data, so anything retained is gone. Never press it before you have an upload.

Before you order a replacement

1. Photograph every lamp, steady or flashing. The pattern vanishes when somebody power cycles the rack.
2. If ERR and I/O flash together, work on the rack and extension bus.
3. Substitute a known good rack supply first.
4. Upload program and data while the machine still talks.
5. Check the front slot for a memory card, and whether its battery was ever changed.
6. On Quantum, read the keyswitch before blaming the logic.
7. Order by the full label reference, including PV and SV.

Related reading on Samonde

• Modicon Quantum and Premium spare parts guide: CPUs and I/O
• Schneider Altivar fault codes: common faults and fixes
• SIMATIC S7 PLC spare parts: CPUs, modules and what to order
• Buying used industrial parts: a practical guide

These racks are past their commercial life and nowhere near the end of their working one. They still run water treatment plants and packaging lines across Europe, and will keep doing it as long as somebody on site can tell a flashing lamp from a steady one. That knowledge thins every year. Write the lamp patterns inside the cabinet door for whoever comes next.