Omron Sysmac NJ/NX — ERR/ALM LED & Event Log Reset

Omron · beginner · 25 min

On a Sysmac NJ/NX controller the ERR/ALM indicator flags a controller error. Go online with Sysmac Studio, read the event log to get the exact event code, fix the cause, then reset the error.

ℹ️ Sysmac controllers keep a full event log — current events plus a history of past ones. That log, not the LED, tells you exactly what happened and how to recover.

What ERR/ALM tells you

On an Omron Sysmac NJ / NX machine automation controller, the ERR/ALM indicator lights or flashes when the controller registers an error — from a non-fatal application event up to a CPU watchdog or hardware error. The controller records every event, with a code and timestamp, in its event log. Some conditions (power-supply errors, wrong power-supply unit) can be distinguished by the indicators alone, but for most errors you confirm the cause by reading the log in Sysmac Studio.

Read the event log

1. Go online with Sysmac Studio (or use a connected HMI on the controller's built-in EtherNet/IP port to view events).
2. Open the Controller Event Log. It shows current controller events and the history of events that occurred before, each with an event code, level, source, and a description with a recommended correction.
3. Read the event code. Note the exact code and level. The level tells you severity (from observation/minor up to major fault); the description names the function module and the corrective action.

Fix the cause, then reset

1. Correct the cause the event names — a device/network fault (e.g. an EtherCAT slave), a variable/logic condition raised by your program, a configuration mismatch, or a hardware/supply issue.
2. Reset the error from Sysmac Studio. Resetting the controller clears the controller errors across all function modules at once. On a CPU with unit version 1.05 or later and Sysmac Studio 1.06 or higher, you can also reset errors for an individual unit.
3. Understand the rule: if the cause of the error has not been removed, the error occurs again immediately after reset. A reset that doesn't stick means the underlying condition is still active.

Fatal errors

A CPU-stop-level error (hardware error, watchdog timer error) may need the cause corrected and the controller re-powered rather than a simple reset. Use the online connection to differentiate a CPU reset, a watchdog error, and a non-fatal error before acting — the troubleshooting manual (Cat. No. W503) maps each event code to its procedure.

Good practice

Build HMI or program access to the event log into the machine so operators can read the event code directly, and keep user-defined events meaningful so application faults are as self-explaining as system events.