Hitachi EH-150 — ERR LED & Clearing Errors

Hitachi · intermediate · 20 min

On a Hitachi EH-150 (HIDIC) PLC the ERR LED won't go out until you actually fix the cause — pressing the clearance switch alone does nothing. Learn how error clearing works, why the CPU's special internal outputs hold the detail, and how to handle a module ERR.

ℹ️ On a Hitachi EH-150 the ERR LED will not extinguish if you press the error-display-clearance switch without eradicating the cause first. Fix the cause, then clear — and note the clearance switch only clears the LED, not the CPU's special internal output error area.

The ERR LED tells the truth

On a Hitachi EH-150 / HIDIC PLC, the ERR LED is honest: it will not be extinguished if you press the error-display-clearance switch without eradicating the cause of the error. So the correct order is always fix first, clear second — you can't clear your way out of a real fault.

How to clear an error properly

1. Eradicate the cause of the error (fix the program, wiring, module, or configuration the error points to).
2. Then press the error-display-clearance switch — or the reset switch — to clear the ERR indication.

If the LED comes back, the cause is still present.

The clearance switch doesn't clear everything

An important subtlety: the error-display-clearance switch is only for clearing the ERR LED display — it does not clear the CPU's special internal output area. The CPU keeps the error information in its special internal outputs, so:

• You can (and should) read the special internal outputs to see exactly what the fault was, even after the LED is dealt with.
• If your program or HMI logic reacts to those special internal output bits, you may need to clear them separately — the LED-clearance switch won't do it.

Module ERR (e.g. EH-ETH)

For an I/O or communication module fault — for example the EH-ETH Ethernet module detecting an abnormality in itself or in communication — the related LED lights. The recovery:

1. Reset the CPU and thereby reboot it.
2. If the module's ERR LED lights again, replace the module. A fault that survives a clean reboot is hardware.

Step-by-step

1. ERR LED on? Don't just press clear — find and fix the cause first (read the special internal outputs for the fault detail).
2. Cause fixed? Press the error-display-clearance switch or reset switch.
3. Need the error history / logic bits cleared? Clear the CPU special internal outputs separately — the LED switch won't.
4. Module ERR (EH-ETH etc.)? Reset/reboot the CPU; if it relights, replace the module.
5. Confirm the ERR LED stays off in normal running.

Never do this

Don't get in the habit of mashing the clearance switch to make the ERR LED go out — on the EH-150 it literally won't clear a live fault, and even when it does clear the LED, the real error info is still in the special internal outputs. Fix the cause, then clear.