OpenPLC Troubleshooting & Default Credentials

The most common OpenPLC issues, in the order they typically occur during a first install. If you're stuck on the login screen, scroll to Default Credentials below.

Default credentials

After fresh install, the OpenPLC web interface (port 8080) accepts:

• Username: openplc
• Password: openplc

Change them in Users settings before exposing the device on any non-isolated network.

Cannot log in — recover credentials

If you changed the password and lost it, reset by editing the SQLite database directly:

# On Raspberry Pi / Linux
cd ~/OpenPLC_v3/webserver
sudo systemctl stop openplc
sqlite3 openplc.db
> UPDATE Users SET password='openplc' WHERE username='openplc';
> .quit
sudo systemctl start openplc

On Windows the database is at C:\OpenPLC_v3\webserver\openplc.db. Use DB Browser for SQLite.

Compile errors

"Variable not declared" — Check the global Configuration variables match what your POU references. %IX0.0 in code requires a Configuration variable mapped to %IX0.0.
"Type mismatch" — Common when assigning DINT to INT or REAL to DINT without explicit conversion. Use DINT_TO_INT(), REAL_TO_DINT().
"Function not found" — OpenPLC supports a subset of IEC 61131-3 functions. Check the ST reference for what is supported.
"Sketch too big" (Arduino targets) — Arduino Uno has only 32 KB flash. Switch to Mega 2560 or use an ESP32.

Runtime not starting

Linux service failed — Check sudo systemctl status openplc and journalctl -u openplc -n 100 for errors.
Port 8080 in use — Another service is using port 8080. Stop it or change OpenPLC port in webserver/scripts/scripts.cfg.
Permission denied on GPIO — On Raspberry Pi, OpenPLC must run as root (or with proper GPIO permissions). The default install handles this; manual installs need sudo.
Compile succeeded but program won't start — Check openplc.log in the webserver folder for runtime errors.

Modbus client cannot connect

Port 502 — verify firewall allows TCP 502. sudo ufw allow 502/tcp on Ubuntu.
Slave ID — OpenPLC defaults to slave ID 1. Some clients ignore it for TCP but it must match for serial Modbus.
Address mapping — OpenPLC %QX0.0 (output bit 0) maps to Modbus coil 1 (1-indexed). Off-by-one is the most common bug.
PLC not running — The Modbus server only responds when the PLC is in RUN mode. Click Start PLC on the Dashboard.