Siemens SIMATIC S7-300 - Manual (Page 33)

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SIMATIC S7-300 Getting Started for First Time Users
Getting Started, 04/2007, 6ZB5310-0NC02-0BA0
3. Set the main switch of the power supply to “ON”.
The DC24V LED on the power supply will light up. All LEDs on the CPU 312C
will light up briefly, and the DC5V LED and the STOP LED will remain lit.