A PWM signal consists of a square wave (pulse) of which the width can be varied (modulated). One period of the square wave consists of a high (1) and low level (0) signal. By 'duty cycle' usually the percentage of the total duration of one period is meant at which the square wave is at level 1. For example at a duty cycle of 25% the signal will be at level 1 at just 25% of the total time. Such a signal also contains just 25% percent of a 100% duty cycle signal. A LED will light up brighter at a 75% duty cycle PWM signal when compared to a LED powered with a 25% duty cycle PWM signal.
To dim LEDS in this manner it is important that the PWM frequency is high enough. When the frequency is too low it would be possible to see the high/low level sequence of the square wave itself! The open collector output module can be configured to generate 16 PWM signals. These signals are generated with a PWM frequency of 1000 Hertz which is more than enough to dim LEDs or Lamps without flickering.
Picture examples: A: 50% duty cycle, B: 50% duty cycle, with a higher PWM frequency, C: 10% duty cycle, D: 80% duty cycle.