Why use open collector?/How do I calculate output Frequency of an Incremental Encoder?！

The only reason that you would use an NPN Open Collector output is when the Supply Voltage is different that the signal logic level required. The open collector format allows the collector of the transistor to be connected to a power source that is different from the main power supply. For example: Power Supply is 5 Volts and the output signal needs to be 24 Volts. 

You must know at least two of the parameters. Frequency = (PPR X RPM) / 60 

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