In most cases optocouplers are driven with an open-collector output, the USB6501 specs say that the outputs are software configurable to open-collector configuration. You can connect the input (LED) of the 4N35 directly to the USB6501 digital output, the cathode should be connected to the output terminal and the anode to +5VDC (common ground with the driver circuit) via a current limiting resistor which limits the output current to the 8.5mA which can be handled by the USB6501. The calculation should be based on the minimum voltage swing, maximum voltage is +5VDC and minimum voltage is the output saturation voltage of the USB6501 (0.8V) plus the LED forward voltage (about 1.2V), so the output voltage swing is 3.0VDC.
You can also connect the anode of the 4N35 input directly to the output terminal of the USB6501 and use the push-pull configuration. The cathode must be connected to GND of the USB6501 output circuitry. In this case the current limiting resistor calculation must be based on
- the maximum output current (8.5mA)
- the minimum output voltage @ 8.5mA (2.0V) minus the LED forward voltage (1.2V).
You also have to know what you want to drive with the 4N35. The current transfer ratio is about 40 p.c. @ 10mA forward current. At If = 8.5mA it is somewhat higher, however you should not expect a maximum current handling capacity of the output transistor of more than 3.5mA. If you need a higher output current you will have to use a buffer circuit.
