Open collector outputs are typically used for signal transmission, alarm and warning
notification, control signal output, etc. Although these outputs allow for a great deal of wiring flexibility, care must be taken not to destroy the circuits via incorrect polarity configurations, over-voltage, transients or current overload.
Most instrumentation indicators or control system inputs require a signal voltage of 0 to 5V or 0 to 24V. Therefore, open collector output circuits must be complemented with a pull-up resistor to function properly.