What does open collector output mean?
Open collector output is normally used in logic circuits and can
be viewed as a common emitter configuration for a BJT transistor
and normally of type NPN. A typical common emitter configuration
requires the emitter to be connected to the ground and a resistor
Rc from Vcc to the collector. When the transistor becomes forward
biased, the collector will pulled the voltage across the resistor
down, from approximately Vcc down to almost 0V, normally in the
order of 0.1V at the collector.
We can see that the output at the collector will swing between
Vcc and ground. In open collector outputs the collector resistor Rc
and Vcc have been left out. This enables the engineer to use any
voltage and any pull-up resistor that will meet the specifications
of the output transistor.
Normally it is acceptable in logic circuits that multiple inputs
may be connected to one output, but not multiple outputs to one
input due to the nature of the active outputs. With open collectors
however multiple outputs may be connected to one input without
damaging the device.
Low state will result in a lower impedance than a high state
since the high state is obtained by current flowing from Vcc
through the pull-up resistor Rc.
Open collector can be used to convert 5V logic to 8V or 12V
logic or down to 3.3V since the logic high will be determined by
the external supply to the pull up resistor.
Open collector also prevents component failure during output
short-circuit or over-current. If an open collector is short
circuited to the ground, it will cause no harm to the device since
all the current will be supplied via the Rc resistor. Short circuit
to a +supply rail can however damage the device.
Open collector is not used that commonly anymore but many
variations of the same concept can be found in modern electronics.
For example: micro processor devices that employ a I2C
(Inter-Integrated Circuit) communications interface make use of a
open-drain lines.
It appears to work very much in the same fashion with pull-up
resistors and all. Open drain is associated with MOSFET transistors
instead of BJT's. In this example an I2C interface will fail if
pull-up resistors are not used. Like open collector, open drain
lines are also passive and does not provide any active high output
of its own. The High output is supplied by Vcc via the Rc or Rp
pull-up resistors. The open collector or open drain will pull the
pull-up resistor voltage down to the ground to create a low
impendence, low logic state.