How to Handle Open Collector Output pins

Open Collector ICs

An Open Collector device is an IC with an output gate that is not connected to the Vcc pin of the chip.
The transistor version is called an Open Collector, while the FET version is called an Open Drain.
An Open Collector IC consists of a circuit with the final driver unconnected, at least to Vcc.
Of course the input [Base or Gate] to the driver is used to receive the signal, while the common pin [Emitter or Source] is grounded.
The unconnected collector pin is still taken up to an IC lead, so the user can attach it in the circuit.
In most cases the pin is attached to Vcc or another voltage via a resistor.
However the lead may also be attached to another open-collector device in addition to the pull-up resistor.
Because the collector is un-attached, the lead could be attached to any circuit voltage to interface with another device.
The graphic shows the schematic of a single gate in a 7407 Integrated Circuit [IC] package.
Note that the output collector 'Y' is not attached to Vcc as the other transistors are.
Resistor values shown are nominal and may not reflect the actual values used.

7407 Open Collector Schematic
Open Collector Logic

Open Collector Output pins should be tied to a valid logic level so the output remains fixed at a valid voltage level when the device turns off.
Wire AND-ing requires a pull-up resistor which needs to function for the range of devices tied to the wire-AND.
The minimum value of pull-up is calculated while only one output is low, the maximum value of pull-up is calculated while all outputs are high.
Both equations are listed below:

RPull-Up {min} = ( [Vcc {max} - VOL] / [IOL - NumInput Unit Loads {Low} * IIL] )

RPull-Up {max} = ( [Vcc {min} - VOH] / [IOH * NumOutput Unit Loads + NumInput Unit Loads {High} * IIH] )

NumInput Unit Loads {Low} = the number of input devices connected to the wired-OR
NumOutput Unit Loads = the number of Open Collector devices connected together
Be sure to watch the maximum sink current of the device.

IC devices use Open Collector outputs for a number of reasons:
... Wiring AND-ing two or more Open Collector device outputs together
... Open Collector outputs allow the device to drive another type of device at a different Vcc.
[High voltage loads], or loads other than 5 volts.
... Open Collector outputs can be used to drive Indicator lamps or relays. [High current loads].

Note the relay coil schematic symbol and incandescent lamp symbol.

TTL Open Collector Gates Driving a Relay and Lamp
Open Collector Logic Gates

Open Collector Outputs, Standard Logic:
7401; Quadruple 2-Input Positive-NAND Gates With Open-Collector Output
7405; Hex Inverter Open-Collector Outputs
7406; Hex Inverter Buffers/Drivers With Open-Collector High Voltage Outputs
7407; Hex Buffers/Drivers With Open-Collector High Voltage Outputs
7409; Quadruple 2-Input Positive-AND Gates With Open-Collector Outputs
7416; Hex Inverter Buffers/Drivers With Open-Collector High Voltage Outputs
7417; Hex Buffers/Drivers With Open-Collector High Voltage Outputs
7433; Quadruple 2-Input Positive-NOR Buffers With Open-Collector Outputs
7438; Quadruple 2-Input Positive-NAND Buffers With Open-Collector Outputs
Note that these devices are also available in the military temperature version; i.e. 5401
Or in any one of the different TTL Families; 74LS04, 74ALS04, 74HC04 and so on.
Other functions like the LM119 Comparator use the term, uncommitted collector to describe an Open Collector.

When to use a pull up resistor;
How to Handle Tri-State Output pins
How to Handle Unused Input pins
How to Handle Bus-Hold Input Pins

Links also show how to calculate a pull up resistor

7409 TTL Open Collector Gate Schematic
Open Collector Wired-AND
........... 7401 TTL Open Collector Gate Schematic
Open Collector Wired-OR

The TTL gates would be using a Vcc of 5 volts, while the pull-up voltage could be any voltage [within current limits of the TTL gates].

Related pages on this site:
. Standard Logic Voltage Thresholds . . Bus Logic Thresholds . . Logic Speed x Power Chart . . Trace Termination . . Ground/Power Planes.

Back to the Logic Design Page.

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Modified 6/13/15
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