An Open Collector device is an IC with an output gate that is not connected to the Vcc pin of the chip. |
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: |
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
Open Collector Wired-AND | ........... |
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].
. Standard Logic Voltage Thresholds . | . Bus Logic Thresholds . | . Logic Speed x Power Chart . | . Trace Termination . | . Ground/Power Planes. |
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