Open collector
Encyclopedia
An open collector is a common type of output found on many integrated circuits (IC). Instead of outputting a signal of a specific voltage or current, the output signal is applied to the base of an internal NPN transistor whose collector is externalized (open) on a pin of the IC. The emitter of the transistor
is connected internally to the ground pin. If the output device is a MOSFET
the output is called open drain and it functions in a similar way.
The output essentially acts as either an open circuit
(no connection to anything) or a connection to ground. The output usually has an external pull-up resistor
, which raises the output voltage when the transistor is turned off. When any transistor connected to this resistor is turned on, the output is forced to nearly 0 volts. Open-collector outputs can be useful for analog weighting, summing, limiting, etc., but such applications are not discussed here.
A three-state logic
device is unlike an open collector device, because it has transistors to source and sink current in both logic states, as well as a control to turn off both transistors and isolate the output.
).
Another advantage is allowing to connect more than one open-collector output to a single line. If all outputs attached to the line are in the high-impedance (i.e., logic 1) state, the pull-up resistor
will hold the wire in a high voltage state. If 1 or more of the device outputs are in the ground (i.e., logic 0) state, they will sink current and pull the line voltage toward ground.
Open-collector devices are commonly used to connect multiple devices to a bus (i.e., one carrying interrupt or write-enable signals). This enables one device to drive the bus without interference from the other inactive devices - if open-collector devices are not used, then the outputs of the inactive devices would attempt to hold the bus voltage high, resulting in unpredictable output.
By tying the output of several open collectors together, the common line becomes a "wired AND" (positive-true logic) or "wired OR" (negative-true logic) gate. A "wired AND" behaves like the boolean AND of the two (or more) gates in that it will be logic 1 whenever (all) are in the high impedance state, and 0 otherwise. A "wired OR" behaves like the boolean OR for negative-true logic, where the output is LOW if any one of its inputs is low. Such circuit constructions are termed "wired-AND" and "wired-OR", respectively.
The open-collector transistor can be rated to withstand a higher voltage than the chip supply voltage. Such devices are commonly used to drive devices such as Nixie tube
s, and vacuum fluorescent display
s which require higher operating voltages than the usual 5-volt logic supply.
SCSI
-1 devices use open collector for electrical signaling. SCSI-2 and SCSI-3 may use EIA-485.
One problem with open-collector devices is power consumption, since they tend to require higher current minimums for correct operation. Even in the 'off' state, they often have a few nanoamps of leakage current (the exact amount varies with temperature).
is an open-drain connection. Open-drain outputs can be useful for analog weighting, summing, limiting, as well as digital logic. An open drain terminal is connected to ground in the low voltage (logic 0) state, but has high impedance
in the logic 1 state. This prohibits current flow, but as a result, such a device requires an external pull-up resistor
connected to the positive voltage rail (logic 1).
Note that microelectronic devices using open drain signals (such as microcontrollers) may provide a 'weak' internal pull-up resistor to connect the terminal in question to a positive voltage source/rail like Vdd of the device. Such weak pullups, often on the order of 100 kilohm, reduce power usage by keeping input signals from floating. External pullups are stronger (perhaps 3 kilohm ) to reduce signal rise times (as with I²C
) or to minimize noise (as on system RESET inputs). Internal pullups can often be disabled for cases where there is an external one, or in other cases where they are not needed.
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
is connected internally to the ground pin. If the output device is a MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
the output is called open drain and it functions in a similar way.
Function
In the picture above, the transistor base is labeled "IC Output". This is an internal output from the internal IC logic to the transistor. From the point of view of the transistor, this is the input which controls the transistor switching. The external output is the transistor collector, and the transistor acts as an interface between the internal IC logic and parts external to the IC.The output essentially acts as either an open circuit
Open circuit
The term Open circuit may refer to:*Open-circuit scuba, a type of SCUBA-diving equipment where the user breathes from the set and then exhales to the surroundings without recycling the exhaled air...
(no connection to anything) or a connection to ground. The output usually has an external pull-up resistor
Pull-up resistor
Pull-up resistors are used in electronic logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected or high-impedance...
, which raises the output voltage when the transistor is turned off. When any transistor connected to this resistor is turned on, the output is forced to nearly 0 volts. Open-collector outputs can be useful for analog weighting, summing, limiting, etc., but such applications are not discussed here.
A three-state logic
Three-state logic
In digital electronics three-state, tri-state, or 3-state logic allows an output port to assume a high impedance state in addition to the 0 and 1 logic levels, effectively removing the output from the circuit...
device is unlike an open collector device, because it has transistors to source and sink current in both logic states, as well as a control to turn off both transistors and isolate the output.
Applications of open-collector devices
Because the pull up resistor is external and need not be connected to the chip supply voltage, a lower or higher voltage can be used instead. Open collector circuits are therefore sometimes used to interface different families of devices that have different operating logic voltage levels or to control external circuitry that requires a higher voltage (e.g., a 12 V relayRelay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
).
Another advantage is allowing to connect more than one open-collector output to a single line. If all outputs attached to the line are in the high-impedance (i.e., logic 1) state, the pull-up resistor
Pull-up resistor
Pull-up resistors are used in electronic logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected or high-impedance...
will hold the wire in a high voltage state. If 1 or more of the device outputs are in the ground (i.e., logic 0) state, they will sink current and pull the line voltage toward ground.
Open-collector devices are commonly used to connect multiple devices to a bus (i.e., one carrying interrupt or write-enable signals). This enables one device to drive the bus without interference from the other inactive devices - if open-collector devices are not used, then the outputs of the inactive devices would attempt to hold the bus voltage high, resulting in unpredictable output.
By tying the output of several open collectors together, the common line becomes a "wired AND" (positive-true logic) or "wired OR" (negative-true logic) gate. A "wired AND" behaves like the boolean AND of the two (or more) gates in that it will be logic 1 whenever (all) are in the high impedance state, and 0 otherwise. A "wired OR" behaves like the boolean OR for negative-true logic, where the output is LOW if any one of its inputs is low. Such circuit constructions are termed "wired-AND" and "wired-OR", respectively.
The open-collector transistor can be rated to withstand a higher voltage than the chip supply voltage. Such devices are commonly used to drive devices such as Nixie tube
Nixie tube
A nixie tube is an electronic device for displaying numerals or other information. The glass tube contains a wire-mesh anode and multiple cathodes. In most tubes, the cathodes are shaped like numerals. Applying power to one cathode surrounds it with an orange glow discharge...
s, and vacuum fluorescent display
Vacuum fluorescent display
A vacuum fluorescent display is a display device used commonly on consumer-electronics equipment such as video cassette recorders, car radios, and microwave ovens. Invented in Japan in 1967, the displays became common on calculators and other consumer electronics devices...
s which require higher operating voltages than the usual 5-volt logic supply.
SCSI
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
-1 devices use open collector for electrical signaling. SCSI-2 and SCSI-3 may use EIA-485.
One problem with open-collector devices is power consumption, since they tend to require higher current minimums for correct operation. Even in the 'off' state, they often have a few nanoamps of leakage current (the exact amount varies with temperature).
MOSFET
The analogous connection used with MOS transistorsMOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
is an open-drain connection. Open-drain outputs can be useful for analog weighting, summing, limiting, as well as digital logic. An open drain terminal is connected to ground in the low voltage (logic 0) state, but has high impedance
High impedance
In electronics, high impedance means that a point in a circuit has a relatively high impedance to other points in the circuit.-Digital electronics:...
in the logic 1 state. This prohibits current flow, but as a result, such a device requires an external pull-up resistor
Pull-up resistor
Pull-up resistors are used in electronic logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected or high-impedance...
connected to the positive voltage rail (logic 1).
Note that microelectronic devices using open drain signals (such as microcontrollers) may provide a 'weak' internal pull-up resistor to connect the terminal in question to a positive voltage source/rail like Vdd of the device. Such weak pullups, often on the order of 100 kilohm, reduce power usage by keeping input signals from floating. External pullups are stronger (perhaps 3 kilohm ) to reduce signal rise times (as with I²C
I²C
I²C is a multi-master serial single-ended computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, cellphone, or other electronic device. Since the mid 1990s, several competitors I²C ("i-squared cee" or "i-two cee"; Inter-Integrated Circuit;...
) or to minimize noise (as on system RESET inputs). Internal pullups can often be disabled for cases where there is an external one, or in other cases where they are not needed.