the stronger the pullup) needs to be, resulting in increased consumption. One problem with open-collector devices is power consumption, since the pullup resistor dissipates power whenever the output is pulled low, and the higher the desired operating speed, the lower the resistor value (i.e. SCSI-1 devices use open collector for electrical signaling. A "wired OR" behaves like the Boolean OR for negative-true logic, where the output is LOW if any of its inputs are low. 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.
If open-collector devices were not used, then the outputs of the inactive devices would attempt to hold the bus voltage high, resulting in unpredictable output.Īctive-low wired-OR / active-high wired-AND circuit using open-drain gates.īy 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. This enables one device to drive the bus without interference from the other inactive devices. Open-collector devices are commonly used to connect multiple devices to one interrupt request signal or a shared bus such as I☬. This wired logic connection has several uses. If one or more device outputs are in the logic 0 (ground) state, they will sink current and pull the line voltage toward ground. If all outputs attached to the line are in the high-impedance state, the pull-up resistor will hold the wire in a high voltage (logic 1) state. This technique is commonly used by logic circuits operating at 5 V or lower to drive devices such as motors, 12 V relays, 50 V vacuum fluorescent displays, or Nixie tubes requiring more than 100 V.Īnother advantage is that more than one open-collector output can be connected to a single line. The open-collector transistor can be rated to withstand a higher voltage than the chip supply voltage. Open collector circuits are therefore sometimes used to interface different families of devices that have different operating voltage levels. Open-collector outputs can be useful for analog weighting, summing, limiting, etc., but such applications are not discussed here.Ī three-state logic device is unlike an open collector device, because it consists of transistors to source and sink current in both logic states, as well as a control to turn off both transistors and isolate the output.Īpplications of open-collector devices īecause the pull-up resistor is external and does not need to be connected to the chip supply voltage, a lower or higher voltage than the chip supply voltage can be used instead (providing it does not exceed the absolute maximum rating of the chip's output). When the transistor connected to this resistor is turned on, the output is forced to nearly 0 volts. The output usually consists of an external pull-up resistor, which raises the output voltage when the transistor is turned off. The output forms either an open circuit (also written "hi-Z" for high impedance) or a connection to ground. ⎏ for a pin that outputs hi-Z L or low-Z H (or ⎑ with an internal pull-down resistor).⎐ for a pin that outputs low-Z L or hi-Z H (or ⎒ with an internal pull-up resistor).On schematic component symbols, the open output is indicated with these symbols: The external output is the transistor collector the transistor forms an interface between the internal IC logic and parts external to the IC. This signal controls the transistor switching. This is a signal from the internal IC logic to the transistor. In the picture, the transistor base is labeled "IC output". 2 Applications of open-collector devices.