Open Collector - Applications of Open-collector Devices

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 voltage levels. 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 tubes, and vacuum fluorescent displays which require higher operating voltages than the usual 5-volt logic supply.

Another advantage is that more than one open-collector output can connect to a single line. 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. If one or more device outputs are in the logic 0 (ground) state, they will sink current and pull the line voltage toward ground. This wired logic connection has several uses.

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.

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).