The output causes the voltage to drop to its low, or ground, state (0 V). In other words, it sources current from the power supply to pull the voltage up to a specified value (typically the operating voltage) so that it can be read as a logic high signal. The basic function of a pull-up resistor is to ensure that when there is no input, a circuit takes on a default value. This is where a pull-up resistor comes in. So there must be a way to manipulate its voltage in order to indicate a logic high signal.
In the on state, it supplies a path to ground, and in the off state, it floats – that is, it is neither high nor low. An open collector driver sinks current from PNP (sourcing) inputs-either the PLC or motion controller. The simplest type of output driver is the open collector, or NPN (sinking) output. The low level output equals the power supply common voltage (0 V) level, effectively “pulling,” or sinking, the load to ground. The high level output is equal to the power supply, effectively “pushing,” or sourcing, current to the load from the power supply. Kuphaldt, Ī push-pull output driver can either sink or source current to the load. Alternatively, a signal “high “ level, typically 2.7 to 5 V, is sufficient to turn on a circuit.
In general, a “low” level, which is typically the same as ground (0 to 0.5 V), means the signal is not at a voltage sufficient to activate a circuit. The “high” and “low” states of a digital signal refer to the signal’s voltage and whether it is able to turn on circuits. This gives the line driver output a high level of noise immunity. This arrangement spreads any noise across both signals, which are evaluated at the receiver as differential, rather than absolute, levels. The complementary signals are exact inverses of their main signals, so when the A signal is in the high state, the A- signal will be in the low state. Line driver outputs are differential, meaning that they produce pairs of signals (A, A- B, B- etc.). Line driver output is referred to as sourcing because it supplies current to the load. A sourcing device provides power and “pushes” the current through the load. A sinking device provides a path for the current to ground and does not provide power to the device. Sinking and sourcing are terms that refer to the direction that current flows when one signal is activating another signal. Open collector output is considered sinking, while the line driver type is considered sourcing, and push-pull output is both sinking and sourcing. Incremental encoders can provide several types of digital output, with the three primary types being open collector, push-pull, and line driver. To understand a pull-up resistor, it helps to first understand how encoder output is generated.
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