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You are here: Home / Encoders / Pull-Up Resistor Basics

Pull-Up Resistor Basics

December 10, 2015 By Danielle Collins Leave a Comment

To understand a pull-up resistor, it helps to first understand how encoder output is generated. Incremental encoders can provide several types of digital output, with the three primary types being open collector, push-pull, and line driver. Open collector output is considered sinking, while the line driver type is considered sourcing, and push-pull output is both sinking and sourcing.

pull-up resistor
Sinking and sourcing are terms that refer to the direction that current flows when one signal is activating another signal. A sinking device provides a path for the current to ground and does not provide power to the device. A sourcing device provides power and “pushes” the current through the load.
Image credit: circuits today.com

Line driver output is referred to as sourcing because it supplies current to the load. Line driver outputs are differential, meaning that they produce pairs of signals (A, A-; B, B-; etc.). 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. This arrangement spreads any noise across both signals, which are evaluated at the receiver as differential, rather than absolute, levels. This gives the line driver output a high level of noise immunity.

pull-up resistors
The “high” and “low” states of a digital signal refer to the signal’s voltage and whether it is able to turn on circuits. 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. Alternatively, a signal “high “ level, typically 2.7 to 5 V, is sufficient to turn on a circuit.
Image credit: Tony R. Kuphaldt, ibiblio.org

A push-pull output driver can either sink or source current to the load. The high level output is equal to the power supply, effectively “pushing,” or sourcing, current to the load from the power supply. The low level output equals the power supply common voltage (0 V) level, effectively “pulling,” or sinking, the load to ground.

The simplest type of output driver is the open collector, or NPN (sinking) output. An open collector driver sinks current from PNP (sourcing) inputs—either the PLC or motion controller. 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. So there must be a way to manipulate its voltage in order to indicate a logic high signal. This is where a pull-up resistor comes in.

pull-up resistor
Typical open collector encoder output
Image credit: Dynapar Corp.

pull-up resistorThe basic function of a pull-up resistor is to ensure that when there is no input, a circuit takes on a default value. 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 output causes the voltage to drop to its low, or ground, state (0 V). This switching of the output pulses on and off creates a square wave.

 

Pull Down Resistor Diagram

 

 

In contrast to pull-up resistors, pull-down resistors limit the current that can flow between the supply and ground and are sometimes used in encoder line driver and push-pull outputs.

 

 

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Filed Under: Encoders, FAQs + basics, Incremental Encoders Tagged With: encoder, encoder output, incremental encoder, pull-up resistor, resistor

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