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Datasheets and User Guides

App Notes

Software & Driver

 

Digital I/O (App Note)

Digital I/O Overview

LabJack DAQ devices have digital I/O lines providing the ability to control external devices and receive input. In most cases, digital sensors are monitored and controlled using some kind of serial protocol, like I2C, SPI, or 1-Wire. At the low-level, digital communication protocols rely on the actual digital I/O electrical configuration and characteristics, so it's good to be familiar with the device datasheet, and different types of digital signals.

In general, digital I/O lines have three states to be aware of:

  • Output-High also known as Active High
  • Output-Low also known as Active Low
  • Input also known as High Impedance or Tri-State

These three states can be sub-classified into two more general modes:

  • Output mode (contains the state Output-High and Output-Low)
  • Input mode

Output Mode:

A digital I/O pin must be configured to operate in this mode when using a LabJack to send an external hardware component a logic high or logic low voltage.  When operating a digital I/O line in this mode it is important to remember that on most of our devices a series resistor is installed to provide over-voltage/short-circuit protection.  Keep in mind, these series resistors limit the ability of the digital I/O lines to sink or source current.

Input Mode:

A digital I/O pin must be configured to operate in this mode when using a LabJack to read a signal produced by an external hardware component.  When operating a digital I/O line in this mode there is a bias voltage that influences the pin's floating behavior.  On most devices this is a 100K ohm pull up resistor to 3.3V.


Refer to the appropriate Digital I/O sections for specific information about each device:

Another important section where device specific information can be found is Appendix A for each device.  It has important voltage and current ratings for both analog and digital input/outputs.