DIO = Digital Input/Output
IO/D/FIO/EIO/CIO/MIO = Somewhat arbitrary monikers for specific DIO ports on different LabJacks.
Digital I/O stands for Digital Input and Output. Digital Inputs allow a microcontroller to detect logic states, and Digital Outputs allow a microcontroller to output logic states.
Each digital I/O on a LabJack can be individually configured to one of 3 states: input, output-high, or output-low.
Digital Input: A digital input detects if a voltage is above/below a specific threshold. If the voltage is higher than some value, the computer will detect the digital input as high/set/1. If the voltage is lower than some value, the computer will detect the digital input as low/clear/0.
Digital Output: A digital output allows you to control a voltage with a computer. If the computer instructs the output to be high, the output will produce a voltage (generally about 5 or 3.3 volts). If the computer instructs the output to be low, it is connected to ground and produces no voltage.