DI, DO, AI and AO are acronyms for, respectively, digital input, digital output, analog input and analog output.
Digital refers to the state of a single-bit, or binary signal. This binary signal typically has two states: on and off. A digital input signal contains the state of an input from such devices as a tank level switch or a door-closed switch. A digital output signal contains the state of an output to such devices as a motor starter or panel signal light.
Analog refers to a continuously changing voltage or current level signal. Using analog to digital (A/D) circuitry, the PLC converts this analog signal to a digital signal represented in the PLC by combining a number of contiguous binary signals together. SCADAPack controllers use 16-bit registers to contain the analog input signal. An example of an analog input signal is a sonar tank level. An example of an analog output signal is a variable speed motor starter.