What is a DI, DO, AI or AO?

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.

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DI Digital Input
DO Digital Output
AI Analog Input
AO Analog Output

  • DI or discrete/digital input is the type of PLC input which used to read status of digital input
    devices. Digital input device only have 2 status, ON or OFF. ON state mean PLC DI point received status in form of electrical power (can be 24 VDC, 120 VAC, etc.) and OFF state mean PLC DI point received no power from devices. Example of Digital input device is switch.

  • DO or discrete/digital output is the type of PLC output which used to control digital output devices. Digital output devices only have 2 status, ON or OFF. ON command mean PLC DO point send command in form of electrical power (24 VDC, 120 VAC, etc.) and OFF command mean PLC DO send no power to devices. Example of digital output devices is relay.

  • AI or analog input is the type of PLC input which used to read value from sensor. AI receive
    value from sensor in form of electrical current (4-20 ma) or electrical voltage (1-5 V). ADC (Analog to Digital Converter) inside PLC will convert current or voltage to digital value in form of 16 or 12 bit word data to further process. Example of sensor is temperature sensor. Temperature sensor will measure temperature and send measurement value to AI channel in form of current or voltage.

  • AO or analog output is the type of PLC output which used to control analog device. AO send command to device in form of electrical current (4 - 20 mA) or electrical voltage. DAC (Digital to Analog Converter) inside PLC will convert digital value (12 bit or 16 bit word data resulted from PLC calculation) to current or voltage and send it to controlled device thru AO channel. Example of AO device is control valve, PLC could control opening percentage of control valve with send 4 - 20 mA current which represent 0 - 100% open command.

author - aan

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