How are analog inputs and outputs addressed in the PLC?

• A. Bit oriented.
• B. Word oriented.
• C. Byte oriented.
• D. Nibble oriented.

Answer: (B)

Analog inputs and outputs carry numeric values that require multiple bits to represent, so they’re addressed as words. Digital I/O is handled at the bit level because each channel is just on or off, but an analog channel is a multi-bit value (for example, a 12- or 16-bit count from an ADC). The PLC stores that value in a word-sized register, which lets you read or write the full measurement or setpoint in one operation. When you take an analog input, the value in its word is then scaled to engineering units for use in control or display. When you drive an analog output, you place the desired value into the corresponding word, and the DAC converts that word into a physical voltage or current. So, the addressing scheme reflects the fact that analog data are multi-bit quantities, hence word oriented.