Analog I/O Capability of PLC

Input/output  capability  for  programmable  logic controllers  comes  in  three  basic  varieties: discrete, analog, and network.

In the early days of programmable logic controllers, processor speed and memory were too limited to support anything but discrete (on/off ) control functions. Consequently, the only I/O capability found on early PLCs were discrete in nature. Modern PLC technology, though, is powerful enough to support the measurement, processing, and output of analog (continuously variable) signals.

All PLCs are digital devices at heart.  Thus, in order to interface with an analog sensor or control device, some “translation” is necessary between the analog and digital worlds. Inside every analog input  module  is  an ADC, or Analog-to-Digital  Converter,  circuit  designed  to  convert  an analog electrical signal  into  a multi-bit  binary word.  Conversely,  every analog output  module  contains a DAC, or Digital-to-Analog  Converter,  circuit to convert the PLC’s digital command words into analog electrical quantities.

Analog I/O  is  commonly available  for modular PLCs  for many different  analog signal  types, including:

  • Voltage (0 to 10 volt, 0 to 5 volt)
  • Current (0 to 20 mA, 4 to 20 mA)
  • Thermocouple (millivoltage)
  • RTD (millivoltage)
  • Strain gauge (millivoltage)

The following photographs show two analog I/O cards for an Allen-Bradley SLC 500 modular PLC system, an analog input card and an analog output card. Labels on the terminal cover doors indicate screw terminal assignments:

Article from Lessons In Industrial Instrumentation by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License

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