Here’s a Quick Way to Understand PLC Inputs and Outputs

The term I/O means Input/Output. I/O can come in two different types; Discrete or Analog. Most people starting out learning about programmable logic controls (PLC) are taught all about discrete inputs and outputs. Data is received from devices such as push-buttons, limit-switches, etc., and devices are turned on such as motor contactors, lights, etc. Discrete Input and output bits are either on or off. (1 or 0) The following program will show a motor control circuit stop-start. Motor off:

Motor on:

Analog inputs Common input variables for analog are temperature, flow, pressure, etc. They are converted to an electrical signal into a PLC analog input. Standard electrical signals are 0 – 20 mA, 4 – 20 mA, 0 – 10 volts DC, and -10 – 10 volts DC. Note: It is recommended that a 4 – 20 mA signal is best. A resistor can be added if a voltage is required to get a voltage input. Analog outputs Common output variables for analog are speed, flow, pressure, etc. They are converted from a word in the PLC to the analog output. The signal range is then outputted to the device to control the position, rate, etc. Standard electrical signals to the device are 4 – 20 mA, 0 – 10 volts DC, and -10 – 10 volts DC. Analog Inputs and Outputs use words to determine the signal going to or from the device. Example: 4 – 20 mA current Input – 8-bit resolution four mA = 000000002 = 0016 20 mA = 11111111= FF16 Example: 4 – 20 mA current Output – 8-bit resolution 0016 = 000000002 = 4 mA FF16 = 111111112 =20 mA For a review of numbering systems, follow the link below: What everyone should know about PLC numbering systems.


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If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLCs are not challenging to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimals, Hexadecimal, ASCII, and Floating Points.

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