A programmable logic controller, or PLC, is the industrial computer that coordinates conveyors, scanners, sorters, lift gates, motors, and safety devices in a warehouse. It connects physical equipment to automation logic through input and output modules. Digital I/O handles simple on or off information, while analog I/O handles changing values such as weight, distance, speed, or pressure.
Understanding the difference helps technicians troubleshoot faster and helps engineers choose the right sensors and actuators.
Key Facts
- Digital input example: a photoeye sends 0 or 24 V to show package absent or present.
- Digital output example: a PLC energizes a 24 V relay or solenoid to start or stop a device.
- Analog input example: a load cell transmitter sends 4 to 20 mA to represent package weight.
- Analog scaling formula: Engineering value = Raw span fraction x Engineering span + Engineering minimum.
- Ohm's law helps check signal circuits: V = IR.
- A common analog current loop uses 4 mA for minimum value and 20 mA for maximum value, with 0 mA often indicating a fault or broken wire.
Vocabulary
- PLC
- A programmable logic controller is an industrial computer that reads inputs, runs logic, and controls outputs in machines and processes.
- Digital I/O
- Digital input and output signals have two states, usually on or off, true or false, or 0 V and 24 V.
- Analog I/O
- Analog input and output signals vary continuously across a range to represent quantities such as speed, position, pressure, or weight.
- Current loop
- A current loop is an analog signaling method, often 4 to 20 mA, that carries measurement information using current instead of voltage.
- Scaling
- Scaling is the process of converting a raw PLC signal value into a real engineering unit such as kilograms, meters per second, or degrees Celsius.
Common Mistakes to Avoid
- Treating every sensor as digital is wrong because many warehouse sensors send a range of values, such as a distance sensor measuring how far a pallet is from a stop.
- Wiring an analog signal into a digital input is wrong because the PLC will only see on or off instead of the actual changing measurement.
- Forgetting the common or reference connection is wrong because both digital and analog circuits need a complete electrical path for reliable signal detection.
- Ignoring signal scaling is wrong because the PLC raw value does not automatically equal the real-world value shown on an HMI or used in control logic.
Practice Questions
- 1 A 4 to 20 mA analog weight transmitter represents 0 to 200 kg. What weight is represented by a 12 mA signal?
- 2 A PLC digital input turns on when it receives 24 V. A photoeye output is measured at 0 V when no box is present and 24 V when a box blocks the beam. If the input is on for 8 seconds during a 30 second test, what fraction of the test time was a box detected?
- 3 A conveyor jam sensor only needs to report whether a package is present at a fixed point, while a weigh scale must report the mass of each package. Which should use digital I/O and which should use analog I/O, and why?