PLC/Programming basics

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Logic is integral to the programming of PLCs.

Logic gates[edit]

  • And gate
  • Or gate
  • Nor gate
  • Nand gate

Programming types[edit]

  • Ladder logic - simplified representation of electronic schematics

Common PLC functions[edit]

Inputs[edit]

  • I (Inputs) - These bits represent the connection to any external control input device wired to the PLC, including switches, buttons and motion sensors.

Outputs[edit]

  • O (Output) - Each output bit can only be used as an output once.

Internal bits[edit]

Internal bits make possible almost limitless necessary logical constructs to be made.

Latches[edit]

Ladder logic symbol(normally closed)

Holds an electrical function on. A momentary push-button, output (from a timer or counter) or an internal bit are capable of triggering a latch. Latches have the same reference as the output on the same rung. Ideally, a latch must be foolproof with an emergency stop from an input. Latches represent virtual relays.

Timers[edit]

  • TON (Timer on) - once powered waits until its preset time to turn on the output.
  • TOF (Timer off) - turns on when powered, then turns off when the preset time is reached
  • RTF (Retaining timer) - are potentially dangerous as they keep count even if the PLC is reset.

Counters[edit]

  • CTU (Counter up) - Every time this counter gets continuous power, this counter counts up. Once a function is reached it outputs.
  • CTD (Counter down) - Similar to CTU, except this counts down.

Sequencers[edit]

Sequencers are capable of using internal (binary) logic bits to shortcut repetitive ladder logic.

See also[edit]