GeezerVison

Logic is integral to the programming of PLCs.

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

• Ladder logic - simplified representation of electronic schematics

I for input and O for output are the generally accepted standard, but some PLC manufacturers may use different letters to describe common items.

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

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

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

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.

• 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.

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

• Design and implementation of PLC systems

• PLC Simulator – browser-based ladder logic editor with scan-cycle visualisation and auto-graded practice exercises; runs with no install, free tier

• Systems engineering