PLC programming controls machines by reading inputs, running logic, and turning outputs on or off. Ladder logic is common in factories because it looks like electrical relay diagrams and is easy to troubleshoot. This cheat sheet helps students connect software instructions to real devices such as sensors, motors, lights, and switches.
It is useful for learning how industrial automation systems make safe, repeatable decisions.
Key Facts
- A PLC scan cycle usually follows this order: read inputs, execute program logic, update outputs, then repeat.
- A normally open contact, often called XIC, is true when its input bit is 1 or on.
- A normally closed contact, often called XIO, is true when its input bit is 0 or off.
- A coil, often called OTE, writes the rung result to an output bit, so Output = rung logic result.
- Series contacts act like AND logic, so Output = A AND B when two true conditions must both be met.
- Parallel contacts act like OR logic, so Output = A OR B when either condition can energize the output.
- A timer done bit turns on when accumulated time is greater than or equal to preset time, so DN = true when ACC >= PRE.
- A counter done bit turns on when count accumulated is greater than or equal to preset count, so DN = true when ACC >= PRE.
Vocabulary
- PLC
- A programmable logic controller is an industrial computer that controls machines by processing input signals and commanding output devices.
- Scan cycle
- The scan cycle is the repeating process in which a PLC reads inputs, solves the program, and updates outputs.
- Ladder rung
- A ladder rung is one horizontal line of ladder logic that represents conditions controlling one or more output instructions.
- Contact
- A contact is a ladder logic instruction that checks whether a bit is true or false before allowing logic power to pass.
- Coil
- A coil is a ladder logic output instruction that turns a bit on or off based on the rung result.
- Preset
- A preset is the target time or count value that a timer or counter must reach before its done bit turns on.
Common Mistakes to Avoid
- Confusing normally open with normally closed contacts is wrong because the ladder instruction tests a bit state, not the physical shape of a switch.
- Forgetting the scan cycle is wrong because outputs usually do not update until the PLC finishes solving the program for that scan.
- Using duplicate output coils for the same address is risky because the last rung scanned can overwrite earlier logic and cause unexpected behavior.
- Placing timer or counter reset logic in the wrong rung is wrong because the accumulated value may clear before the done bit can be used correctly.
- Treating series and parallel branches the same is wrong because series contacts require all conditions to be true, while parallel branches require at least one true path.
Practice Questions
- 1 A PLC scan time is 12 ms. About how many scans occur in 1 second?
- 2 A TON timer has PRE = 5.0 s and ACC = 3.2 s. Is the timer done bit true or false?
- 3 A counter has PRE = 8 and ACC = 8 after a sensor pulse. What is the state of the counter done bit?
- 4 A motor start rung uses a start pushbutton, a stop pushbutton, and a seal-in contact. Explain why the stop instruction is usually placed in series with the motor coil.