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Worksheet: ladder logic

Name:Class:Date:

Material: the free ladder logic simulator at examrobotic.com (examrobotic.com/en/tools/ladder-logic-simulator). Time: about 40 minutes.

Aim: read ladder networks, understand the scan cycle and relate the program to the real wiring of the push buttons.

1. Start-stop with seal-in

  1. In the program, the stop appears as a normally open contact, even though the push button in the panel is normally closed. Why?
  2. Which contact makes the seal-in? Where is it placed relative to Start?
  3. Press Start and Stop together. Which one wins? Would it change if you put the stop after the seal-in?

2. Forward-reverse

  1. Which contact stops both direction outputs being on at once? Is it normally open or normally closed?
  2. Is this interlock in the program enough, or is it needed in the wiring too? Why?

3. Timer and counter

  1. In the TON example, the lamp comes on if the part has been in front of the sensor for 3 s. What happens if the part leaves after 2 s?
  2. In the CTU example, how many parts are needed for «box full»? What is the Reset input for?

4. The double-coil mistake

  1. Turn on switch A only. Does the lamp light? Why?
  2. How should it be written so the lamp lights from A or from B?

Answers (for the teacher)

  1. The button is NC in the panel (for safety: a loose wire stops the machine), so the input is 1 while it is not pressed. The program uses an NO contact on that input, which conducts while the input is 1.
  2. A contact of the output itself (Motor), in parallel with Start.
  3. Stop wins, because it is in series before the parallel branch. If the seal-in bypassed the stop as well, the stop would no longer work with the motor running.
  4. A normally closed contact of the other output, in series with each coil.
  5. It is needed in the wiring too (auxiliary contacts and, if possible, a mechanical interlock): the program cannot know if a contactor has welded, nor does it control the milliseconds of the arc.
  6. The TON resets and the lamp does not light.
  7. 5 parts. Reset sets the counter to zero for the next box.
  8. No: on every scan, the network for B writes the same output afterwards and sets it to 0. The last write wins.
  9. A single network with switches A and B in parallel and a single Lamp coil.