Question

QUESTION 1 a. Why is a stop button wired closed, and a start button wired open? b. The inputs to a PLC are a normally closed stop button and a normally open start button. The (2 marks) contacts used in the ladder logic to represent them are both normally open. Why are they both the same? (2 marks) (2 marks) c. Explain the terms contacts and coils with respect to the PLC. d. Explain the terms memory map, assembly language, source code, object code, polling, and reset with respect to the 68HC12 microcontroller. (6 marks)

Answer 1

Ans1 a)By using  a normally start button wired open  in the ladder logic, and a stop button wired closed . By doing so, the input will be not produce hazardous situations under failure. It plays an important when it comes to the risk assessment process. The same is also true with safety relays and contacts in safety circuits.

Ans1 c) Below is the explanation for terms contacts and coils -

1.These are discrete programming elements, dealing with Boolean (1 and 0; on and off; true and false) variable states.

2.Contacts and coils are often associated with the physical Inputs & Outputs on the PLC.

The fundamental element of a ladder diagram is a "Contact". A contact has only two states: open or closed. An open contact breaks the current flow whereas a closed contact allows current to flow through it to the next element.

3.Each contact in a Ladder Diagram PLC program represents the reading of a single bit in memory, while each coil represents the writing of a single bit in memory.

4.Discrete input signals to the PLC from real-world switches are read by a Ladder Diagram program by contacts referenced to those input channels.

5.Similarly, discrete output channels – referenced by coil symbols in the Ladder Diagram – must also bear some form of address or tag name label.

6.The simplest contact is an On/OFF switch which requires external force (e.g. the human hand) to activate it. Limit switches are those small switches that are placed at certain location so that when a mechanical device moves towards it, the contact will be closed and when the device moves away from it, the contact will be open.

If a contact is connected to a load and the contact is closed, the load will be turned on.