Answered Essay: 1- The CPU system forms the interfaces through which field devices are connected to the controller.

1- The CPU system forms the interfaces through which field devices are connected to the controller.
True

2- The programming device must always be connected to the controller to run the program.
False

3- One disadvantage of fixed I/O PLC system is its lack of flexibility.
True
you are limited in what you can get in the quantities and types dictated by the packaging.Also for some models, if any part in the unit fails, the wholeunit has to be replaced.

4- The input and output modules execute the user defined program during the scan cycle.
True
The scan cycle consists of three steps: input scan, executing program(s), and output scan.

Input Scan: A simple way of looking at this is the PLC takes a snapshot of the inputs and solves the logic. The PLC looks at each input card to determine if it is ON or OFF and saves this information in a data table for use in the next step. This makes the process faster and avoids cases where an input changes from the start to the end of the program.

Execute Program (or Logic Execution): The PLC executes a program one instruction at a time using only the memory copy of the inputs the ladder logic program. For example, the program has the first input as ON. Since the PLC knows which inputs are ON/OFF from the previous step, it will be able to decide whether the first output should be turned ON.

Output Scan: When the ladder scan completes, the outputs are updated using the temporary values in memory. The PLC updates the status of the outputs based on which inputs were ON during the first step and the results of executing a program during the second step. The PLC now restarts the process by starting a self-check for faults.

5- The central processing unit:
A. looks at the inputs, makes the decisions based on the program, and sets the outputs.

6- The programming device:
D. All of the above.

7- The programming device:
D. All of the above.

8- The symbol —( )— in a ladder logic diagram represents a:
B. virtual relay coil.
Coil represent relays that are energized when power flows to them. When a coil is energized it causes a corresponding output to turn on.

9- The symbol —| |— in a ladder logic diagram:
A. can be thought of as a normally open contact.
The normally open is true when the input or output status bit controlling the contact is 1.

10. It is a relay ladder diagram for modified process.It is the original process control circuit for mixing operation. The change requires that the manual pushdown button control should be permitted to operate any pressure but not unless the specified temperature setting has been reached.

11.It represents normally closed(NC). It is true when the input or output status bit controlling the contact is 0.