PLC Chapter 5

Analog Electrical Signal

Can take any shape and represent an infinite number of possible values

In what manner are data files organized?

Data files are organized by the type of data they contain

Identify the sequence of events involved in a PLC scan cycle.

During a PLC scan cycle, the processor reads all the inputs, takes these values, and energizes or de-energizes the outputs according to the user program.

A normally open limit switch is to be programmed to control a solenoid. What determines whether an Examine-on or Examine-off contact instruction is used?

If the contacts need to close to energize the output, then the Examine If Closed is used.

Compare the way horizontal and vertical scan patterns examine input and output instructions.

In the horizontal scan pattern, the processor examines input and output instructions from the first command, top left in the program, horizontally, rung by rung. In the vertical scan pattern, the processor examines input and output instructions from the top left command entered in the ladder diagram, vertically, column by column and page by page.

When are input branch instructions used as part of a ladder logic program?

Input branch instructions are used in an application program to allow more than one combination of input conditions.

"A normally closed pushbutton is connected to a PLC discrete input." Does this mean it must be represented by a normally closed contact in the ladder logic program?

No

Digital Electrical Signal

Operate using discrete ON or OFF signals that have only two possible values.

What does the memory map for a typical PLC processor consist of?

Program and data files

Program Test Run

Program: Used to enter a new program or update an existing one in the internal RAM memory Test: Used to operate or monitor the user program without energizing any outputs. Run: Used to execute the user program with input devices being monitored and output devices energized accordingly.

What are the different types of data files that are used by the SLC 500 controller.

Status date file, Binary Date, Timer data , counter data, control data , integer, floating point data

"A normally closed pushbutton is connected to a PLC discrete input" Identify the reason for that.

The action of the field devices should be separated from the action of the PLC bits.

What does the address assigned to an instruction indicate?

The address assigned to an instruction indicates what PLC input is connected to what input device and what PLC output will drive what output device.

What information is stored in the input image table file?

The input image table file stores the status of the input terminals for the controller. Stored in bits

What information is stored in the output image table file?

The output image table file stores the status of the output terminals for the controller.

Identify the difference between the functions of the PLC program and data files.

The program files are the part of the processor memory that stores the user ladder logic program, whereas the data files store the information needed to carry out the user program.

Match the following descriptions with their corresponding standard PLC languages as defined by the International Standard for Programmable Controllers.

- Ladder Diagram: A graphical depiction of a process with rungs of logic, similar to the relay ladder logic schemes that were replaced by PLCs. - Instruction List (IL): A low-level, text-based language that uses mnemonic instructions. - Structured Text (ST): A high-level, text-based language such as BASIC, C, or PASCAL specifically developed for industrial control applications. - Function Block Diagram (FBD): A graphical depiction of process flow using simple and complex interconnecting blocks. -Sequential Function Chart (SFC) :A graphical depiction of interconnecting steps, actions, and transitions.

Identify the factors that enter into the length of the scan time.

- The Speed of the processor module -The length of the ladder program - The type of the instructions executed -The actual ladder true/false conditions

Identify the matrix limitations that may apply to certain PLCs.

-A maximum of seven parallel lines -Ten series contacts per rung

In what way does an internal output differ from a discrete output?

Unlike a discrete output, an internal output does not directly control an output field device.

Under what condition does a ladder logic rung have logic continuity?

When there is a continuous path of true conditional instructions in a rung

Compare the way memory space is assigned and referenced in rack-based and tag-based PLCs.

With rack-based addressing a fixed device with a fixed address is used, whereas tag-based addressing is more flexible and uses text-based addressing.

Answer the following with regard to the Examine If Closed instruction: What is another common name for this instruction What is this instruction asking the processor to examine? Under what condition is the status bit associated with this instruction 0? Under what condition is the status bit associated with this instruction 1? Under what condition is this instruction logically true? What state does this instruction assume when it is false?

a) Another name for the Examine If Closed instruction is the Examine-On instruction. b) This instruction asks the PLC's processor to examine if the contact is closed c) The status bit associated with the XIC is 0 when the address associated with the XIC is false (0). d) The status bit associated with the XIC is 1 when the address associated with the XIC is true (1). e) This instruction is logically true when the status of the input (physical) device or internal (logical) relay address associated with that bit is true (1). f) When the XIC instruction is false it will assume a normally open state just like an open relay conta