PLC TOPIC 5

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.

Explain the purpose of Windows based programming software such as RSLogix.

RSLOGIX is used to develop and monitor ladder logic programs.

Draw the symbol and state the equivalent instruction for each of the following: NO contact, NC contact, and coil.

11. NO= -[ ]- =XIC NC= -[/]- =XIO Coil= -( )- =OTE

A normally closed push button is connected to a PLC discrete input. Does this mean it must be represented by a normally closed contact in the ladder logic program? Explain why or why not.

A normally closed pushbutton connected to a PLC discrete input does not have to be represented by a normally closed contact in the ladder logic program. The action of the field devices should be separated from the action of the PLC bits.

In what manner are data files organized?

Data files are organized by the type of data they contain.

Outline 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.

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 instruciton are used in your application program to allow more than one combination of input conditions.

List eight different types of data files used by SLC 500 controller.

Output data file, input data file, status data file, binary data file, timer data file, counter data file, control data file, integer data file, floating point data file.

List four 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 instructions executed The actual ladder true/false conditions

Identify two matrix limitations that may apply to certain PLCs.

Two matrix limitations that may apply to certain PLCs is a maximum of seven parallel lines and ten series contacts per rung is possible.

Briefly describe each of the following PLC modes of operations: Program Test Run

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

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

The memory map for a typical PLC processor consists of program files and data files.

What information is stored in the output image table file? In what form is this information formed?

The output image table file stores the status of the output terminals for the controller. Information in the output image table is stored in the form of bits.

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.

List the five standard PLC languages as defined by the International Standard for Programmable Controllers, and give a brief description of each.

Ladder Diagram (LD) -a graphical depiction of a process with rungs of logic, similar to the relay ladder logic schemes that were replaced by PLCs. 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. Instruction List (IL) -A low-level, text-based language that uses mnemonic nstructions. Structured Text (ST) -A high-level, text-based language such as BASIC, C, or PASCAL specifically developed for industrial control applications.

Compare the function of the PLC program and data files.

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

What information is stored in the input image table file? In what form is the information stored?

The input image table file stores the status of the input terminals for the controller. Information in the input image table is stored in the form of bits.

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

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

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 contact.

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

Answer the following questions with regards to the Output Energize instruction: What part of an electromagnetic relay does this instruction look and act like? What is this instruction asking the processor to do? Under what condition is the status bit associated with this instruction 0? Under what condition is the status bit associated with this instruction 1?

a) OTE instruction looks and operates like a relay coil. b) This instruction signals the PLC to energize (switch on) or deenergize (switch off) the output associated with it. c) If a true logic path cannot be established or rung conditions go false, the OTE I instruction is deenergized (0) and the output device wired to it is switched off . d) If a true logic path is established with the input instructions in the rung, the OTE instruction is energized (1) and the output device wired to its terminal is energized.

Answer the following with regard to a ladder logic rung: Describe the makeup of a ladder logic rung. How are the contacts and coil of a rung identified? When is the ladder rung considered as having logic continuity?

a) The basic makeup of a ladder logic rung consists of a set of input conditions, represented by contact instructions, and an output instruction at the end of the rung, represented by the coil symbol. b) Each contact or coil symbol is referenced with an address that identifies what is being evaluated and what is being controlled. c) A ladder rung is considered to have logic continuity when at least one left-to-right true logical path exists.