1. Identify the following elements associated with the preparation of a programmable logic controller program.(assessment criteria 3.1) a. Statement list
b. Function diagrams
c. Graphical programming languages
In PLC statement list: - This is a textual programming language this can be used to generate the code section of logic blocks in PLC program preparation. Its syntax for statements is similar to assembler language and consists of instruction followed by addressed on which on the instruction acts. A example of statement list used in plc programming.
Function diagrams: - It is a graphical language for PLC program. It describes the function between I/O variable and O/P variables. A function is described as set of elementary blocks. I/O variable and O/P variables are connected to blocks by connection lines. Input and output both blocks will be wired together with connection lines, or links. Single line may be used to connect two logical points of the diagram.
An input variable and an input of a block
An output of a block and an input of another block
An output of a block and an output variable
Graphical programming language:- grafcet (graphe fonctionnel de commande etape transition) is a symbolic, graphic language that represents the control programme as steps or stages in the machine or process. It lets users create programs by manipulating program elements graphically rather than by specifying them textually. As the IEC 1131 standard’s sequential function charts (SFCs), which allow several PLC languages to be used in one control programme.
2. Write a programme using logic functions based on relay ladder logic for e.g a motor controlled by stop and start push button switches and for which one signal light must be illuminated when the power is applied to the motor and another when it is not applied.(assessment criteria 3.2) Solution:-
The figure shown is an example of a ladder diagram for a circuit that is used to start and stop a motor using push buttons. In the normal state button 1 is open and push button 2 closed. When button 1 will be pressed the motor circuit is completed and the motor starts and the signal light will be on. Also, the holding contacts wired in parallel with the motor close and remain closed as long as the motor is running. Thus when the push button 1 is released, the holding contacts maintain the circuit and hence the power to the motor. To stop the motor, button 2 is pressed. This disconnects the power to the motor and the holding contacts open. Thus when push button 2 is released, there is still no power to the motor so the signal light will be off. Thus we have a motor which is started by pressing button 1 and stopped by pressing button 2. stop start switch
3. Evaluate the following range and type of advanced functions of programmable logic controllers. a. Less than
b. Greater than
c. Binary to BCD conversion(assessment criteria 3.3)
In data comparison
The data comparison instruction gets the PLC to compare two data values. Thus it might be to compare a digital value read from some input device with a second value contained in register. PLCs generally can make comparison for:-
Less than (< or LESS)
Equal to (= or EQU),
Less than and equal to (or GRT),
Greater than and equal to (>=or GEQ),
Not equal to(NEQ)
Binary to BCD conversion
A code in which a string of four binary digits represents each decimal number 0 through 9 as a means of preventing calculation errors due to rounding and conversion. For example, since the binary equivalent of 3 is 0011 and the binary equivalent of 6 is 0110, 36 is represented as 0011 0110. Example:-
How to convert binary to BCD.
STEP 1: convert the binary number to decimal
STEP 2: convert decimal number to BCD...
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