PLC lab PLC lab PLCLab Q1) List and explain the architecture of the PLC componen
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PLC lab PLC lab PLCLab Q1) List and explain the architecture of the PLC components. 02) Write a PLC program to implement the conditional logic statements (a), (b) and (c) below A PLC output is to switch on if any of three inputs is switched on. A PLC output is to switch on if any one of three inputs is switched on but not two or more A PLC output is to switch on if any two outputs are switched on, but not the third. a. b. c. Q3) Select the one correct statement below regarding the ladder diagram S1 S2 alam alam ignt a. Closing switch S2 switches on the alarm b. Closing S1 and S2 switches on the alarm. c. When the alarm is on, so is the light. d. Closing switch SI switches off the light Q4) A counter is used to keep track of the number of vehicles in a parking lot. As vehicles enter the lot through an entrance gate, the counter counts up. As vehicles exit the lot through an exit gate, the counter counts down. When the lot is full a sign at the entrance gate turns on indicating the lot is full. (You will design the capacity of the parking Q5) draws the ladder diagram for the wiring diagram below for a fan and light on the time mentioned Fan LightExplanation / Answer
FIG:ARCHITECTURE OF PLC
a. The CPU
The internal structure of the CPU depends on the microprocessor concerned. In general they have:
1 An arithmetic and logic unit (ALU) which is responsible for data manipulation and carrying out arithmetic operations of addition and subtraction and logic operations of AND, OR, NOT and EXCLUSIVE-OR.
2 Memory, termed registers, located within the microprocessor and used to store information involved in program execution.
3 A control unit which is used to control the timing of operations.
b. The Buses
The buses are the paths used for communication within the PLC. The information is transmitted in binary form, i.e. as a group of bits with a bit being a binary digit of 1 or 0, i.e. on/off states. The term word is used for the group of bits constituting some information. Thus an 8-bit word might be the binary number 00100110. Each of the bits is communicated simultaneously along its own parallel wire. The system has four buses:
1. The data bus carries the data used in the processing carried out by the CPU. A microprocessor termed as being 8-bit has an internal data bus which can handle 8-bit numbers. It can thus perform operations between 8-bit numbers and deliver results as 8-bit values.
2.The address bus is used to carry the addresses of memory locations. So that each word can be located in the memory, every memory location is given a unique address. Just like houses in a town are each given a distinct address so that they can be located, so each word location is given an address so that data stored at a particular location can be accessed by the CPU either to read data located there or put, i.e. write, data there. It is the address bus which carries the information indicating which address is to be accessed. If the address bus consists of 8 lines, the number of 8-bit words, and hence number of distinct addresses, is 28 = 256. With 16 address lines, 65 536 addresses are possible.
3 The control bus carries the signals used by the CPU for control, e.g. to inform memory devices whether they are to receive data from an input or output data and to carry timing signals used to synchronise actions.
4 The system bus is used for communications between the input/output ports and the input/output unit.
C. Memory
There are several memory elements in a PLC system:
1 System read-only-memory (ROM) to give permanent storage for the operating system and fixed data used by the CPU.
2 Random-access memory (RAM) for the user’s program.
3 Random-access memory (RAM) for data. This is where information is stored on the status of input and output devices and the values of timers and counters and other internal devices. The data RAM is sometimes referred to as a data table or register table. Part of this memory, i.e. a block of addresses, will be set aside for input and output addresses and the states of those inputs and outputs. Part will be set aside for preset data and part for storing counter values, timer values, etc.
4 Possibly, as a bolt-on extra module, erasable and programmable read-only-memory (EPROM) for ROMs that can be programmed and then the program made permanent. The programs and data in RAM can be changed by the user. All PLCs will have some amount of RAM to store programs that have been developed by the user and program data. However, to prevent the loss of programs when the power supply is switched off, a battery is used in the PLC to maintain the RAM contents for a period of time. After a program has been developed in RAM it may be loaded into an EPROM memory chip, often a bolt-on module to the PLC, and so made permanent. In addition there are temporary buffer stores for the input/output channels.
d. Input/output unit
The input/output unit provides the interface between the system and the outside world, allowing for connections to be made through input/output channels to input devices such as sensors and output devices such as motors and solenoids. It is also through the input/output unit that programs are entered from a program panel. Every input/output point has a unique address which can be used by the CPU. It is like a row of houses along a road, number 10 might be the ‘house’ to be used for an input from a particular sensor while number ‘45’ might be the ‘house’ to be used for the output to a particular motor. The input/output channels provide isolation and signal conditioning functions so that sensors and actuators can often be directly connected to them without the need for other circuitry.
Question 3 Solution: B
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