1. Chapter 2 – PLC Hardware Components
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2. Recap
Programmable logic controllers (PLCs) are specialized industrial computers.
It is used to execute specific functions such as sequence control, on/off control, arithmatic operations, and other functions.
PLCs were originally made to replace hard-wired relay control systems.
Advantages of using PLCs:
Eliminates the hard wiring
More reliability
Lower cost
Flexibility
Communication capability
Easy troubleshooting
Faster response time
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3. PLC Architecture & Components (1)
The central Processing Unit contains the microprocessor and the memory
Central Processing Unit (CPU)
Input/Output
Memory
Microrpocessor
Question: What microprocessors are usually used by Siemens PLCs?
Siemens PLCs usually use Intel mircoprocessors
Question: What microprocessors are usually used by Allen Bradley PLCs?
Allen Bradley PLCs usually use Motorolla mircoprocessors
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4. The I/O Section
The section to which all field devices are connected and provides the interface between them and the CPU.
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5. The I/O Section
The hardware assembly that houses I/O modules, processor modules, and power supplies is referred to as the chassis.
A logical rack is an addressable unit consisting of 128
input points and 128 output points.
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6. The I/O Section
Addressing
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7. The I/O Section
Addressing
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8. Addressing The I/O Section The smallest unit is the bit
1 byte = 8 bits
1 word = 2 bytes
1 Double Word = 4 bytes = 32 bits
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9. Addressing The I/O Section Type – Input/Output
Slot – Physical location of I/O Module
Word and Bit – Identifies the terminal connection
EX: I0.5
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10. Inputs : The I/O Section Limit Switch Reed Sensors Pushbuttons
Ultrasonic sensors
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11. Outputs : The I/O Section Motor Starter Relay Signal Column Cylinder
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12. Types of Memory RAM: Random Access Memory. Fastest but volatile
ROM: Read Only Memory
PROM: Programmable ROM, this memory is non erasable once porgrammed.
EPROM: Erasable PROM, this memory is erasable using a strong UV radiation dose.
EEPROM: Electrically Erasable PROM, can be erased while in the circuit.
Flash memory: non-volatile high capacity memory
Note: Other forms of memory storage include Non-stationary (rotating) disks such as Hard Disk Drive, Floppy Disk, and Compact Disk (CD). These forms are normally not used in PLC systems.
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13. Memory Usage
Load Memory: A flash type of memory, this part of memory contains the user program and the hardware configuration. In Siemens PLCs built after 2002, this memory is implemented in the memory card.
Work Memory: A Random Access Memory (RAM) that is used to temporarily store portions of the program code to execute them. After that portion of the program code is executed, it is erased from the work memory
System Memory: A Random Access Memory (RAM) that is used to store variables when running and executing the program such as variables of timers, counters, memory bits,..etc.
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14. Human Machine Interface (HMI)
connected to communicate with a PLC and to replace pushbuttons, selector switches, pilot lights, thumbwheels, and other operator control panel devices
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15. PLC Architecture & Components (2)
Siemens PLCs have all or some of the following components:
Power Supply
Central Processing Unit: contains the following
- Microprocessor
- EEPROM/ RAM memory storage
- MicroSD card Slot (after 2002
- Lithium battery (in models before 2002)
- MPI connection: for MPI interface
- DP connection: for Profibus interface (Not always provided)
- Ethernet connection : for Profinet connection (not in S300)
- Status indicators: SF, Run, Stop, …
- Mode selector: Stop, Run, MRES
Input/Output Modules: can be fixed (such as on the S314 stations) or mudular (such as in the S315 station in the lab).
Other modules: such as the interface and communication modules
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16. Real Time
Definition: A system operates in real time when it reacts to an event that occurs on the field within a time constraint.
Examples:
1. The airbag system: must respond to a collision in real time so that airbags are deployed in the right time.
2. Real-Time surveillance and tracking in UAVs (Drones): cameras are connected to a network (“online“) for processing to respond to events that take place.
3. Non-real time ( “offline“) surveillance of the Sonar underwater drones: sonar scans are recorded to be processed after submarine emerges from water.
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17. a) It responds before the time it is supposed to
Example: A mechatronic system does not respond in Real Time, what could be a possible discription of this system?
a) It responds before the time it is supposed to
b) It responds in a time after it is supposed to
c) It records data to be processed later by another system component.
d) All of the above are possible
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18. Background
Why binary?
Easier to have 2 states On or OFF, thence simpler electronics
The smallest unit is the bit
1 byte = 8 bits
1 word = 2 bytes
1 Double Word = 4 bytes = 32 bits
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19. Review: Decimal Vs. Binary
How many bytes are there in 1 KByte?
Solution: 1KB = 210 Bytes = 1024 Bytes
Decimal
Binary
1 kilo = 1000
1 Kilo = 210=1024
1 mega = 1000,000
1 Mega = 220=1,048,576
How many bits are there in 2 KBytes?
Solution: 2×1024×8 = 16,384 bits
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20. Decimal and Binary numbers
Decimal numbers have a base of 10
Binary numbers have a base of 2
Decimal
Binary
Base 10
0, 1, 2, 3, 4…. 8, 9 2
0, 1
kilo
103 = 1000
Use small “k”
210 = 1024
Use capital “K”
Mega
106 = 1000,000
220 = 1,048,576
Gega
109 = 1000,000,000
230
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21. Decimal and Binary Numbers
Example 1: Fill in the blank: 1 µm = ______ meter 1 Kg = _______ grams 6 MHz = __________Hz 6 MB = __________ Bytes 1 Nibble = ______ bits 1 Byte = _______ bits 1 word = ________ bytes = ______ bits
10 -6
1000
6,000, 000
6,291,456 4 8 2 16
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22. Example 2: How many bytes are there in 16 KB?
16 KB = 16 * = 16,384 Bytes
How many bytes are there in 1 MB?
1 MB = 220 = 1,048,576
How many bits are there in 24 KB?
number of bytes = 24 * 1024 = 24,576
but there are 8 bits in a byte
thus, number of bits = 24,576 * 8 = 196,608 bits
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23. Introduction to computer programming
Compiler
Assembler
4th generation
3rd generation
LabView, SPSS,
Simulink
2nd generation
C++, C, Java,
Visual basic
Ladder Logic,
Matlab, Basic,
Pascal,..etc
1st generation
Assembly
Language
Machine Code
Punch cards
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24. What type of memory is volatile and therefore only used of storing variables? Only one answer
ROM
FLASH
RAM
EPROM
PROM
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