PLCs general Flashcards
Name the five basic components of a PLC system!
• The PLC processor, or controller • I/O (Input /Output) modules • Chassis or backplane • Power supply • Programming software that runs in a PC
What does the processor do?
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Stores the control program and data in its memory
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Reads the status of connected input devices
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Executes the control program
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Commands connected outputs to change state based on program execution
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For example: Turn a light on, start a fan, adjust a speed, or temperature
What do the I/O Modules do?
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Physically connect to field devices
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Input modules convert electrical signals coming in from input field devices such as pushbuttons, to electrical signals that the PLC can understand.
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Output modules take information coming from the PLC and convert it to electrical signals the output field devices can understand, such as a motor starter, or a hydraulic solenoid valve.
I/O that is remotely located from the PLC requires a network connection back to the PLC
Sketch an Input Module!
Input modules interface directly to devices such as switches and temperature sensors.
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Input modules convert many different types of electrical signals such as 120VAC, 24VDC, or 4-20mA, to signals which the controller can understand.
Whats the difference between analogue and discrete input modules - when do you use what?
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Discrete modules use only a single bit to represent the state of the device. For example, a switch is either open or closed. Therefore, the bit is either a 0 (switch is open) or a 1 (switch is closed). Discrete modules are also known as Digital modules.
•Analog modules use words to represent the state of a device. An analog signal represents a value.. For example, the temperature could be 5, 9, 20, 100, etc degrees. Analog modules use a value, such as 52, rather than a 0 or 1 to represent the state of the device.
operation voltage of discrete modules and number of possible input devices per module!
Discrete modules come in a variety of types, such as 24VDC or 120VAC. You can buy discrete modules that allow you to typically connect anywhere from 2 to 32 devices, with the most popular being 16 devices.
Analog Modules - typical voltage/performance of operation and nubmer of inputs per module.
Analog modules come in a variety of types, such as 4 to 20 mA or 0 to 10 VDC. You can buy analog modules that allow you to connect anywhere from 2 to 16 devices.
Storage of words - difference analog and discrete inputs
Since it takes 1 word to represent a number, a 16 point analog module requires 16 words of memory in the controller to store the value of all the numbers on the module. Each word in a PLC takes 16 or 32 bits (depending on the PLC), therefore it takes 16 or 32 times the amount of PLC memory to store analog points vs. digital points.
Output modules - what do they do and usual mode of operation?
Output modules interface directly to devices such as motor starters and lights
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Output modules take digital signals from the PLC and convert them to electrical signals such as 24VDC and 4 mA that field devices can understand
How do components of the PLC communicate with each other? Which kind of chassis/backplane are possible? Draw them!
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Modules are installed in the same chassis as the PLC and communicate over the chassis backplane
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Modules are designed to “plug” into each other. The interconnecting plugs form a backplane. There is no chassis
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Modules are built into the PLC. The modules come together in one physical block. The backplane in this case is transparent to the user.
What forms of power supply are there?
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Power supply modules that fit into one of the slots in a chassis
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External power supplies that mount to the outside of a chassis
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Stand alone power supplies that connect to the PLC or I/O through a power cable
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Embedded power supplies that come as part of the PLC block.
What kind of devices can be connected to the PLC over the Network interface?
Network Interface Most PLCs have the ability to communicate with other devices. These devices include computers running programming software, or collecting data about the manufacturing process, a terminal that lets an operator enter commands into the PLC, or I/O that is located in a remote location from the PLC. The PLC will communicate to the other devices through a network interface.
Name the parts of the plc Cabinet!
Name the great PLC manufactureres and their softwares!
Siemens, Allen-Bradley/Rockwell, etc…
How to program and install a PLC program?
The most popular language used to program a PLC is ladder logic.
What does this rung mean?
If the Start Button is on, turn the Motor on.
If the Start Button is off, then turn the Motor off.
The first instruction is as an “eXamine If Closed”, or “XIC” instruction.
In this example, if the actual Start Button is on, then the value of all the XICs named Start Button, in the program will be true (also known as a ‘one”, or “closed”). If the start button is off (not on) then the value of the Start Button XICs will be false (also known as a “zero”, or “open”).
The second instruction is known as an Output Energize, or “OTE”. This is an output instruction.
This instruction turns on if the logic to the left of the OTE is true. If the logic is false, then the output will be turned off. The OTE commands a physical output located on a output module to turn either on or off.
What about multiple XICs?
If there are multiple XIC’s on the rung, then all would have to be on for the rung to be true.
What about multiple OTEs?
If there are multiple OTE’s on the rung, then all would be turned on or off based on the rung condition (true or false).
What is the input and what the output of a ladder program?
What other kind of addresses (except from I/O addresses) are there?
Data addresses store a value used for functions like timers, counters, or calculations.
