PLCs Flashcards
How are PLCs usually selected?
According to the number of input and output points they can service.
What side of an I/O point are switches and sensors connected to on a PLC?
Input
What might be connected to the output of an IO point within a PLC?
Pilot lights
Contactors and relays
True or false: the programming, memory, and communication requirements on a PLC remain the same regardless of size.
False. In larger systems, these requirements increase.
How many IO points could you expect to see in a “Large” system?
120 000
What devices comprise a control system?
Input devices
Controller
Output devices
What device receives information from an input device and decides upon a response?
Controller
What device of a control system would a solenoid be? (In/out/control)
Output device
What device in the control system would a thermostat be? (In/out/control)
Input
What are the advantages of using PLCs?
Lower overall cost
Programmability
Reliability
Expandability
What factors contribute to the cost advantages of a PLC? (Vs. Control panels using relays)
- less maintenance (no mechanical parts wearing out)
- troubleshooting is easier and reduces down time
- physically smaller
- smaller energy requirements
What factors contribute to the programmability of PLCs?
- using software increases level of process automation
- can be updated without changes in hardwiring
- program can be revised while system in operation
- one program can be used for several identical processes
What factors contribute to the reliability of PLCs?
solid state technology
- able to operate in many environments - static electronic switching (no movingparts)
What is a communication data highway?
A link between two PLC systems (as in adding another system to expand, and linking the two with a communication data highway)
What are the major hardware components of a modular PLC system?
Power supply
Backplane
Processor (CPU)
I/O modules
What type of power is supplied to the PLC and its components?
DC power
Which specific devices does the power supply feed power to? Which devices does it NOT?
Supplies power to the processor (CPU) and I/O modules.
Does NOT supply power to field I/O devices
True or false:
I/O devices are electrically isolated from backplane, and CPU but connected to I/O modules and PLC power supply
False. electrically isolated from backplane, CPU, and PLC power supply
Connected to IO modules
What is the function of a backplane?
1) Allows communication btwn CPU and and other components
2) Distributes power for internal operation of processor and IO modules
What hardware component is the power and communication bus for PLC components?
Backplane
If the PLC does not have a chassis, how is the continuity of the backplane maintained?
By connectors on the sides of the modules and at the end plates which snap together.
What kind of PLC is not designed to be readily taken apart and replaced?
Shoebox PLC
What is a common application of shoebox PLCs?
Often used to replace small relay control systems.
What makes a shoebox PLC unique?
Do not have separate power, processor, or I/O modules
What are the primary functions of a PLC processor?
1) read and store status of all input devices
2) execute user program by solving programs logic
3) update status of all the outputs
What a processor scan?
The sequence by which a processor carries out its functions
what are the types of memory used in PLCs?
RAM, ROM, EEPROM
What type of memory requires backup battery supply to keep stored data or to keep programs in tact when power is removed?
Volatile
Which type of memory is considered permanent?
Non-volatile
Which type of memory is easily altered to update or change programs?
Volatile
What does RAM stand for?
Random access memory
What kind of memory is RAM memory?
Volatile (usually)
What type of RAM memory is provided a memory chip with a self-contained battery within the package
Non-volatile RAM
What does ROM stand for?
Read only memory
What type of memory is ROM?
Non-volatile
(RAM/ROM) systems cannot be altered or written to by a user. Ie. Program fixed at manufacture.
ROM
The basic input/output system is stored in ________ (RAM/ROM). Why?
ROM. so the PLC recognizes its components when it is first turned on.
What does EEPROM stand for?
Electrically erasable programmable memory
Is EEPROM considered volatile or non-volatile?
Non-volatile
What’s an example of a type of EEPROM?
Flash memory cards or USB drives
Processors utilizing EEPROM memory ____ (can/cannot) be reprogrammed
Can, even though it is considered to be non-volatile.
What must happen before a new program is loaded on an EEPROM processor?
Memory chip must be erased
TRUE OR FALSE: One start button could be used to operate a number of motors or other equipment
True
TRUE OR FALSE: The voltage for the input signals is provided by the controller
False; is provided by a separate source.
How is the signal voltage isolated from the controllers processor?
Optoisolators or
Small relays and filter circuitry
What is the processor itself isolated from?
Voltage transients or
Magnetic interference
What kinds of controls are special I/O modules used for?
Position control High speed counters Temperature control Proportional integral derivative control Fuzzy logic control Identification sensors Cam positioners Analogue I/O
What is the advantage remote IO modules offer?
Reduce wiring btwn PLC and input/output devices. Requires only one cable back to PLC instead of individual cables from each IO device.
What is a data highway?
Local area network cabling that connects a number of PLCs, or PLCs and personal computers, allowing them to exchange programs and data.
What allows one PC to change programs of several PLCs?
Data highway
What is the difference between the terms “discrete” and “analogue” with regards to PLCs?
Discrete = device is on or off Analogue = device operates over a range of values of voltage or current
How does an optoisolating device operate?
Changes the input from a discrete device (to an input module) into a light beam
Light beam electrically isolates I/O modules from signal circuits
Protects PLC from high voltage of discrete input circuit
What is a solid state electronic switch called for dc? Ac?
DC = transistor AC = triac
What is the function of a solid state electronic switch?
Activated by a light (signal from processor) and closes the circuit to a discrete output device
When would an interposing relay be used?
For output circuits that require larger currents.
What are the two most common methods of sending an analogue signal?
1) proportional voltage
2) proportional current
What device must be used in order for a PLC to receive an analogue signal?
Special analogue input card
What is the most common analogue voltage standard?
0V to 10V dc
0V = 0%
10V = 100%
What is the most common analogue current standard?
4mA to 20mA
Why is the min mA analogue signal NOT 0A?
Allows PLC to tell the difference between a zero % signal and a broken wire condition
What kind of output card is used to achieve proportional control?
Analogue output card
What is the most common type of cable used in analogue PLC applications?
Shielded twisted pair
What size conductor would you expect to see in analogue cables?(large/small)
Small. As small as 22AWG. (Current levels are small)
What is used to prevent electrical interference and distortion of analogue signals?
SHIELDED cable
How do you ensure the shield acts as a “drain”?
Ground only one end of the shield.
What is the common programming device?
PCs
What are the 5 main types of languages used for programming PLCs? Which are graphic-based, and which are text-based?
Graphic: Ladder diagrams (LD) Function block diagrams (FBD) Sequential functions (or flow) chart (SFC) Text: Structured test (ST) Instruction lists (IL)
Which language is most often used to program PLCs?
Ladder diagram
Which language resembles a schematic diagram?
Ladder diagrams
What language is typically used in Europe?
FBD function block diagrams
How is each function block labelled in function block diagrams?
Based on boolean logic
What language is used for large-process controls where several complex functions take place in sequence?
Sequential function chart (SFC)
Which language is similar to BASIC language?
Structured text
Which language is more closely related to machine code used to make a microchip function?
Instruction lists
Which language is similar to assembler language programs?
Instruction lists
