PLCs Flashcards
What is the purpose of the backplate of a PLC?
- supply power to the modules
- data communication for the modules
What are the four main components of a PLC?
Input, CPU, Programming Device, Output
What is the input section?
Designed to sense the state of the field devices that are wired to the input terminal
What are the three categories of input mm output devices?
Discrete, Analog, Pulse-generating
What are the four basic functions of the input module?
Termination, signal conditioning, isolation, indication
What is the purpose of signal conditioning in the input?
Incoming signals rectified - bridge rectifier
Noise and denounce filter
Threshold level detection
What is the purpose of the optical coupler (isolation) of the input module?
To isolate the PLC from potentially high voltages. Optocoupler can withstand very high and very low voltage differences, protecting the PLC from voltage surges
What are the two types of DC inputs?
Sinking and sourcing
What is a sinking type module?
The DC positive source is switched (ex. Limit switch) into the input terminal, negative or common source is connected to -V of the terminal
What is a sourcing type DC module?
The DC 0V source is switched into the terminal.
The DC positive source would be connected to +V
Indicate the wire colours
Blue - Negative
Brown - Positive
Black - switched output
Define Address
A character and number combination that identifies a particular and unique
area of memory
Define Analogue
A mechanism in which data is represented by continuously variable
physical quantities. A voltage or current signal that continuously changes in a smooth
gradual progression over a specific range would be an analogue signal
Define Bit
Binary digit, the smallest particle of data. A bit can have a logical value of 1 or 0
Define Bit
Binary digit, the smallest particle of data. A bit can have a logical value of 1 or 0
Define Branch
a possible alternate path for the flow of “control power flow” in ladder logic. Instructions that are placed on different branches with a common end point are said to
be “logically OR’d”
Define Data Table
An area of memory which stores the status of individual bits or groups of bits. A data table is part of a computer or PLC memory map.
Define Data Word
a group of bits: in most PLCs the word size is 16 bits.
Define Image Table
A section of the data table, associated with real (hardware) inputs and outputs.
Define MCR (CMR)
Master Control Relay. A relay used to shut off power to the I/O but not normally to the CPU
Define Software
The computer program, which PLC manufacturers sell, to allow the writing and monitoring of PLC programs.
Define Upload
To copy a PLC program from the PLC into the computer so that it can be monitored, modified, and/or documented
Define Variable
A program element associated with a hardware address or an internal memory bit
Define Alias Tag
A tag that references memory defined by another tag. An alias tag may refer to another
alias tag or a base tag. An alias tag may also refer to memory within another tag by referencing a member of a structure tag, or a bit within an integer tag or member
Define Base Tag
A tag that actually defines the memory where a data element is stored.
Define Controller Tags
Apply to the entire controller, and can be used by all programs. In other words, the data in a controller tag is available to every task or program within the controller application. These tags can be seen as global variables
Define Program Tags
Contain data that is used exclusively by the routines within an individual program. This means that program tags can have the same name in more than one program, allowing programs to be copied and reused more easily. These tags can be seen as local variables
Define Tag
A named area of the controller’s memory where data is stored. It is the basic mechanism
for allocating memory, referencing data from logic, and monitoring data
What are the four basic functions of the output module?
Termination, noise suppression, isolation, indication
Discrete output modules are available in how many bits?
Available in 8-bit to 32-bit configurations
The output circuit includes which four components?
Logic, isolation, switch, filter
What are the main four components of the CPU?
PLC power supplies, memory, microprocessor, battery
What are the two main sections of PLC memory?
Executive and user memory
What is executive memory?
Pre-installed system program by the factory and is never changed by the user
What is user memory?
User program memory and data storage memory (RAM and ROM)
What is volatile memory/Random access memory?
Read/write memory, fast read/write speed, RAM is used for both user program and data storage, battery back up is required (indicating light when it needs to be replaced)
What is non-volatile memory/read only memory?
Read Only (ROM)
Programmable read only (PROM)
Ultraviolet ProgrammabLe read only (UVPRIM-EPROM)
Electrically erasable programmable read only (EEPROM)
Flash memory
What are the five DATA types?
Bool, INT, SINT, DINT, REAL (or floating point)
Which type of data uses only a single bit?
Bool. Uses 1 = ON, and 0 = OFF
Which type of data uses 8 bits (a “byte”)
SINT
Which type of data uses 16 bits (“a word”)
INT
Which type of data uses 32 bits (“a double word”)
DINT
Which type of data has the most available storage?
REAL
What are the four functions of a PLC communication device?
Program entry, documentation, run mode, and monitoring.
What are the three types of programming devices?
Hand held, full size dedicated terminals, personal computer terminals
What the is current rating of a typical TRIAC?
2 amps
What is leakage current? How is it stopped?
-TRIACs leak a small amount of current. Can be a shock hazards and can cause outputs to appear on after the output is off.
-Resistor is connected across the load, approximately 10000ohms and 10watts
-imposing relays may be needed if loads are higher than 2 amps
What is a watchdog timer?
Used for processors internal self diagnostics.
Timer is set for slightly longer than the program scan time, if program scan completes before timer then the system is good, if scan does not complete before the timer then the processor triggers a fault condition.
What is another term commonly used for Upload?
Backup
What is the tolerance of common PLC timers?
+0.001 seconds (one millisecond)
What are the three points that make PLC’s very maintainable?
PLC’s provide current status of field devices, they are modular and can be replaced easily and quickly, Electricians can easily access the ladder logic programs
What is the purpose of the microprocessor in the PLC system?
Controls the activities of the PLC
What are the five main functions of the microprocessor?
- monitors state of on/off of inputs
- solves ladder logic
- controls state of outputs (on/off)
- communicates with other devices
- manages memory, updates timers and counters and internal registers
What are the four steps in the operating cycle?
- Determine status - on/off
- Interpret logic
- Update outputs (turn on/off devices)
- Communication and housekeeping
Are I/O updates synchronous?
Yes
What component is largely replaced by PLC’s?
Electromechanical relays
What does ‘field’ or ‘real-world’ devices refer to?
Any device that is physically wired to the PLC
What determines the total scan time of a PLC?
Total number of I/O’s, program length, program content
On larger PLC systems, what is the purpose of the PLC power supply module?
To power the PLC modules
From the computer to the PLC is upload or download?
Download
From the PLC to the computer is upload or download?
Upload
What is XIC?
Examine if closed, examine on -] [-
What is XIO?
Examine if open, examine off -]/[-
The latch instruction sets the data bit when ______
enabled
The latch instruction does not change the status of the data bit when _______
disabled
The data bit remains set until it is bleared by _______
unlatch instruction
When the one-shot is enabled, what happens?
The storage bit (one-shot) is cleared, the ONS instruction enabled remainder of the rung
When the one-shot is disabled, what happens?
The storage bit _one-shot) is set, the ONS instruction disables the remainder of the rung
T/F? Leading-edge one-shot replay always activates as soon as input is true.
True
What are the stesp for changing a program in PLC?
Go offline, make changes, stop PLC, download new program to PLC
What is run mode?
When PLC in run mode its reading inputs, solving logic, setting outputs. Real outputs are energized based on logic program
What is stop mode?
When PLC is stopped it does not read inputs, solves logic, or sets outputs. All outputs are reenergized.
What is test mode?
PLC is running, reads inputs and solves logic, outputs are logically set. Real outputs are not energized.
What is program mode?
Controller scan is stopped, programmer develops or edits the user program.
What is offline programming mode?
Manipulating the project files or folders in the PC only, PC is not communicating with the PLC, any changes made to the ladder logic program does not affect the PLCs operation
What is online programming mode?
PC and PLC are communicating, allows the user to watch the logic program work
