PLC’s Flashcards
The XIC instruction is true when the external input device that controls it ________ (provides/does not provide) voltage.
Provides
The examine if off instruction is true when the external input device that controls it _______ (provides/does not provide) voltage.
Does not provide
What binary bit will be represented when the XIO instruction is false?
1
If a normally closed push button (which is not pressed) is wired to a PLC as a stop button, a logic _______ (true/false) condition will exist at the corresponding XIO ladder logic instruction.
False
List the 4 major sections of a PLC system
Input
CPU
Programming
Output
List 3 main reasons PLC’s are preferred over other control sections
Flexibility; easy to add control sections
Reliably; no moving parts
Accuracy; timers are more accurate than conventional
List three types (categories) of devices that can be wired to an input
Discrete; PB’s, limit switches, float switches
Analogue; potentiometer, rheostat
Pulse; shaft encoder
What are the two main types of PLC memory?
RAM; executive, factory set, non volatile
ROM; user, volatile
List two factors that should be taken into account regarding the physical environment for a PLC installation
Temperature
Humidity
Dust
Shock and vibration
List two devices that can be used for programming a PLC
PC
Laptop
Handheld programmer
Name the device that is used in both input and output module circuitry to provide electrical isolation
OPTO isolator (coupler)
What is the best practice for grounding a single shielded cable?
Ground/bond at one end only (controller)
NO SHIELD IN THE FIELD
List the three main PLC programming instructions used with relay ladder logic
XIC; examine if closed
XIO; examine if open
OTE; output energize
What is the difference between an internal dummy relay instruction and a real output energize instruction?
There is no physical field connection. It only exists in the logic program
While in run mode, a Micrologix 1000 PLC will allow an instruction to be changed from XIC to XIO. (T/F)
False
While in run mode, a PLC will allow a timers preset value to be changed. (T/F)
True
A “single scan test” mode is a convenient way of testing the logic and function of your program while you are still offline. (T/F)
False
Output modules having ‘dry contacts’ may switch both AC and DC loads. (T/F)
True
The user memory (program) will normally be retained in memory on the PLC’s in our lab if the normal AC power supply is lost. (T/F)
True
An on-delay timer’s address could be O:0/1. (T/F)
False
An output thats addressed B:0/2 can be wired to an indicating light in the field. (T/F)
False
Why would it not be considered good practice to wire a normally open stop button to an XIO.
Its not fail safe. It wont stop if PB fails or a loose wire occurs
A sinking type input module will ______ (send/receive) current from/to the input field device.
Receive
What is an MCR sometimes used for?
Low voltage protection
The CPU of a PLC has three main jobs. What are they?
Read inputs
Solve logic
Write outputs
What could happen if a PLC’s battery were to become depleted?
Loss of user data
What is meant by discrete I/O?
On/off only
What should be considered when running wires to inputs?
Use shielded wire
Tie ground to one end only (controller)
Does a latching relay provide low voltage protection?
No. It will remain latched after restoration of power
When would you not want to use a master control relay in conjunction with a PLC?
Where low voltage protection in not desirable
Other than analogue inputs, what other type of inputs are available?
Discrete
Pulse
Of all the advantages of PLC’s, which is the most important?
Maintainability
Give a brief description of the CPU section.
Its the brain of the PLC
Includes: processor, power supply, memory
What is a bit?
0 or 1
What is a byte?
8 bits
What is a word?
16 bits
Which type of memory is used as a form of non-volatile memory?
RAM
Which type of memory deals with the status of field devices?
ROM
Which type of memory deals with the user program and data?
ROM
Which type of memory is not changed by the user?
RAM
How is the memory size required for a PLC system determined?
Determined by the size of the program and the number of inputs and outputs
List three types of backup non-volatile memory for the PLC user memory
EEPROM
UVPROM
NOVRAM
List three types of programming language for PLC’s
Ladder logic
Boolean
Structured text
Function block
What is a one shot?
An instruction that will only stay on for one operating cycle (scan)
What type of timers are available with the PLC we used in the lab?
TON (on delay)
TOF (off delay)
RTO (retentive on delay)
What type of counters are available with the PLC used in the lab?
CTU (count up)
CTD (count down)
List five different functions that PLC application software provides
Program ladder logic
Upload and download programs
Document ladder programs
Edit or modify logic programs
Change or modify controller operation
What does the programming section do?
Program entry
Documentation
Ability to initiate run mode
What does the output section do?
Switches outputs on or off
Provides surge protection and electrical isolation
Describe a PLC
Its a solid state control system that monitors the status of devices connected as inputs. Based upon a user written program, stored in memory, it controls the status of devices connected as outputs
Where could you find the status of each input?
Data table
Whats an I/O update?
Reading inputs and writing to outputs based on the program
