Chapter 2 - Central Processing Unit Flashcards
What are the CPU components?
RAM and ROM memory, power supply, battery back up
What is the executive program?
Supervisory programs that are permanently stored in the memory. It allows the microprocessor to perform all of its control, processing, communication and other functions.
What are the four basic operations of a microprocessor?
Input and Output operations -allow PLC to communicate with the outside world.
Arithmetic and Logic - performs operations such as add, subtract, AND, and OR functions
Reading or changing memory contents
Jump operations- allow the user program to be bypassed when requested
What are the components of the microprocessor?
Executive Memory, Arithmetic Logic Unit (ALU) Program Counter (PC) Internal Register (IR) The read/write (r/w) memory The I/O ports
What is the program counter?
Keeps track of which step or line number of the program that the microprocessor is currently executing. When branches or jumps occur, the pc is set forward or backwards to keep track of execution times
What is the internal register?
Stores data as commanded by the ALU. It receives data from the executive memory and stores it until the ALU requests them.
What is the read/write memory?
Stores the program written by the user. It also stores the general parameters for total system operations. Contains memory units called registers.
What are I/O ports?
Used by the microprocessor to communicate with other circuits in the PLC system. Transfers data using buses.
Address bus?
Contains 16 unidirectional lines. Allows a microprocessor to select an address, which is then decoded, and the appropriate memory location or I/O device is chosen. It’s sole purpose is to activate devices memory locations at the proper times for further use by the data bus.
Data bus?
Is bidirectional and is typically 8,16,32 or 64 lines. Used to transfer bits out of or into the microprocessor. Can be used to read or write data.
Reading data?
Retrieving data from memory locations
Writing data?
Storing data into memory locations.
Latches?
Peripheral devices that provide the microprocessor additional circuitry to store data until the proper cycle time of the processor is reached.
Address latch?
Receives an address location and stores it until the microprocessor commands it to be sent. The command signal is sent via a strobe line.
Data I/O Latch?
Temporarily stores data being sent to the output system.
CPU Diagnostics?
Controlled by the executive program.
Self checking on startup checks for: memory ok, processor ok, battery ok, and power supply ok
User initiated diagnostics - checks all memory and tests communication ports. Testing the communication ports ensures the validity of the data being sent to input/output subsystems
Multiprocessing?
Involves dividing control between two processors to increase operating speed.
Control microprocessor - complex computations and data manipulation.
Logic microprocessor - executes timing, logic and counting operations and looks after application programs
Memory Capacity?
The total number of binary digits (bits) that can be stored in RAM memory.
Total memory system components?
Executive memory and control application program. Executive memory is permanent, Application memory is temporary.
Classes of memory devices?
Volatile- data is lost when power is lost, usually has a battery back up
Non-volatile- data is permanent, not lost with power loss.
Semiconductor memories?
Bipolar: transistor-transistor logic (TTL), constructed using bipolar transistors. More complex circuitry limits the memory capacity.
MOS: use metal oxide semiconductor field effect transistors or MOSFETs. MOS is slower but requires less power, good for volatile memory.
Memory Types?
Read Only Memory (ROM)
Random Access Memory (RAM) - static & dynamic
Programmable Read Only Memory (PROM)
Erasable Programmable Read Only Memory (EPROM)
Electronically Erasable Programmable Read Only Memory (EEPROM)
Flash Memory
Memory Utilization?
The number of memory locations required to store each type of instruction.
Bit- single digit of data
Words - group of bits
Register - storage location for words
File/Table - consecutive group of registers
I/O Tables?
Refers to the location where the status of an input or output device is stored. Constantly being revised and updated by CPU
Internal storage bits?
Software coils not accessible at the I/O rack. Have a portion of CPU memory set aside for them
Storage Registers?
Store data from timers, counters and analog modules obtained from analog and input devices (switches, encoders) in binary form.
The 3 parts of processor memory?
The Data Table
The User Program
The Message Storage Area
The data table?
Stores factory configured data that is permanent (ROM), and is 128 words (128x16 bits). The file storage and additional presets store information accumulated by timers, counters, sequencers and other file functions (Size varies)
The user program?
A.K.A the application memory. Contains the program instructions, subroutines and shares the file or bit storage with the data table (contains information on coils and contacts).
Message storage area?
Stores user programmed messages that are displayed on the CRT screen or printed out in hard copy form.
Scan function?
Reading the status of all I/O’s, examining application program instructions and executing the control program
Scan time?
The amount of time it takes the CPU to read the program stored in RAM, execute the control program and update all the outputs at The I/O rack.
Scan delay?
When an input signal changes states faster than the PLC scan time.
Scan interruptions?
Interrupt processing function- stops PLC scan due to a high priority event
Event interruption- executes everytime a pulse is detected
Time interruption- increases scan time in direct proportion to the # of times the interruption is executed during one scan cycle
Full Wave Bridge (FWB) Rectifier?
Coverts AC to DC voltage
Line conditioner?
Prevents AC voltage spikes from affecting the power supply
Filter circuit?
Smooths out the pulsating DC voltage from the rectifiers
Regulator Circuit?
Ensures a constant voltage level at the output of the power supply, regardless of the loading conditions.
