Chapter 2 - Central Processing Unit

Question 1

What are the CPU components?

Answer 1

RAM and ROM memory, power supply, battery back up

Question 2

What is the executive program?

Answer 2

Supervisory programs that are permanently stored in the memory. It allows the microprocessor to perform all of its control, processing, communication and other functions.

Question 3

What are the four basic operations of a microprocessor?

Answer 3

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

Question 4

What are the components of the microprocessor?

Answer 4

Executive Memory, 
Arithmetic Logic Unit (ALU)
Program Counter (PC)
Internal Register (IR)
The read/write (r/w) memory
The I/O ports

Question 5

What is the program counter?

Answer 5

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

Question 6

What is the internal register?

Answer 6

Stores data as commanded by the ALU. It receives data from the executive memory and stores it until the ALU requests them.

Question 7

What is the read/write memory?

Answer 7

Stores the program written by the user. It also stores the general parameters for total system operations. Contains memory units called registers.

Question 8

What are I/O ports?

Answer 8

Used by the microprocessor to communicate with other circuits in the PLC system. Transfers data using buses.

Question 9

Address bus?

Answer 9

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.

Question 10

Data bus?

Answer 10

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.

Question 11

Reading data?

Answer 11

Retrieving data from memory locations

Question 12

Writing data?

Answer 12

Storing data into memory locations.

Question 13

Latches?

Answer 13

Peripheral devices that provide the microprocessor additional circuitry to store data until the proper cycle time of the processor is reached.

Question 14

Address latch?

Answer 14

Receives an address location and stores it until the microprocessor commands it to be sent. The command signal is sent via a strobe line.

Question 15

Data I/O Latch?

Answer 15

Temporarily stores data being sent to the output system.

Question 16

CPU Diagnostics?

Answer 16

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

Question 17

Multiprocessing?

Answer 17

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

Question 18

Memory Capacity?

Answer 18

The total number of binary digits (bits) that can be stored in RAM memory.

Question 19

Total memory system components?

Answer 19

Executive memory and control application program. Executive memory is permanent, Application memory is temporary.

Question 20

Classes of memory devices?

Answer 20

Volatile- data is lost when power is lost, usually has a battery back up

Non-volatile- data is permanent, not lost with power loss.

Question 21

Semiconductor memories?

Answer 21

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.

Question 22

Memory Types?

Answer 22

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

Question 23

Memory Utilization?

Answer 23

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

Question 24

I/O Tables?

Answer 24

Refers to the location where the status of an input or output device is stored. Constantly being revised and updated by CPU

Question 25

Internal storage bits?

Answer 25

Software coils not accessible at the I/O rack. Have a portion of CPU memory set aside for them

Question 26

Storage Registers?

Answer 26

Store data from timers, counters and analog modules obtained from analog and input devices (switches, encoders) in binary form.

Question 27

The 3 parts of processor memory?

Answer 27

The Data Table

The User Program

The Message Storage Area

Question 28

The data table?

Answer 28

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)

Question 29

The user program?

Answer 29

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).

Question 30

Message storage area?

Answer 30

Stores user programmed messages that are displayed on the CRT screen or printed out in hard copy form.

Question 31

Scan function?

Answer 31

Reading the status of all I/O’s, examining application program instructions and executing the control program

Question 32

Scan time?

Answer 32

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.

Question 33

Scan delay?

Answer 33

When an input signal changes states faster than the PLC scan time.

Question 34

Scan interruptions?

Answer 34

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

Question 35

Full Wave Bridge (FWB) Rectifier?

Answer 35

Coverts AC to DC voltage

Question 36

Line conditioner?

Answer 36

Prevents AC voltage spikes from affecting the power supply

Question 37

Filter circuit?

Answer 37

Smooths out the pulsating DC voltage from the rectifiers

Question 38

Regulator Circuit?

Answer 38

Ensures a constant voltage level at the output of the power supply, regardless of the loading conditions.