Computer Electronics

Underneath every computer is the same handful of building blocks: a processor that executes instructions, memory that holds data and programs, buses that move information between them, and a clock that keeps everything in step. This covers the hardware architecture — the counterpart to the Computer Applications page, which covers ports, networking, and software. It also covers block diagrams and flow charts, programmable logic controllers, programming concepts and computer languages, and peripherals and the interface cards and chips that connect them.

The processor (CPU / MPU)

The CPU (Central Processing Unit) — the microprocessor when built on one chip — is the brain that fetches, decodes, and executes instructions. Inside it are three core sections: the ALU (Arithmetic Logic Unit) that does the math and logic, the control unit that sequences operations, and a set of registers — tiny fast storage cells for the data being worked on right now.

The CPU runs a relentless fetch–decode–execute cycle: fetch the next instruction from memory, decode what it means, execute it, repeat — billions of times a second.

Counting input and output separately gives the five classic functional sections of a computer: input, output, memory, control and the arithmetic/logic unit (Wikipedia, Computer, groups input and output together as I/O). Two registers keep the cycle on track: the program counter holds the memory address of the next instruction to fetch, and the instruction register holds the instruction being decoded and executed (Wikipedia, Program counter).

The system bus

The CPU connects to memory and I/O through the system bus — a set of parallel lines, traditionally grouped into three:

The clock & timing

Everything in a synchronous computer marches to a clock — a steady square-wave from a crystal oscillator that synchronizes every operation. Each tick steps the processor through its cycle. Clock speed (in MHz/GHz) is roughly how many cycles run per second — higher generally means more instructions per second, though architecture matters too.

Memory types

Data representation

Computers represent everything — numbers, characters, instructions — as combinations of 0s and 1s (binary). A single 0/1 is a bit; 8 bits make a byte, which can represent 2⁸ = 256 distinct values. Characters are encoded in schemes like ASCII.

Microprocessor vs. microcontroller

Block diagrams and flow charts

Competency 15.2 asks you to differentiate how the computer block diagram and flow charts are used. The two drawings answer different questions. A block diagram shows a system's principal parts as blocks, joined by lines that show how the blocks relate; the CPU, memory, I/O and buses in section 1 are one. It leaves out the circuit detail on purpose, and that makes it the map for hardware troubleshooting: it narrows a fault down to one block before you open the schematic for that block (Wikipedia, Block diagram). A flow chart shows a process: the steps of a program or a procedure in the order they happen, and the decisions that send it down one path or another. It is used to analyze, design, document and follow a program or process (Wikipedia, Flowchart).

Flow-chart symbols are standardized in ISO 5807, Documentation symbols and conventions for data, program and system flowcharts. The ones you will meet most are these (Wikipedia, Flowchart):

Section 2 of the Technician Procedures guide covers using flow diagrams for work and troubleshooting procedures (competency 6.9).

Programmable logic controllers (PLCs)

Competency 15.5 asks you to explain programmable logic controllers and list their uses. A programmable logic controller (PLC) is an industrial computer that has been ruggedized: built to tolerate dust, moisture, heat and cold, with immunity to electrical noise and resistance to vibration and impact. PLCs came from car manufacturing. In 1968 General Motors' Hydramatic division asked for an electronic replacement for hard-wired relay control systems, which were hard to change whenever the process changed; the first PLC, the Modicon 084, followed in 1969. Changing the process became a matter of changing the program instead of rewiring relays (Wikipedia, Programmable logic controller).

A PLC runs its program in a repeating scan: read the inputs, execute the program, write the outputs, then start again. One scan typically takes tens of milliseconds, and a PLC is a hard real-time system: it must respond within a set time. The original PLC language is ladder logic, which strongly resembles a relay-logic schematic, so it reads like the relay panels it replaced. The IEC 61131-3 standard adds function block diagram, sequential function chart and structured text. PLCs control manufacturing processes such as assembly lines, machines and robotic devices (Wikipedia, Programmable logic controller).

Basic programming concepts

Competency 15.6 asks you to describe basic programming concepts. A program is a list of instructions that the computer carries out in order. It usually takes input, processes it and produces output, and it keeps the values it is working on in variables: named storage locations whose value can change while the program runs (Wikipedia, Variable). Any program, however large, can be built from just three control structures, as Böhm and Jacopini showed in 1966 (Wikipedia, Structured program theorem):

Writing a program is a cycle. Edit the source code; translate it into machine code by compiling or assembling it (section 10); load it into the target's memory (an Arduino upload, in the Digital Electronics guide's section 10, writes it into the microcontroller); then test and debug it. Debugging is finding the root cause of a fault (a bug) and fixing it, with tools such as breakpoints that pause the program, stepping through it one step at a time, and printing the values of variables (Wikipedia, Debugging). A flow chart (section 7) is a common way to plan the steps before writing the code.

Computer languages and how they relate

Competency 15.7 asks you to describe the reasons for different computer languages and their relationships. A CPU runs only machine code: each instruction is a pattern of bits from that processor's instruction set, and instruction sets differ between CPU families (Wikipedia, Machine code). Every other language is translated down to machine code before or while it runs.

Why so many languages? The levels trade closeness to the hardware for convenience. Machine code and assembly work instruction by instruction on one processor, which ties the program to that architecture; high-level languages are easier to read and write and are portable between machines, because a compiler or interpreter does the translation for each one (Wikipedia, Assembly language). Others are built for one job, such as ladder logic for machine control.

Peripherals and interface devices

Competency 15.8 asks you to define a peripheral and list various types; 15.9 asks you to explain the reasons for using interface devices, chips and cards and name common types. A peripheral is an auxiliary device that the computer uses to move information in or out. It is controlled by the computer but is not one of its core components, the CPU and memory (Wikipedia, Peripheral).

Most peripherals cannot connect to the CPU's bus directly: the bus carries parallel data at its own levels and timing, while a printer, a network or an instrument expects its own standard. An interface device sits between them and converts one to the other. On a PC it is often an expansion card, a circuit board plugged into a motherboard slot (PCI, or today PCI Express) to add an interface or a function the motherboard lacks (Wikipedia, Expansion card). The same circuit is often built onto the motherboard instead, as most network interfaces now are.