Intro to Computer Systems

Chapter 2: System Basics

Data Links

Between every internal component or subsystem in a computer (processor, memory, expansion cards, etc.) lies a data link. There are a variety of link types, each designed for a particular purpose in mind (or, in the case of expansion links, designed on purpose to support a wide variety of potential use cases).

A computer's functionality can also be expanded using external devices known as peripherals. These include common computing devices such as keyboard, mice, printers and modems. Perhiperals are connected through an interface port on the computer: there are a number of interface types, each best suited to a particular task: low or high speed, hot pluggable (can be connected/disconnected while the computer is on), cheap or expensive to implement, etc.

Although these connection schemes are all different, there are a few common attributes that are shared by all.

Topology - Bus vs. Point-to-Point

A topology refers to how the signal ends are connected to one another. Most all modern signal interfaces use either a bus (shared data path), or point-to-point (dedicated data path) topology. They can be summarised in the following diagram:

Comparing a data bus to a point-to-point data link
Comparing a data bus to a point-to-point data link

As we mentioned earlier, a "bus" is an electrical path onto which devices can be connected. Unlike a "point-to-point" system, where each device is guaranteed its own set of wires, a "bus" shares the wires across all devices connected to it.

This brings about the issue of the bus being congested - where so many devices try to communicate on the bus, it simply can't handle the volume of information. The bus interfaces are typically designed such that they are sufficiently fast that the data demands do not normally overwhelm the bus in this way. Issues arise, however, in time when increasing computing and data demands mean that buses are pushed further than originally intended or designed for.

Data Transfer - Parallel vs. Serial

A "serial" interface means that data is transferred along a wire one bit at a time. Along with this "data" wire, there are a few other connections, such as an electrical ground and some signal wires to control the flow of information.

A "parallel" interface means that instead of data being transmitted one bit at a time, there are a number of signals travelling along many wires at the same time.

Serial interfaces use a single high-speed wire to transfer data. Parallel interfaces use many wires, but each wire runs at a slower rate.
Serial interfaces use a single high-speed wire to transfer data. Parallel interfaces use many wires, but each wire runs at a slower rate.

Due to the decreased number of wires and the unique way in which the electrical signals are transmitted, serial cables can span a far longer distance before the electrical signals degrade.

In the past, when computer clock (and respectively, the interface) speeds were relatively slow, parallel interfaces were far faster than their serial counterparts.

Virtually all new-generation interface connections are serial in nature: with modern technology, the interface can be run incredibly quickly, while still allowing the long cable length typical of a serial data transfer scheme.

Interface Connection - Hot vs. Cold Swap

A hot-swappable interface connector means that the device can be safely plugged and unplugged while the computer is switched on. This is usually achieved through two means:

A cold-swappable device should only be plugged and unplugged after switching off the computer. If this is done while the computer is on, there is a risk of the electrical connections shorting out, causing potential damage to the device and/or the computer port.

Most external cold-swappable connectors and computer interfaces are usually equipped with some form of short-circuit protection, which vastly reduces the risk of disaster if a cold-swappable device is hot-swapped.

This protection shouldn't be relied upon, though.

There is also the issue in that a computer might rely on a cold-swappable device being present at all times, if it was available at startup. If it is disconnected while the computer is on, it may try to communicate with it later on (on the assumption it's still there) causing a system crash or other undesirable behaviour.

Almost all internal connections in a computer are cold-swappable, and offer little or no short circuit protection.

Never work on the insides of a computer that's switched on.

There is also the concept of a warm-pluggable device, usually in portable computers. This means that a component is unsafe to plug in or remove when switched on, but okay when the computer is in sleep mode. The most common such device are laptop batteries: a good proportion of laptops can remain in sleep mode without a battery installed, just long enough to remove and replace the battery. This allows the user to replace the battery without having to shut down and restart the entire system.