Intro to Computer Systems

Chapter 6b: Peripheral Expansion

General-Purpose Expansion

As computer technology developed, and the amount of data they dealt with increased, it was clear that the legacy parallel and serial connections were not keeping up. Efforts were made to enhance the speeds of these interfaces, however the writing was on the wall: their time had past, and new peripheral interface standards were required.

USB (Universal Serial Bus)

The most ubiquitous of these new standards is the Universal Serial Bus (USB), developed by Intel in 1995. It is a high-speed serial interface which was designed to be as flexible as possible: modems, printers, scanners, digital cameras, hard disks -- it allowed just about anything.

A USB hub can split one port to connect multiple devices.
A USB hub can split one port to connect multiple devices.

The USB specification also called for a small amount of power to be made available along the wires. This allowed enough power for small devices (mainly keyboards and mice, but also some larger peripherals) to operate without the need for a power cord.

The USB specification was updated to version 2.0 in 2000 to include a faster "high speed" variant. This was to cater for high bandwidth devices that were starting to appear in the marketplace. A further revision in 2008, USB 3.0, added a 5 Gbit/sec "superspeed" option for modern, high-bandwidth devices such as high capacity mass storage devices.

A table of the various USB speeds is below:

Variant Name
Peak Speed
Typical Use
USB 1.1 Low Speed
1 Mbit/sec
keyboards and mice
USB 1.1 Full Speed
11 Mbit/sec
printers, scanners, older digital cameras
USB 2.0 High Speed
480 Mbit/sec
digital cameras, hard disks, MP3 players
USB 3.0 Superspeed
5 Gbit/sec
new portable mass storage

Power over USB

A standard USB 1.0/2.0 connector can supply up to 2.5 Watts of power per port, for powering up devices such as keyboards, mice, and mass storage devices. (There are also unpowered USB ports, which can only supply 0.5 Watt.) USB 3.0 can supply up to 4.5 Watts.

With its use in connecting portable digital accessories such as digital cameras, phones, and tablets, the USB connector has also been used as a defacto power and charging cable standard. With the popularity of this use case, the USB standard has since included a Battery Charging Specification to standardise the use of the port for these tasks.

Some devices, such as some pocket-size hard disks, and optical drives, can draw more than the standard 2.5W of power; without any special protocol, trying to exceed this power output may damage the port of the computer. There are various standards in place to signal a device that's capable of requesting additional power, such as:

Larger devices, such as tablets, may need up to 10W of power to charge sufficiently quickly; this kind of power draw falls outside even the USB Battery Charging Specification, and can only be safely supplied by special chargers.

A FireWire 400 connector.
A FireWire 400 connector.

FireWire

Another modern connection standard, FireWire (developed by Apple Computer) is in common use. It is also known by the names IEEE 1394 and i.Link. This standard is used most often in two niches:

Firewire is similar to USB; however it has two primary differences:

A faster 800 Mbit/sec standard has been released for specialist applications, however it is not very common in general computing.

Thunderbolt

A Thunderbolt device with two ports, that allow for daisy chaining. Not all devices have two ports to allow this feature.
A Thunderbolt device with two ports, that allow for daisy chaining. Not all devices have two ports to allow this feature.

Thunderbolt is a next-generation external expansion interface developed by Intel. In essence, it is both a DisplayPort and PCI Express interface transmitted over a serial cable. It was originally designed to be carried over fibre-optic cabling, however in its first commercial application in Apple notebook computers, the standard was implemented over conventional copper wires. (Despite the cheaper copper wiring implementation, the cables are still fairly expensive as they include circuitry inside the connectors.)

The interface supplies 10W of power to peripherals, and has two high-speed (10 Gbit/sec each), and one low-speed data link. The newer Thunderbolt 2 standard allows the two high-speed data links to be aggregated, allowing for a single device to have a peak bandwidth of 20 Gbit/sec.

The physical (copper wired) Thunderbolt connector is idential to a Mini DisplayPort connector, which allows for it to be used with DisplayPort monitors, as well as Thunderbolt peripherals that make use of the PCI Express interface on offer.

Interface Peripherals

Given the ubiquity of USB (and Super Speed USB 3.0), Thunderbolt's primary purpose has been for ultra high speed external peripherals such as laptop docking stations and high-end external disk storage (most consumer grade external drives' bandwidth requirements are met by USB 3.0). If the peripherals can support it, the interface can be "daisy chained" so that up to six devices can be connected to a single port.

The dual purpose nature of the interface port (DisplayPort monitors and external PCIe peripherals) places some restrictions on how the peripherals can be daisy chained - a DisplayPort monitor can only be added at the end of a chain.