What USB Can Do

USB is a likely solution any time you want to use a computer to communi-cate with a device outside of the computer. The interface is suitable formass-produced, standard peripheral types as well as small-volume designs,including one-of-a-kind projects.To be successful, an interface has to please two audiences: the users who want to use the peripherals and the developers who design the hardware and write the code that communicates with the device. USB has features to please both.Benefits for UsersFrom the user’s perspective, the benefits of USB are ease of use, fast and reli-able data transfers, flexibility, low cost, and power conservation.

Ease of Use

Ease of use was a major design goal for USB, and the result is an interface that’s a pleasure to use for many reasons:

One interface for many devices. USB is versatile enough to be usable with a variety of peripheral types. Instead  of having a different connector type and supporting hardware for each peripheral, one interface serves many.

Automatic configuration. When a user connects a USB peripheral to a PC, Windows detects the peripheral and loads the appropriate software driver. The first time the peripheral connects, Windows may prompt the user to insert a disk with driver  software, but other than that, installation is auto- matic. There’s no need to restart the system before using the peripheral.

Easy to connect. With USB, there’s no need to open the computer’s enclo- sure to add an expansion card for each peripheral. A typical PC has four or more USB ports. You can expand the number of ports by adding hubs with additional ports.

Easy cables. USB cable connectors are keyed so you can’t plug them in wrong. A cable segment can be as long as 5 meters. With hubs, a peripheral can be as far as 30 meters from its host PC. USB connectors are small and compact in contrast to typical RS-232 and parallel connectors. To ensure reliable operation, the USB specification includes detailed requirements that all cables and connectors must meet.

Hot pluggable. You can connect and disconnect a USB peripheral when- ever you want, whether or not the system and peripheral are powered, with- out damaging the PC or device. The operating system detects when a peripheral is attached and readies it for use.

No user settings. USB peripherals don’t have user-selectable settings such as port addresses and interrupt-request (IRQ) lines so there are no jumpers to set or configuration utilities to run. Frees hardware resources for other devices. Using USB for as many peripherals as possible frees up IRQ  lines for the peripherals that require them. The PC dedicates a series of port addresses and one IRQ line to the USB host controller, but individual  peripherals don’t require additional resources or any PC programming that involves specifying port addresses or detecting hardware interrupts. In contrast, peripherals with other interfaces may require dedicated port addresses, an IRQ line, and an expansion slot.

No power supply required (sometimes). The USB interface includes power-supply and ground lines that provide a nominal +5V from the com- puter’s or hub’s power supply. A peripheral that requires up to 500 milliam- peres can draw all of its power from the bus instead of having to provide a power supply. In contrast, peripherals that use other interfaces may have to choose between including a power supply inside the device or using a bulky and inconvenient external supply.

Speed

USB supports three bus speeds: high speed at 480 Megabits/sec., full speed at 12 Megabits/sec., and low speed at 1.5 Megabits/sec. The USB host con- trollers in recent PCs support all three speeds. The bus speeds describe the rate that information travels on the bus. In addition to data, the bus must carry status, control, and error-checking sig- nals. Plus, all peripherals must share the bus. So the rate of data transfer that an individual peripheral can expect will be less than the bus speed. The the- oretical maximum rate for a single data transfer is about 53 Megabytes/sec. at high speed, 1.2 Megabytes/sec. at  full speed, and 800 bytes/sec. at low speed. The USB 1.0 specification defined low and full speeds. Low speed was included for two reasons. Mice require flexible cables to make the devices easy to move around. Low-speed cables don’t require twisted pairs or as much shielding and thus can be more flexible than full/high-speed cables. Also, low-speed devices can often be manufactured more cheaply. Full speed was intended to replace most other peripherals that used RS-232 (serial) and parallel ports. The data-transfer rates attainable at full speed are comparable to or better than the speeds attainable with earlier interfaces. High speed became an option with the release of version 2.0 of the USB specification.

Reliability

The reliability of USB is due to both the hardware and the protocols for data transfer. The hardware specifications for USB drivers, receivers, and cables ensure a quiet interface that eliminates most noise that could cause data errors. The USB protocol enables the detecting of errors in received data and notifying the sender so it can retransmit. The detecting, notifying, and retransmitting are done in hardware and don’t require any programming or user intervention.

Low Cost

Even though USB is more complex than earlier interfaces, the components and cables are inexpensive. A device with a USB interface is likely to cost the same or less than an equivalent device with an  older interface or a more recent interface such as IEEE-1394.

Low Power Consumption

Power-saving circuits and code can automatically power down USB periph- erals when not in use yet keep them ready to respond when needed. The reduced power consumption saves money, is environmentally friendly, and for battery-powered devices, allows a longer time between recharges.

Wireless Communications

USB originated as a wired interface, but options now exist for wireless devices that use USB to communicate with PCs.