IO Interfaces

An I/O interface is a hardware circuit inserted between a group of I/O ports and the corresponding device controller. It acts as an interpreter that translates the values in the I/O ports into commands and data for the device. In the opposite direction, it detects changes in the device state and correspondingly updates the I/O port that plays the role of status register. This circuit can also be connected through an IRQ line to a Programmable Interrupt Controller, so that it issues interrupt requests on behalf of the device.

There are two types of interfaces:

Custom I/O interfaces

Devoted to one specific hardware device. In some cases, the device controller is located in the same card [11 that contains the I/O interface. The devices attached to a custom I/O interface can be either internal devices (devices located inside the PC's cabinet) or external devices (devices located outside the PC's cabinet).

[1] Each card must be inserted in one of the available free bus slots of the PC. If the card can be connected to an external device through an external cable, the card sports a suitable connector in the rear panel of the PC.

General-purpose I/O interfaces

Used to connect several different hardware devices. Devices attached to a generalpurpose I/O interface are always external devices.

Just to give an idea of how much variety is encompassed by custom I/O interfaces — thus by the devices currently installed in a PC — we'll list some of the most commonly found:

Keyboard interface

Connected to a keyboard controller that includes a dedicated microprocessor. This microprocessor decodes the combination of pressed keys, generates an interrupt, and puts the corresponding scan code in an input register.

Graphic interface

Packed together with the corresponding controller in a graphic card that has its own frame buffer, as well as a specialized processor and some code stored in a Read-Only Memory chip (ROM). The frame buffer is an on-board memory containing a description of the current screen contents.

Disk interface

Connected by a cable to the disk controller, which is usually integrated with the disk. For instance, the IDE interface is connected by a 40-wire flat conductor cable to an intelligent disk controller that can be found on the disk itself.

Bus mouse interface

Connected by a cable to the corresponding controller, which is included in the mouse.

Network interface

Packed together with the corresponding controller in a network card used to receive or transmit network packets. Although there are several widely adopted network standards, Ethernet (IEEE 802.3) is the most common.

13.1.2.2 General-purpose I/O interfaces

Modern PCs include several general-purpose I/O interfaces, which connect a wide range of external devices. The most common interfaces are:

Parallel port

Traditionally used to connect printers, it can also be used to connect removable disks, scanners, backup units, and other computers. The data is transferred 1 byte (8 bits) at a time.

Serial port

Like the parallel port, but the data is transferred 1 bit at a time. It includes a Universal Asynchronous Receiver and Transmitter (UART) chip to string out the bytes to be sent into a sequence of bits and to reassemble the received bits into bytes.

Since it is intrinsically slower than the parallel port, this interface is mainly used to connect external devices that do not operate at a high speed, like modems, mouses, and printers.

Universal serial bus (USB)

A recent general-purpose I/O interface that is quickly gaining popularity. It operates at a high speed, and may be used for the external devices traditionally connected to the parallel port and the serial port.

PCMCIA interface

Included mostly on portable computers. The external device, which has the shape of a credit card, can be inserted into and removed from a slot without rebooting the system. The most common PCMCIA devices are hard disks, modems, network cards, and RAM expansions.

SCSI (Small Computer System Interface) interface

A circuit that connects the main PC bus to a secondary bus called the SCSI bus. The SCSI-2 bus allows up to eight PCs and external devices—hard disks, scanners, CD-ROM writers, and so on—to be connected. Wide SCSI-2 and the recent SCSI-3 interfaces allow you to connect 16 devices or more if additional interfaces are present. The SCSI standard is the communication protocol used to connect devices via the SCSI bus.

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