3.3. External Device Controls

External devices provide input, output and accumulation of information in the PC, interact with the processor and the OP via the system or local bus, as well as through the input-output ports. They are located both outside the system unit (keyboard, mouse, monitor, printer, external modem, scanner) and inside it (disk drives, device controllers, internal fax modems). Often external devices are called peripheral, although in the narrow sense the term "peripheral" means a part of devices providing input and output of information (keyboard, coordinate manipulators, scanners, printers, etc.).

Most external devices for IBM-compatible PCs are controlled by controllers that are installed in motherboard expansion slots. A controller is a card that controls the operation of a specific type of external device and ensures their connection to the system board. Most controllers are system expansion cards , an exception could be port controllers and drives on floppy and hard magnetic disks embedded directly into the motherboard. In the early models of IBM-compatible PCs, these controllers were usually located on a separate board called a multiplatform or multi-card. Sometimes other controllers, including video adapters and sound cards, are built into the motherboard in laptops.

Expansion cards, called daughterboards, are installed on the motherboard. They are intended for connecting additional devices to the PC bus, and the motherboard usually has 4 to 8 expansion slots. In accordance with the processor width and the parameters of the external data bus of the motherboard, they are 8-, 16- and 32-bit.

Child boards are divided into two types:

1) full-length, i.e. the same length as the motherboard;

2) half-size, i.e., two times shorter.

Expansion connectors can be installed any daughter boards, if they are consistent with the bus management, bit and power.

A serial port transmits information one bit at a time, and devices such as a mouse, external modem, and plotter are connected via serial ports.

The most important types of expansion cards are:

1) video adapters (necessary for normal PC operation);

2) internal modems (required for using internal modems);

3) sound cards (designed for multimedia systems);

4) LAN adapters (required when using a computer in a local computer network environment).

In addition to the above, other types of expansion cards are used:

• scanner control;

• tape drive control;

• SCSI interface;

• virtual reality device controllers;

• ADC;

• bar code readers;

• control light pen;

• communications with large computers;

• accelerator boards.

The PC provides special input-output controllers, which is implemented through the input-output ports.

The serial port transmits information one bit at a time, and the parallel port transmits information byte-by-byte. Through serial ports connect devices such as a mouse, external modem and plotter.