Serial ports

Conventional serial ports—more properly referred to as RS-232 serial ports—were the most common way to attach mice to x86 computers through the mid-1990s. The serial port is a general-purpose port, and so can be used for many devices, albeit only one device per port. For this reason, most computers came with, and many still include, two RS-232 ports. Some computers, though, have only one RS-232 port.

Modern computers use 9-pin serial port connectors, as shown in Figure 15.1. Many older computers used at least one larger 25-pin connector, however. As with the full-size 5-pin DIN and smaller PS/2 keyboard connectors, adapters are available to connect 9-pin devices to 25-pin plugs, and vice versa. Figure 15.3 shows a 9-pin serial cable and a 9-pin-to-25-pin adapter. Despite the reduction in the number of pins, the 9-pin variety of serial port can be used for almost all the same tasks as can the 25-pin port; the additional 16 pins don't add any substantial functionality, and certainly nothing that's used by mice.

Figure 15.3

Serial connectors come in 9- and 25-pin varieties, and adapters let you convert from one type to another.

Figure 15.3

Serial connectors come in 9- and 25-pin varieties, and adapters let you convert from one type to another.

One of the reasons for the demise of 25-pin connectors on motherboards is the similarity of these connectors to 25-pin parallel port connectors. Serial and parallel ports differ in gender, however. On the computer side, parallel ports use a male connector on the cable and a female connector on the computer. For serial ports, this relationship is reversed. Because the cables used by serial mice are permanently attached to the mouse, you needn't be concerned with the gender of mouse cables on the mouse end.

Chapter 15

A wide variety of serial mouse protocols have emerged over the years, the most common of these being Microsoft, Logitech, and Mouse Systems, each named after the company that originated the standard. Some mice support more than one protocol by use of a switch on the mouse's bottom or side. You select the mouse protocol you want to use at some point during Linux installation. If you choose the wrong protocol or change your mouse after installing Linux, you might need to adjust your XFree86 configuration, as described later in this chapter.

USB Ports

Universal Serial Bus (USB) is the latest craze in external device connectors. USB is substantially faster than RS-232 serial, which is a great boon for many applications. Mice and keyboards, though, require relatively little in the way of speed, so USB's speed improvements aren't terribly important for these applications. USB does have other advantages, though. Most importantly, all the USB devices attached to a computer (up to 127) consume a single interrupt. In theory, then, switching from a standard keyboard and PS/2 or serial mouse to USB devices can save you an interrupt, because you need only one interrupt rather than two for these devices. Unfortunately, the standard PC keyboard interrupt is so deeply engrained in the PC architecture that it's impossible to disable it on most computers, even when a USB keyboard is used instead. If you have other USB devices, however, such as a USB digital camera or modem, you might be able to save an interrupt by putting your mouse on the USB port, as well.

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