The connector used to attach your mouse to the system depends on the type of interface you are using. Three main interfaces are used for mouse connections, with a fourth option you also occasionally might encounter. Mice are most commonly connected to your computer through the following three interfaces:

• Serial interface

• Dedicated motherboard (PS/2) mouse port

• USB port

Serial

A popular method of connecting a mouse to older PCs is through the standard serial interface. As with other serial devices, the connector on the end of the mouse cable is typically a 9-pin male connector; some very old mice used a 25-pin male connector.

Only a couple of pins in the DB-9 or DB-25 connector are used for communications between the mouse and the device driver, but the mouse connector typically has all 9 or 25 pins present. Because most older PCs come with two serial ports, a serial mouse can be plugged into either COM1 or COM2.

The device driver, when initializing, searches the ports to determine to which one the mouse is connected. Some mouse drivers can't function if the serial port is set to COM3 or COM4, but most newer drivers can work with any COM port 1–4. Because a serial mouse does not connect to the system directly, it does not use system resources by itself.

Instead, the resources are those used by the serial port to which it is connected. For example, if you have a mouse connected to COM2, and if COM2 is using the default IRQ and I/O port address range, both the serial port and the mouse connected to it use IRQ3 and I/O port addresses 2F8h–2FFh.

Motherboard Mouse Port (PS/2)

Most computers include a dedicated mouse port built into the motherboard. This practice was introduced by IBM with the PS/2 systems in 1987, so this interface is often referred to as a PS/2 mouse interface. This term does not imply that such a mouse can work only with a PS/2; instead, it means the mouse can connect to any system that has a dedicated mouse port on the motherboard.

From a hardware perspective, a motherboard mouse connector is usually exactly the same as the mini-DIN connector used for newer keyboards. In fact, the motherboard mouse port is connected to the 8042-type keyboard controller found on the motherboard. All the PS/2 computers include mini-DIN keyboard and mouse port connectors on the back.

Most computers based on the semiproprietary LPX motherboards and all ATX-series motherboards use these same connectors for space reasons. Most Baby-AT motherboards have a pin-header type connector for the mouse port because most standard cases do not have a provision for the mini-DIN mouse connector.

If that is the case, an adapter cable is usually supplied with the system. This cable adapts the pin-header connector on the motherboard to the standard mini-DIN type connector used for the motherboard mouse.

Connecting a mouse to the built-in mouse port is the best method of connection on systems that don't have USB ports because you do not sacrifice any of the system's interface slots or any serial ports, and the performance is not limited by the serial port circuitry.

The standard resource usage for a motherboard (or PS/2) mouse port is IRQ12, as well as I/O port addresses 60h and 64h. Because the motherboard mouse port uses the 8042-type keyboard controller chip, the port addresses are those of this chip. IRQ12 is an interrupt that is usually free on most systems, but if you use a USB mouse, you can probably disable the mouse port to make IRQ12 available for use by another device.

Hybrid Mice

Hybrid mice are those designed to plug into two types of ports. Although a few low-cost mice sold at retail are designed to plug into either the serial port or the PS/2 port, most mice on the retail market today are designed to plug into either the PS/2 port or the USB port.

These combination mice are more flexible than the mice typically bundled with systems, which are designed to work only with the PS/2 or USB port to which they attach. Circuitry in a hybrid mouse automatically detects the type of port to which it is connected and configures the mouse automatically.

Serial-PS/2 hybrid mice usually come with a mini-DIN connector on the end of their cable and an adapter that converts the mini-DIN to a 9- or 25-pin serial port connector, although the reverse is sometimes true on early examples of these mice. PS/2-USB mice usually come with the USB connector on the end of their cable and include a mini-DIN (PS/2) adapter

Sometimes people use adapters to try to connect a serial mouse to a motherboard mouse port or a motherboard mouse to a serial port. If this does not work, it is not the fault of the adapter. If the mouse does not explicitly state that it is both a serial and a PS/2-type mouse, it works only on the single interface for which it was designed.

Most of the time, you find the designation for which type of mouse you have printed on its underside. A safe rule of thumb to follow is if the mouse didn't come with an adapter or came bundled with a system, it probably won't work with an adapter.

USB

The extremely flexible USB port has become the most popular port to use for mice as well as keyboards and other I/O devices. Compared to the other interfaces, USB mice (and other USB pointing devices such as trackballs) have the following advantages:

• USB mice move much more smoothly than the traditional PS/2-type. This is because the frequency with which the mouse reports its position is much higher. A typical PS/2 mouse has a reporting rate of about 40Hz, whereas an average USB-wired mouse has a reporting rate of 125Hz (most USB wireless mice have a reporting rate of 40Hz–50Hz). Several utilities are available to test and adjust the mouse frequency.

• Mice with the most advanced features are usually made especially for the USB port. One example is the Logitech iFeel mouse, the first mouse with an optical sensor plus force feedback. It vibrates gently as you move the mouse over clickable buttons on Web pages, software menus, and the Windows desktop, and it's made especially for USB.

• USB mice and pointing devices, similar to all other USB devices, are hot-swappable. If you like to use a trackball and your computing partners prefer mice, you can just lean over and unplug the other users' pointing device and plug in your own, or move it from PC to PC. You can't do that with the other port types.

• USB mice can be attached to a USB hub, such as the hubs contained in some USB keyboards, as well as standalone hubs. Using a hub makes attaching and removing your mouse easy without crawling around on the floor to reach the back of the computer. Many computers now have front-mounted USB ports, letting you easily attach and remove a USB mouse without the use of an external hub.

Although the early USB mice were decidedly on the premium end of the price scale, low-cost USB mice are now available for less than $20. If you want to use a USB mouse at an MS-DOS prompt, in Windows safe mode, or in some other environment outside of normal Windows 98 or later, make sure that USB Legacy mode is enabled in your PC's BIOS.

Legacy mode enables non-USB-aware systems to recognize a USB keyboard and mouse. A fourth type of connection, the bus mouse (referred to by Microsoft as the Inport mouse), used a dedicated adapter card and is now obsolete.