A peripheral device is any piece of hardware used for input, output, storage, or communications purposes.

The Keyboard

The keyboard formats found on any DOS or Windows PC can be grouped into one of four catagories:

• XT: 83 keys, 10 function keys, numberic keypad and cursor control keys combined, keyboard processor in keyboard.
  
  
• AT: 84 keys, with the addition of the SysRq key, a larger Return key, and the keyboard processor on system board.
  
  
• Enhanced: 101keys, which includes 12 function keys, cursor and screen control keys, and numeric keypad. Enhanced is the most common keyboard type.
  
  
• Ergonomic: Enhanced keyboard with built-in wristrest, and arched or bowed keyboard shape; may also separate into segments.
  
  

     Two types of key mechanisms are used in PC keyboards: mechanical and capacitive. A mechanical keyboard provides a more positive tactile click to the key action. A capacitive keyboard, also called a membrane keyboard, is common on name-brand keyboards. It is more reliable than a mechanical keyboard but more expensive as well. In place of metallic contacts, it uses a capacitive membrane module to generate signals.

     A keyboard is connected to the PC with one of two connector types: a DIN 5 pin connector or a mini DIN 6 pin, also known as a PS/2 connector. These connectors are also called DIN-5 and mini-DIN-6 connectors, respectively. DIN connectors have a notch or key slot that prevents an incorrect connection. The keyboard is typically assigned IRQ 1 and I/O address 060h.

     Keyboards, like all peripheral devices, require several types and layers of software to help them work. One of these software layers is the device driver. Keyboards and other input devices are relatively unsophisticated as a rule and don't require much beyond the two fundamental layers: firmware and device driver.

• Firmware: Stored on a ROM chip either inside the device or on the interface card installed on the motherboard. The firmware contains hardware and BIOS interace information, such as IRQ, COM port assignments, and boot sequence support data.
  
  
• Device driver: The software that communicates directly with the device itself and interacts with the operating system during normal operations. Device drivers are started during the system boot sequence from either the CONFIG.SYS in DOS and Windows 3.x or by the Windows 95 device management utilities.
  
  

To determine whether a keyboard electrical failure is in the keyboard or on the system board, do the following:

• Power off the computer.
  
• Unplug the keyboard connector from the motherboard.
  
• Power on the computer.
  
• Use a digital multimeter to check the voltages of the connector pins.
  

     If any voltages are out of range, in the vicinity of +2 volts to +5.5 volts, depending on the pin, the problem is likely in the keyboard circuits of the motherboard. Otherwise, the problem is likely in the keyboard.

     Keyboard related problems may also occur during the POST process. Should the problem occur early in the POST process, you hear beep codes for the particular BIOS on that computer. If the failure occurs later in the POST or boot sequence, an error code in the 300 - 399 number range displays, indicating a keyboard error. The most common reason for this error is a keystroke detected during the POST processing. This could be a stuck key.

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The Mouse

     Four types of mouse units are available for the PC. These units differ in how they connect to the computer. The four mouse types are:

• Serial: Connects via a 9 or 25 pin serial port usually with a DB-9 or DB-25 connector.
  
  
• PS/2: Connects with a mini-DIN 6-pin plug to a port usually mounted on the motherboard.
  
  
• Combination: Connects with either a mini-DIN (PS/2) 6-pin connector to a port on the motherboard or with a DB-9 or DB-25 connector through an adapter card.
  
  
• BUS: connects to its own adapter card. Bus mouse units usually connect with a mini-DIN 6-pin plug to an adapter added to the PC specifically to support the mouse.
  
  

     The most commonly used mouse units are the serial and PS/2 types. In addition to connecting the mouse to the PC, the mouse needs to have a device driver. The mouse is typically assigned IRQ 12 and an I/O address of 238h. IRQ 12 is actually assigned to the motherboard's in-port, but because that is where the mouse is typically plugged, it defaults to the mouse. The mouse, like most input devices, is not assigned a DMA channel.