Keyboard Data Line Failure
Explanation: The data line is one of the control lines between the keyboard and the keyboard controller that interfaces the keyboard to the motherboard.
Early computers relied upon an Intel 8042 keyboard controller to interface to the keyboard. The controller on the motherboard actually interfaces with a similar controller inside the keyboard using a specialized serial link. Today’s computers merely emulate the 8042 for compatibility instead of actually putting one on the motherboard. As computers evolved, some Super I/O chips were further integrated, incorporating into them the keyboard control functions as well. This of course eliminates the need for a separate keyboard controller chip.
The keyboard controller provides the following functions to the PC:
- Keyboard Control and Interpretation: The keyboard controller is the circuit that controls the keyboard. When the controller receives a scan code from the integrated controller in the keyboard, it signals the processor using the hardware interrupt dedicated to the keyboard (IRQ1). The processor then interprets the key pressed and processes the command. In addition, the controller also handles the keyboard’s typematic rate, the rate at which keys automatically repeat when a key is held down for a period of time.
- PS/2 Mouse Support: Perhaps surprisingly, the keyboard controller handles input from the integrated PS/2-style mouse found on today’s machines. This is the manner in which IBM handled the original design when they created this standard with their PS/2 line.
- Access to the High Memory Area: The “high memory area” is the first 64K of extended memory. Access to this particular part of memory can be made by DOS despite the fact that it is not part of the traditional 640K of conventional memory, due to a strange design flaw in the original IBM AT. When the 640K DOS limit became constricting, using this area to save on conventional memory became advantageous. Access to this area is via the “A20 line” (21st address line) which is controlled using the keyboard controller on older systems. Many newer chipsets now incorporate this support natively, to increase performance.
Diagnosis: There is a problem either with the keyboard or with the keyboard controller on the motherboard.
- Troubleshoot the keyboard. Make sure that the keyboard is properly connected to the computer and/or reseat the plug into the back of the computer and then restart the system. If necessary, disconnect the keyboard and move it to another machine if possible to test it. If something has been spilled on the keyboard, discard it and replace it.
- Troubleshoot the keyboard controller by attaching a keyboard from another machine. If the problem goes away, it’s the keyboard, if it doesn’t, then there’s a problem either with the port where the keyboard attaches to the system or the keyboard controller on the motherboard.
- If none of the above resolves the issue, troubleshoot as a motherboard failure and replace the motherboard.
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