Input Devices - Upgrading and Repairing PCs - page 942

If the cable is removable, you can test it for continuity with a digital multimeter (DMM).
DMMs that have an audible continuity tester built in make this procedure much easier
to perform. To test each wire of the cable, insert the DMM's red pin into the keyboard
connector and touch the DMM's black pin to the corresponding wire that attaches to the
keyboard's circuit board. Wiggle the ends of the cable as you check each wire to ensure
no intermittent connections exist. If you discover a problem with the continuity in one of
thewires,replacethecableortheentirekeyboard,whicheverischeaper.Becausereplace-
ment keyboards are so inexpensive, it's almost always cheaper to replace the entire unit
thantogetanewcable, unlessthekeyboardisanexpensive unitlikeanolderIBMModel
M. You can get replacement cables (and other parts) for older IBM Model M type key-
boards from www.clickykeyboards.com .
For more information about using DMMs for testing hardware, see Chapter 20 , “ PC Dia-
gnostics, Testing, and Maintenance . ”
Many times you first discover a problem with a keyboard because the system has an error
during the POST. Many systems use error codes in a 3xx numeric format to distinguish
the keyboard. If you encounter any such errors during the POST, write them down. Some
BIOS versions do not use cryptic numeric error codes; they simply state something such
as the following:
Keyboard stuck key failure
This message is usually displayed by a system with a Phoenix BIOS if a key is stuck. Un-
fortunately, the message does not identify which key it is!
Ifyoursystem displays a 3xx(keyboard) error preceded bya two-digit hexadecimal num-
ber, the number is the scan code of a failing or stuck keyswitch. Look up the scan code
in the tables provided in the Technical Reference section onthe topic'sDVD to determine
whichkeyswitch istheculprit. Byremoving thekeycap oftheoffendingkeyandcleaning
the switch, you often can solve the problem.
Forasimpletestofthemotherboardkeyboardconnector,youcancheckvoltagesonsome
of the pins. Using Figure 15.8 (shown earlier in the chapter) as a guide, measure the
voltages on various pins of the keyboard connector. To prevent possible damage to the
system or keyboard, turn off the power before disconnecting the keyboard. Then, unplug
the keyboard and turn the power back on. Make measurements between the ground pin
andtheotherpinsaccordingto Table15.2 ,shownearlierinthechapter.Ifthevoltagesare
within these specifications, the motherboard keyboard circuitry is probably okay.
If your measurements do not match these voltages, the motherboard might be defective.
Otherwise,thekeyboardcableorkeyboardmightbedefective.Ifyoususpectthatthekey-
boardorcableistheproblem,theeasiestthingtodoistoconnectaknown-goodkeyboard