Keyboard no worky
Charles Steinkuehler
charles at steinkuehler.net
Fri Jul 23 21:58:10 CDT 2004
Jonathan Hutchins wrote:
>> My son's computer gets a no keyboard found error at boot up. We've
> tried two different keyboards to no avail. The amazing thing is, it
> says no keyboard found, press F1 to continue. How stupid is that? If
> there is no keyboard where the hell do I press F1?!
>> Any ideas to try, or do we need to replace the MB?
>
> If you were to plug in a keyboard, you could then press F1, and the system
> would continue.
>
> Assuming this is a standard XT or PS/2 keyboard connection, it's quite
> possible that the keyboard jack on the motherboard is bad. If the system
> has USB ports, and the bios supports it (check for updates), you may be
> able to use a USB version without resorting to resoldering the actual
> keyboard port on the MB.
Typically, if the KB/Mouse port goes bad, the problem is with the 5V
supply to the port, rather than with the two communication lines or the
ground.
In the old-old days, a short on the 5V power provided at the keyboard
port would typically fry a trace on the motherboard.
In the old-days (circa early PS/2 keyboards), a small fuse (typically
surface-mount devices, located near the PS/2 connectors) was added to
keep the smoke from escaping in the event of a short on the power line.
You can tell if either of the above happened by putting a volt meter
between the 5V and GND pins of the PS/2 connector. If you see 5V when
the computer is on, you've got some other problem. If the fuse (or a
trace) is blown, you'll typically see 0V with no keyboard plugged in, or
a couple of volts (typically between 1-2V) if a KB *IS* plugged in
(caused by leakage from the data lines which are pulled high to the
power pin of the KB microcontroller).
More modern systems include a "resettable circuit-breaker", typically a
"fuse" made of polymer material who's resistance goes up dramatically as
it gets hot. When it's cool (normal operation), it will easily pass
current, but if there's a short the extra current heats up the material,
causing its resistance to increase, thus limiting the current drawn to a
safe level.
If you've blown a fuse (IMHO the likeliest scenerio), wiring around it
with wire, or replacing it (with either a similar SMT part if you can
find one, or by wiring in a larger more readily avaialble fuse type)
should get your KB port running again.
--
Charles Steinkuehler
charles at steinkuehler.net
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