Keyboard no worky

[Charles Steinkuehler]

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