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1992-07-16
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TIP #: 014
KEYWORDS: Zenith, Keyboard, Error Message
UPDATED: July, 1990
PROBLEM: The error message "Error, keyboard not responding or not connected"
occurs on a Zenith computer when doing a cold boot. This problem
is intermittent, and may occur with Zenith or clone keyboards.
SYMPTOMS: The above error message appears when doing a cold boot of the
computer system. It may not happen all the time. It may occur
with Zenith or clone keyboards.
SOLUTION: The problem is in the keyboard reset circuitry. A look at the
schematic for a Zenith ZKB-2 keyboard shows that pin 1 of the
keyboard processor is used to provide a power-up reset to the
processor itself. When power is first applies, capacitor C8 is
totally discharged. This capacitor is charged through the keyboard
processor towards five volts. Because of the capacitance, the
charging is delayed and is held low long enough to provide the
power-on reset. However, if the capacitor charges up too far
towards five volts before the processor is ready, the reset signal
is missed.
A related factor may be the amount of memory and peripherals that
you have in your computer. My problem surfaced after adding memory
and a larger hard drive. It may be that the five volt supply does
not come up as fast as normal due to the increased load.
In my Zenith keyboard there is a 2.2 uf capacitor at C8.
Electrolytics are notorious for changing their value dramatically
with age. I also believe that raising the value from 2.2 to
something in the 4.7 - 10 uf range would allow a much better chance
for the keyboard processor to be properly reset. I also had a
clone keyboard which would never be "seen" by the Zenith Z386-25
machine that I was using it on. This unit had a 4.7 uf tantalum
capacitor on the processor reset pin. This had to be increased
substantially in order for the keyboard to work properly. I went
as high as 35 uf with no problems. The keyboard took a relatively
long time to reset, but it worked fine, every time.
If you do decide to replace the capacitor, observe proper polarity
for either electrolytics or tantalums. If you are a hacker type,
give it a try. Otherwise you can do what I did and order an
upgraded BIOS, SCP, and then find out that a fifty cent part was
giving you grief.
NOTE 1: Originally it looks as if the computer was supposed to
provide the reset signal for the keyboard, but in my ZKB-2
that jumper was not installed. My guess is that the reset
signal is no longer supplied by the computer.