Re: CBM Keyboard not working

From: Marko Mäkelä (
Date: 2000-01-17 14:51:44

Hello Brian,

> Thanks for the reply!  It's a Commodore CBM 4032.  When I turn it on, It
> makes a shrill beeping sound for about a second then displays all the
> correct bootup info.  It also shows "ready>" at the bottom.

Really "ready>" and not "ready."?  The code for the ">" sign would be
0x3e, while "." is 0x2e.  This could mean a failure in the memory chip
connected to the processor's data line D4 (1 << 4 == 0x10).  The chip
probably has faulty bits also at other addresses, but luckily the system
starts up pretty far nevertheless.

> I opened it up and and probed the back of the keyboard connector with a
> multimeter.  When I press a key, the voltage will drop by a few tenths of a
> volt.

I don't think it's a problem with the keyboard.  The keyboard lines are
arranged in a matrix.  If you really suspect the keyboard, you can measure
the resistance between a row and a column line (disconnect the keyboard
from the mainboard first) and press the corresponding key.  The resistance
sould be around 100 ohms (anyway less than 1 kilo-ohm).  And if it really
was a keyboard problem, I'd rather suspect the electronics that drives and
reads out the keyboard matrix lines.

I sent a copy to the cbm-hackers list, in case someone else has any ideas.
You can send your further questions to the list address.


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