Question about older keyboards.

From: Louis Schulman <louiss_at_gate.net>
Date: Tue Dec 11 17:57:04 2001

On Tue, 11 Dec 2001 19:13:38 +0000 (GMT), Tony Duell wrote:

#OK, the '159 is the open-collector version of the '154. It's a 16 output
#decoder/with open collector outputs. I know it well.
#
#It takes in a 4 bit number, and (in this case) selects one of 16 logical
#columns of keys. Presumanly the keyboard is scanned in software and thus
#the 4 bit number comes from an output port somewhere.
#
#> So, if the keyboard seems to be OK, where would I next look for the cause of
#> a few random characters being generated on startup? Any tips here will help.

Well, if this was a keyboard problem, the 74159 itself could be bad. I just fixed an Exidy Sorcerer keyboard,
which is made in this very manner (using a 74L154), where some key combinations did not give the right
results. The 74L154 itself had an internal short.

But, I think we need more information on the machine. Random characters on startup don't sound like a
keyboard problem. In some cases, this could be normal. What happens after startup? Do the keys work?

In the Sorcerer, random characters on startup indicate a problem in the video RAM. But again, it is hard to
guess a cause for a problem without knowing what hardware we are talking about, and having a much
better description of the problem.

#
#Look the the connections on the keyswitches that do _not_ go to the '159.
#Where do they go to? Presumably via some kind of cable to a chip on the
#mainboard of whatever machine this is. What's that chip?
#
#Do you still get random characters if the machine is turned on with the
#keyboard totally disconnected?
#
#It may not even be a keyboard problem. It might be a RAM/ROM/CPU problem
#(the machine might not be running the monitor program correctly).
#
#-tony
#

Louis
Received on Tue Dec 11 2001 - 17:57:04 GMT