Distinguishing shift lock and left shift

From: Marko Mäkelä <msmakela_at_gmail.com>
Date: Thu, 5 Mar 2015 08:23:36 +0200
Message-ID: <20150305062336.GA1577@x220>
On Wed, Mar 04, 2015 at 04:26:59PM -0500, Jim Brain wrote:
>On March 4, 2015 at 2:41 PM David Wood <jbevren@gmail.com> wrote:
>>I've heard of (but not seen in the wild) a method to separate the
>>shiftlock key from left shift. As I remember, if you drive the line 
>>for shift against the key, the resistive element of the left shift key 
>>will read true, while the shift lock key's switch will not.
>I've heard that as well, but I don't know how you would figure it out 
>with the 6526.

I have seen that in a C64 demo by some Finnish group, iirc. I also 
repeated it myself. If you drive both the row and the column as outputs 
(one outputting 1, the other outputting 0), pressing normal keys cannot 
affect the reading of the port that is outputting 1. But, pressing the 
shift lock will force the output to 0.

I remember measuring the resistance of the keys on a normal C64 keyboard 
and on the C128D keyboard, some 20 years ago. The C128D keys had a 
resistance around 70 or 80 ohms, if I remember correctly. The C64 would 
have a higher resistance. I am not fully sure, but it could have been 
that due to the low resistance of the normal keys, on the C128D keyboard 
the left shift would be detected as shift lock by the above-mentioned 


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Received on 2015-03-05 07:00:05

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