At 10:29 PM 10/20/01 +0100, you wrote:

>I hadn't forgotten but I was hoping that I could somehow isolate them, hence
>my original post.  It's beginning to look like I will need to off load other
>I/O, in order to gain enough pins for the encoders.  The problem is they
>will need 6 dedicated pins that I really can't afford.

You can certainly isolate them with diodes. Using TTL threshold inputs,
you can use two diodes per encoder (6 total). Then two inputs (with resistors
to ground) are dedicated to the encoders, and you have 3 output pins connected
to the encoder commons (let's assume the shells are nicely grounded).
If you don't follow this, e-mail me.

Now, you've mentioned 21 keys, so I'll assume you have at least 10 port pins
already dedicated to the keyboard. (6 x 4 or 5 x 5 or 7 x 3). If you use
3 pins to drive the encoders and the remaining 7 as inputs, 2 for the
encoders,
that allows you to have 15 more keys (including the momentary ones in the
encoders, which don't require diodes).  Or, if you use the 6 x 4 method,
you can have a 4 resistor pulldown network, 6 diodes or (3 SMT duals),
3 encoders, and up to 12 keys without any additional components (including
the encoder switches). If you want the full complement of switches, as
implied by the x-y matrix, you'll have to isolate at least some of the
switches from the encoder inputs with diodes, but that may not be
necesssary.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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