At 03:37 14/11/99 -0500, you wrote:

...
Now my simple question is: how can I simulate a rotary shift decoder in a
simple manner, so that when I key, the freqency is for example 10 Hz higher
and then moves down again to the original frequency. I have no idea how
this is normally realized with the shift encoder, however, I saw that a 5V
signal is simply pulled to ground over a resistor. If I just key one of
those encoder port to ground, I already get the frequency shift keying I
need, however, after keying a couple of times down, the freqnecy is moving
up or down (in those 10Hz jumps). What I need, I assume, is a kind of time
sequence to simulate the rotary shift encoder first into one direction and
then to the other.
...

An alternative way to generate FSK might be to alter the frequency of the
reference oscillator, this should work for small frequencyshifts (10Hz or
less).
Of course the shift will be a bit frquencydependent but if you stick to the
'digital mode' or visual-CW segments this will be acceptable.

eg. : assuming that you use a 10MHz reference oscillator, a 10 Hz shift on
137600Hz will require a 726.7Hz shift on 10MHz. If you stay within the
freuency range of 137400 to 137800 for digital modes or visual-CW then the
error on the 10Hz shift will limited to 1Hz.

If your reference ocillator is not 'canned' you will need most likely only
need to add 1 capacitor that is switched to get FSK.

73, Rik  ON7YD