[newbie] Input to SID

Hi,

i'm trying to get the whole picture of what sid does and doesn't do and
there are dark parts on my map that i can't light from the documentation.
The main question that bothers me now is:

-how do you get input into the running application, from a parallel or
serial port for example. I investigated the "system monitor" (tksm) and
didn't manage to manipulate any value. Is it possible to write some
user-interface in tcl/tk that somehow invokes commands on sid-components
and simulates signals?

Thanks for your help
Greetings Malte