Sounds feasible to me. I'm just not sure if it's the best solution to your
problem. I was wondering if you could use a FET to charge a capacitor when
the line is idle and use that as a the reference for a comparator.

I'll try and sketch it but no promises...



                                                    0V
                                                    |
           |           |--/\/\/--|                 <
           |           |         |                  >
           |-----/\/\/---|\      |        Diode    <
           >             | \     |           .     |
  r(sense)<              |  \____|___________|\|___|__|\
           >             |  /        |       |/|      | \
          <              | /         |       '        |  \_____P2
           |----/\/\/----|/          -|               |  /
           |                          |<---P1   ______| /
           |                         -|         |     |/
           |                         |__________|
                                     |
                                     |
                                   -----
                                   -----
                                     |
                                    0V

I hope that's right. When the op-amp output exceeds V(ref) by 0.7 volts (1
diode drop) your data has arrived so switch off the FET and start sampling.
When the line is low, turn the FET on to maintain the voltage on the capacitor.

Requires 2 pins, a FET and a dual FET input op-amp.

Hope this is some help. I'm not too hot on analog design so perhaps someone
on the list can simplify it a bit. I can't help thinking that a simple
analog filter should be able to do the same job without a FET.

Keith.
==========================================================
Keith Dowsett         "Variables won't; constants aren't."

E-mail: kdowsett@rpms.ac.uk
   WWW: http://kd.rpms.ac.uk/index.htm