10 Km will provide quite a volt drop (obviously depending on the current) especially when all 128 switches are on. You will probably need some method of encoding the switches and that will take some more current. If you can provide sufficient power then here are some possibilities.

One technique is to AC couple the signal onto the lines, while the DC power supply (low output impedance) is isolated from the AC by means of a gyrator or low pass filter (normally a coil). This technique is used in Foundation Fieldbus. (They have some app notes on their web site http://www.fieldbus.org/) You can find the circuit for a gyrator here: http://www.epanorama.net/documents...rator.html

If you search the archives of the design ideas in EDN and Electronic Design, I have a recollection of some ideas that were published on this topic.

The MC14469 (http://www.ee.washington.edu/stores/Da...c14469.pdf ) is unfortunately obsolete. However if you look at Fig 4, there is a technique that also mixes signal with power.

Also try looking at Analog Devices AD537 Voltage to Freq converter especially Fig 6 of the app note (http://www.analog.com/static/im...AN277.pdf) that allows for a 2 wire connection. You could possibly adapt this. In fact it may be possible to configure a different voltage (and hence frequency) for each switch, but for 128 and with the possibilty of multiple switches this is probably impractical.

Of course there are always modems that can be transformer coupled for the signal and draw DC power a la the telephone.

-Aubrey