I am building a multi-drop communication system that will communicate over power lines (12V DC). After some discussion and research, I've decided to use FSK. The system will have 1 master and multiple slaves. I want it to be full duplex (so the master can always be transmiting and 1 slave at a time can be responding). The master will transmit at one frequency and the slaves on another, I want to couple the outputs onto the +12V line. I'm going to use center frequencies of ~100k & ~150k. I'm going to use a CD4046 (74xx4046) PLL chip.
My question is what is the best way to overcome the loads on the power line. The loads I have are the power source (which will most likely be a battery), the voltage regulators (at each of the slaves), the FSK receivers, and the FSK transmitters. From some testing that I've done, it appears that the output impedance from the PLL chip is about 1k. From the data sheets it looks like the input resistance to the receiver is about 400k ohms, and the input sensitivity is about 1/2 volt. (the input resistance goes down and the sensitivity goes up when the supply voltage increases)
So far I have come up with the following solution (this is where I'm open to suggestions). I can isolate the battery with an inductor (not isolate, but increase the impedance looking towards the battery) and I can do the same with the voltage regulators. (but this has a downside if there is a quick demand for more current, and I would need to use large inductors to get any effective resistance). The output of the VCO can be turned off, but I don't know what it's impedance is (if it goes high it would help, but if it stays at 1k, then I might need to add some other kind of buffer).
I would like to be able to have a lot of nodes on the line (maybe as many as 99), so if anybody has any suggestions, please post.
ps. - I've seen other companies that sell a multi-drop FSK modem that can couple on to any lines (power, pbx, etc.) and they don't seem to have any problems with loading, so there must be a way to do this.