RS485 over DC power line (+12 v)

hello
I find out that we can send data (RS485) on DC power line (+12 , GND).
now I have 4 questions:
1- can I couple RS485 ICs (ADM485) to bus with Capacitors for block DC ?
2- what is value of termination resistor? (how calculate)
3- what length cable I can use? (RS485 + DC power(12v))
4- I have one master uc and many client, now, how many client I can use on RS485 buses?

thank you for your time!

http://www.sparkfun.com/datasheets/Sensors/Current/PLC-UAR-HS.pdf
http://www.sss-mag.com/pdf/485appno.pdf

thank you . I should read this application note.

If I understand the question correctly, you are intending to send the data bits directly over the power line, without any modulation. That gonna be problematic.

Particularly, you are considering an AC coupling of RS485 data. But the standard RS485 bus signal can't be AC coupled, because it's not a DC balanced signal, it has a dedicated idle level, and the duty cycle of character frames varies between 10 and 90%. Even more problematic, you can't couple an AC signal to the power lines without decoupling the low impedance DC path for AC, e.g. by chokes. Transformers are another option. Possibly, the signal drivers must be considerably stronger than standard RS485. The feasibility of a solution depends also on the current rating of the DC supply.

In any case a kind of modulation or encoding of the original serial data stream to form a DC balanced signal would be needed.

thank you for your complete explain.
but what should I do?:sad: I need send data over DC power line (maximum 20v DC)
I think about DTMF but I can't use this because it is very slow.
my cable is approximately 1500 meters.
what is your idea?
thanks

---------- Post added at 13:31 ---------- Previous post was at 13:22 ----------

Also I see a page about this from Texas Instruments , please see it.
data over dc power line using RS485(2 wire) - Industrial Interface Forum - Industrial Interface - TI E2E Community

Yes, the suggested circuit from TI forum is also mentioning manchester encoding to generate a DC balanced signal. Alternatively, you should consider V.23 modulation (1200 Baud FSK) as it's used for the Hart protocol in industry.

ok, so I find out that I need a FSK modulation now you said I can use V.23 modulation. I should design this project in very low PCB and very chip for example can I use XR2206 for FSK modulation and XR2211 for demodulation?
and another question is with this ICs what length of cable I can use?

thank you for your time

FSK modulation/demodulation with Exar chips should generally work, but I'm not sure about the achievable speed. Cable length shouldn't be restricted at kHz carrier frequencies, you should be aware of interferences, e.g. generated by VFD inverters, if it's an industrial application.

have you thought of using something like a TDA5051A home automation modem which can be used on any two wire system, e.g.
www.michat.com/pdf/GB/Faqs.pdf