>> Does anyone know of a link to somewhere for power line and serial
 >> transmission on the same two wires.

 P> I'm not aware of any URL's, but there are four approaches I've heard
 P> of to accomplish this task:

 P> [1] The end of the wire supplying the power modulates the voltage on
 P> the cables; the recipients send their return data by varying their
 P> current draw.  In this case, communication may be as fast or as slow
 P> as desired without either end needing to store energy.

 P> [2] Current is supplied through an inductor or gyrator (or, in a
 P> pinch, a resistor); data is output capacitively coupled to the wire.
 P> This method works excellently for analog signals (it's how telephones
 P> work) but it will most likely require extra circuitry to exchange
 P> digital data.

 P> [3] The device supplying the current sends data by briefly stopping
 P> the current flow; the device at the other end must then have a
 P> capacitor large enough to keep the device powered through the outage.
 P> The power-receiving device may return data by sourcing power to the
 P> unpowered line.  Note that if the line is loaded excessively, however,
 P> the device could lose power.

 P> [4] The device supplying the current reverses the current flow to send
 P> data. The downstream device then uses a full-wave rectifier to power
 P> itself This approach eliminates the need for a large cap, while
 P> providing very good noise immunity.  Unfortunately, there isn't a
 P> really good way of returning data.

 P> For a PIC-based application, I'd guess that number 3 is probably the
 P> best.

Number 3 technique can be used only on small low-current systems, due to
each slave should be able to power all system during sending data.

Number 2 technique requires simple modem circuits and software and can use
 almost all your program memory in small PICs, such us 12C508/509.

We use combination of number 1 and number 4 methods for really big PIC-based
networks: till 500 controllers, drawing about 1 mA each.

System have one master device, supplying all slaves. Data from master to slave
sended with line polarity reverses. Supplying current is limited to necessary
 value. Each slave have rectifier bridge, diode D1, small electrolytic capacitor
 after that and low-current voltage stabilizer (LM78L05, for example).
 Stabilizer isn't necessary for small-distance systems with low wire resistance.
 In our case it was about 5000 meters wires with resistance about 500 Ohm - 2.5
 Volt fall on wires.
Slave devices send data to master with number 1 method - by short-time
line shortenings. One external transistor and current-limiting resistor before
 diode D1 was used for this purpose. Master device have line voltage comparator
 and receive data from slaves with line voltage changes. There are some problem
 on master device with data receiving, if the total number of connected devices
 can be varied, or their supply current varies depended from external
 situations. A/D converter on master device with program level autocalibration
 seems to be good solution in this case. (Master is only one for
all system and it's cost have no very big value). Possible communication speed
 depends from the total current supplied. In our application 100 baud speed was
 enough and we didn't try to increase it.

Another application of this technique was small networks (about 100 meters) with
 about 20 slaves. We use about 1200 baud speed at this networks without
problem.

I plan to made some application of this technique available on the www in near
future.

Best regards

Alexey Vladimirov  avlad@mail.ormix.riga.lv

http://www.ormix.riga.lv/eng/mchip/mchip.htm
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