Modulating data onto a DC supply line

[uoficowboy]

Hi - I'm working on a bussed system where I will have a number of boards spread out over a fairly small distance (maximum length from first to last node would probably be under 10 meters). I would like to connect these boards with a two wire bus - GND and VCC/data. There could be as many as 100 nodes, all daisy chained together. I need any of these boards to be able to send or receive. I'd like 1KBps, but less is OK too. Parts count needs to be as small as possible.

My general idea is to capacitively couple an output from my microcontroller (possibly buffered) onto the VCC line. Then capacitively couple some sort of band pass filter onto the VCC line, and then amplify that.

My worry is that the power supply will be too stiff and that there will be no signal available to the band pass filter.

Has anybody gone down a similar path before? Any advice?

 

[jmpac]

Why are you using capacitive coupling, instead of inductive coupling, to take out the AC signals from the VCC wire.

Regards.

 

[betwixt]

It can and does work but you have to be aware of some of the pitfalls.

If your boards have their own voltage regulators you can modulate the unregulated supply and sense it at the regulator input. For example, if each board has a 5V regulator and your supply is 9V, you can heavily modulate the 9V without upsetting the regulated lines.

If the supply is already at the voltage you need, you can isolate the incoming supply with a low resistance RF choke (so the DC conditions are not changed) and extract the signal capacitively from the supply. It might help to use a carrier frequency rather than just putting raw data on the lines.

I don't think lmpac's suggest will work well, as more nodes are added and the load on the signal increases, it would become difficult to extract enough to be used. With capacitors and feeding a high input impedance amplifier the loading would be far less.

I've used the principle to control banks of lights before.

Brian.

 

[etgpsp]

If you refer to fisher rosemount smart pressure transmitter documents.it uses fsk modulation on the dc line to tansmit data between several device and monitoring system by use of only two wire and all of them are paralel to gether.for better search result use the "hart"protocol.