Bus over power supply

[Ln_mark7930]

Hi boys,
Is It possible to transfert data over power supply wire's ?
I want connect two PIC16f628 through ONLY 2 wires and send/receive data inside 12V DC whitch are the same o RX/TX. How Can I do it?
What are the component's ? What is the circuit?
Anybody would help me?

[Ttelmah]

Yes.
Many dozens of different ways. Depends on a lot of other things, such as just how much power is involved, what data rate is involved, unidirectional/bidirectional, etc. etc..
At the end of the day, two fundamentally different 'basic' ways of going. First, is that you feed power from one unit to the other, have a diode, and sufficient 'reservoir' capacitance at the second device, then modulate the supply rail directly. This is most suited to low power application, and is at heart, the basis of many of the 'one wire' systems. Second, is that you have enought inductance at the supply source, that high frequencies can be modulated onto the line, then feed tones onto the wire, with capacitors from the devices. The same capacitors can be used to monitor these tones, while blocking lower frequencies or DC. Basically involves using a simple 'modem' chip. This is the basis of systems like the power-line modems.
Then within these two basic 'types', you have a wide variety of actual implementations.

In the 'old days', Texas did a simple modem chip, that handled this well, for low data rates, but this has been obsolete for several years. However modern chips for 'powerline' modems, usually integrate more functionality for higher rates, and many are now available.

Best Wishes

[KaraMuraT]

Of course you can, but you'll have to deal more things you imagine right now.

First of all you have to split DC and concerning data signal, which will have different frequencies as your UART speed changes.

But the real problem is, you have RX/TX signals. As the power supply has only the +12V (you can not use GND) you should pick one of the signals. That means your solution will be just one way.

If you think that's not enough the best solution will be adding 2 wires for RX and TX. In fact the other way is so complicated, you'd better to chose adding 2 wires to your design.

But for a brain storm, here is my ideas:

Transmitter:

You should have a transistor to trigger some signal on power supply. But you can not use it directly since you will cut the +12 therefore your PIC will reset over and over again. So, you have to use a small signal like 1V. You have to add this 1V signal with a transistor logic. (some resistors, maybe zener diode). You can add this signal with a transformer or directly. But as the 12V is a rough power source, you have to be careful about the currents. You can easily damage your circuit.

Receiver:

Receiver should filter the exactly right frequency. Because your signal probably will be a square wave, you'll have a lot of harmonics. Also your other circut and your power source will be generating other noises.

So prepare for some sleepless nights.

After filtering the right frequency (assuming that nothing generates noise on the same frequency) you should make the signal a bit squareish. You'll notice that your signal will be as impulses, not square waves. Therefore your circuit may not be able to get the data, because it should last some time according to RS232 definitons. To make it work so, you should add some capacitors and resistor. (you may also need some transistors)

Than you may have a working RS232 signal.

As you can see, this is a rough idea. I know it will have more components, more concepts and more pain.

I still suggest you to use 2 wires. Believe me, much easier, much cheaper
_________________
/// KMT
/// www.muratursavas.com

[Ln_mark7930]

Hi boy, thank's for reply.
This is my situation: I have got a alarm circuit and two PIR sensors.
Those sensors are connect to the circuit with only 3 wires (12Vdc-GND-SIGNAL).
In my application I must the sensors connecting third wire to ground with a remote relay.
To do this difficult operation I must send into 12V Dc wire some kind of signal and the receiver, when recognize this information, must close its contacts. Then, communications is unidirectional, current is very low, voltage of power supply is only 12 V direct current, I CAN'T use 4 or more wires because all wires and cables are into walls, and i have a lot of microchip PIC and Electronics components.

I thinks this method to transmitter: one (maybe power) NPN transistor is connected between 12 Vdc and ground through some low resistors.
When I provide a positive signal on transistor's base, current flow to emitter and the 12 become less.I must send only 1 or 2 byte, then i would not damage nothing.

The receiver circuit (maybe comparator) must be able to recognize energy defect and translate it in a digital signal (0 - 5 V dc)

If someone want contact me to complete or to help me, my e-mail address is : marco.p@lab-net.it. I'm always available.
I can send you some circuit that I think to do, some ideas, a little project...

[FvM]

My preferred method would be to use an existing "powerline" bus standard, or to modfiy it according to your needs. The modem option can be found with the HART standard in the automation world, but I think it's involving too much hardware for a simple application.

The M-Bus (meter bus) operation principle, that uses bus voltage switching for downstream and current switching for upstream seems flexible enough to apply it to a wide range of supply voltage and current levels.

[Ln_mark7930]

With a few components (about 15) i trasmitted digital data inside 2 wires togheter at power supply with success.
Circuit is very very simple.

I tested circuit until 100 meter and it work fine.

[KaraMuraT]

If it's not a problem, I'd like to see your design.
_________________
/// KMT
/// www.muratursavas.com

[Ln_mark7930]

I send you an e-mail to your personal address, with some explain and the drawing (jpeg).
Please read it and answer to me.
Thank's

[lord anubis]

Hi,

Because I have a two wire cable in a wall and can't remove or replace it and need to do some controlling 8 meters away behind concrete, and being new here I am very curious about this.

Therefor I was searching around and I just read this topic.

Did there came up finally a scheme. Or is there a scheme that you where playing with?

If so, may I ask you kindly, to send me a copy, or show it here, of that and results.

With kind regards,

LA

[temtronic]

Yes, it is definitely possible to have full duplex communications of just two wires which are also power. Download the Motorola MC14469 Addressable UART datasheet for how it can be done. For short distances power and data is easy. However the greater the distance you'll have to lower the baud rate and be careful about power losses, however with ultra low power PICs it's a LOT easier than doing it with 400 series CMOS 30 years ago.
Depending on how you configure your two wires, it is possible to get up to 24 miles, yes MILES and achieve full duplex communications. I've been doing that for the past 30 years as well. In fact I only need one wire, the second was actually a spare.

[5440]

If I recall this is what X10 is based on?

In my line of work they can transmit it over HV transmission lines, miles long at 230kV, using what's called Power Line Carrier (PLC). Although this is now old technology and fiber optic cables are now used for new installations.