Avr twi protocol with power 2 wire

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Helo
I would like to use twi avr protocol or rs485 but over 12v/2a dc power.
I have only 2 wires.
How i can do this?
Have anyone any schematic?
The cable length is about 150-200 meter utp or single alarm cable.
Thank you

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A I2C bus spends most of its time with the lines at high level. i.e. when the bus is idle.

You simply steal power during this time, but you can never guarantee that all transactions are 'short and sweet'.

The I2C protocol requires at least SCL, SDA, GND even if you create an artificial VCC.

Why not buy 4-core cable? Or regular ethernet cable. Do you really need 2A?

David.

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You could try current loop or something along the lines of HART whereby they impress fsk signal on top of the DC. Google is your friend.

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Yep ... a current loop works well. Vaisala uses one to interconnect up to 4 wind sensor sites around airports over many kilometres of 2 wire loop.

Ross McKenzie ValuSoft Melbourne Australia

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The lines are installed before many years so it is not possible to change them.
What do you mean current loop can you please explain it?or post a schematic?
Also I can use any protocol that i want so it is not necessary to use something common i can make and my homebrew maybe protocol.
To give some more info about project here is what I have to do.
In old building there is a control center with bus system 2 wires (now not available)who is failed.
The sensors are a single on/off contact ,like magnetic switch and work with 2 wires who cary dc 12v , the info of the switch on or off and the id of the sensor.
Now I can change the sensors with new one who have 2 wires for dc and 2 for contact with compatible control center but it is not possible to change the wiring.
So i have to build one module master who create the bus system and 20 slaves module one for every sensor.

Also I am thinking about dtmf tones like telephone line.

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Current loop is where you modulate the current. The control panel for my central heater uses this method. A transistor is used to switch a resistor so the current gets switched between, say, 4 and 20mA. 4mA is the idle current and 20 mA is the current when the transistor is on. You can send usart data using this method. The supply end has a resistor that you measure the voltage drop. A comparator is used to detect the change in voltage across this resistor.

Schematics? Sorry, you'll have to find these yourself.

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There is a scheme that is commonly used with marine hardware that is essentially a coaxial cable. Electrically, its a two-wire system.

At the power supply end, add an inductor in series (the DC power supply current passes through the inductor). Couple the data into this line with a capacitor to the load end of the inductor. This probably means that you would need to use a coding like Manchester.

At the load end, you do much the same. Another inductor between the line and the power supply circuity. Couple data in/out through a capacitor connected to the line end of the inductor.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net