One wire serial comms on +5V line

Hello ! I have a little problem. I have two devices interconected by two wires ( the power lines GND and +5V ) and I have to transmit small ammount of data between them. I must not use rf modules. The cable length is 2 metters. In fact the master device poewres the slave. I have to put a pic in the slave and send some data to a pic in the master device using the +5v line. I thought about poewring the slave device with 9 volts ( and inside the slave to pass the voltage through 7805 ). Before 7805, in the slave, to put a transistor, and in the master device, on the power line, another transistor,In this way, I think I can modulate somehow the voltage, and, using a kind of Manchester coding scheme, .... to solve somehow the problem. This is just a theoretical ideea. Does anyone have another ideea ? Or do you think that my ideea might work ? Thank you very much for your time.

On the Master send the 5volt line + 1volt or more (as Offset)+ Serial data modulated on with a sufficient amplitude. At the other end, use an LDO voltage regulator... not 7805.
You'll extract your serial data with a simple voltage comparator between your voltage regulator output and it's input.

This could work great... not a fan of that method but could work.

Ok, I've got the ideea, but it's still unclear to mehow should I do this. Why do I have to use LDO ? And how do I modulate the data, through a transistor ? or ..

At the MASTER end, take the Comms pin to Vdd via a realively low-value Resistor - say 1K. The MASTER will normally be left in Receive (Input) mode until it wishes to Transmit. When transmitting, always idle HIGH and immediately on completion switch back to INPUT (Receive) mode.

At the SLAVE end, choose one of the Microchip micropower PICs. Take the comms line to your desired I/O pin. Connect a Diode from this pin (Anode) to the PICs Vdd pin (Kathode). Connect a Capacitor (at least 100uF) along with 100nF for decoupling between Vdd and Vss. The SLAVE is always in Receive (Input) mode until commanded by the MASTER to Transmit - and when doing so, always idle HIGH.

The main storage Capacitor at the SLAVE end will charge through the Resistor at the MASTER end. This will provide operating power for the SLAVE during the communications interchange. You may need to allow a couple of Seconds at Power-up for the SLAVE to come alive. If you have difficulty here, you may need to incorporate a Microprocessor PSU Supervisor IC to bring the PIC alive once the storage Capacitor has charged. Keep the communications interchange SHORT... MASTER sends a SEND DATA instruction to the SLAVE and the SLAVE then responds with a short burst in reply. Sending Tolstoy's "War and Peace" is not an option... that Capacitor will discharge long before you get to Chapter two... Don't attempt to send at high data rates... that low value Resistor (needed to charge the supply Cap) will round-off your nice pulses... so 1200 baud or lower is recommended.

Component count is two Capacitors and one Resistor... and if you're unlucky, a supervisor IC.

Thank you very much Melanie. This is a great ideea. Hope it works.