Serial comms on VCC line

[MegaADY]

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 ? Any schematic would beHIGHLY apreciated ).
Thank you very much for your time.

[Melanie]

I did explain the theory and operation on this thread...

http://www.picbasic.co.uk/forum/showthread.php?t=1761

Schematic appended.

[keithdoxey]

I did explain the theory and operation on this thread...

http://www.picbasic.co.uk/forum/showthread.php?t=1761

Schematic appended.

Hi Melanie,

I just took a look at the thread you referenced and got a strange feeling of Deja Vu.

Reading the start of the thread I got the feeling I had read it before, then I realised.

Both threads were started by the same person asking exactly the same question (apart from one extra sentence)!

Aside from that, can I ask how you remember what you have typed before and in what thread. I know you are a dab hand with the search function but remembering which of your post is which is a pretty impressive feat!

[Melanie]

Ah... in this case it was easy...

MegaAdy earlier today asked me for a schematic off-list reminding me of that thread... as I was about to reply I noticed he had posted the question again on-list as well, so I posted the schematic publicly. So no great feat of mental prowess in this instance... and all this from a girl who mislays her car keys a dozen times a week!

[Kamikaze47]

Just looking at that schematic, it may be necicary to note that you will get a voltage drop accross the R1 resistor as well as a voltage drop across the diode. You would want to make sure the voltage getting to the slave pic is sufficient.

As for the large capacitor, you can get 5v, 0.047F (or 47000uF) caps. They are small, quite cheap, and will maintain power for your PIC for a number of seconds.

[Luciano]

Don't worry Melanie, we can fix your car keys problem...

[Melanie]

Surgically implanted obviously! Great idea. Now all I got to do is get all the locks in my life changed to be chip proximity activated.

Kamikaze...

The Voltage drop across R1 will be negligible once C1 has charged since the micropower PICs draw only a few uA. However I do have issue with Caps that size... they'll take forever to charge and the voltage rise-time at the Slave end will be so slow that the PIC will not initialise for the best part of a minute! Just work out the CR time constant!

[keithdoxey]

Ah... in this case it was easy...

MegaAdy earlier today asked me for a schematic off-list reminding me of that thread...

You disappoint me Melanie

After all those superb code examples, I thought you had some magical formula for finding previously written information

So no great feat of mental prowess in this instance... and all this from a girl who mislays her car keys a dozen times a week!

Good to know you suffer from the same problems as the rest of us.

I have lost count of the number of times I have put down a screwdriver and two minutes later when I go to pick it up it isnt there!

Nice solution to the problem BTW. My initial thoughts were for a choke at each end and use a carrier to modulate a tone onto the power rail. Much higher component count and cost.

Regards

[MegaADY]

Thank you for schematic Melanie ! You were right, I posted this question some months ago. You didn't understand my setup here. So, once again,
I have two devices ( not pics ). A master device, and a slave device that get power from the slave (5v, 3-400 mA ). Now, I want to put a PIC in the slave device which should send to the pic I put in the master device a few bytes ( 1200 baud ). The problem is the power consumption of the slave ( 4-500mA). And I can not use too big caps like ... 4700u@25v. I can not test the schematic right now, but I got the ideea. But do you thing it will work in the context I explained you ? The power on that slave device ( and the transmitting pic also ) should be kept as close as it can to 5V. Thank you .

[Kamikaze47]

i can draw you a circuit diagram that should work, but just to clarify that this is what you want to do:

power the slave pic via the master pic with a +ve and ground wire
transmit data from the master to the slave via those wires
no transmission from the slave to master (one way from master to slave only)

is that right?