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'[EE]: One wire serial between devices with no comm'
2004\01\13@064336 by Ben Jackson

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I'd like to have a serial (one way would be okay, two way better) interface
between two PICs with different power supplies.  One would probably be
on a regulator powered by a wall wart and the other would be battery
powered.  The battery powered device will be on a rotating platform,
so I'd like to connect just through the metal axle it's rotating on.

I'm a software guy, not an EE guy, so if this idea is stupid for some
obvious reason, please enlighten me.  Here's my circuit (one half, one
for each side):

                VCC
                 +
   TX    R1_     |
   o----|___|----|-----.
         100     |     |
                 |     |  C1
                  \|   |  ||  WIRE
               Q1  |---o--||---o
                  <|      ||
   RX            |       1uF
   o-------------o
                 |
                .-.
             1k | |R2
                | |
                '-'
                 |
                ===
                GND

Q1 acts as an envelope detector and provides a logic level signal to RX.
TX is high-Z when receiving.  When transmitting, RX sees what got sent
and the 'slave' device can release the line if it sees an error.

TX needs to be driven without a DC component, which can probably be done
by manchester encoding bytes before sending them via the UART (start/stop
are already balanced).  It might be necessary after a quiescent period
to drive 0xAA (or 0x55) to frame the message and 'balance' the line.

--
Ben Jackson
<spam_OUTbenTakeThisOuTspamben.com>
http://www.ben.com/

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2004\01\13@065826 by Mauricio Jancic

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My opinion is that you MUST have minimum 2 wires. If you do not, the signal
from the "source" will not be detectable by the "destination" circuit, since
it has no point of comparing that signal.
So, in this aspect, imagine you have a battery power circuit, completly
isolated from ground, lets imagine it is over a thick rubber base, ok? Well,
you can conect a wire to the AC wall outlet and connect it anywhere in your
circuit with no damage to the circuit (yes, perhaps, to yourself:) but the
circuit will not be harm, since the 120VAC are not referenced to any
ground/neutral connection....

That's my opinion, perhaps someone will produce a much more clear
explanation.

Mauricio

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2004\01\13@072733 by Wouter van Ooijen

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How about using light (preferrably IR)? There are cheap IR-receivers
available. Also google for 'propellor clock' for sites that have solved
your problem in one way or another.

Wouter van Ooijen

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2004\01\13@075924 by cisco J. A. Ares

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You have to remember that "voltage" is a short name for "potential
difference" (sorry if this is not the usual english name, but it is the
closest translation from my native language), so you must have two
different electric potentials to have a voltage.

It is analog to altitude: an altitude difference makes a potential
energy in mechanics, you always have to say that your altitude relates
to a known level (sea level, for example or ground potential in
electricity).

Hope this helps.
Francisco

Ben Jackson wrote:

{Quote hidden}

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2004\01\13@080544 by Jonathan Johnson

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You could RF couple it....?

Gnite All,


JJ


-----Original Message-----
From: pic microcontroller discussion list
[PICLISTspamKILLspamMITVMA.MIT.EDU]On Behalf Of Ben Jackson
Sent: Tuesday, 13 January 2004 10:43 PM
To: .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
Subject: [EE]: One wire serial between devices with no common ground


I'd like to have a serial (one way would be okay, two way better) interface
between two PICs with different power supplies.  One would probably be
on a regulator powered by a wall wart and the other would be battery
powered.  The battery powered device will be on a rotating platform,
so I'd like to connect just through the metal axle it's rotating on.

I'm a software guy, not an EE guy, so if this idea is stupid for some
obvious reason, please enlighten me.  Here's my circuit (one half, one
for each side):

                VCC
                 +
   TX    R1_     |
   o----|___|----|-----.
         100     |     |
                 |     |  C1
                  \|   |  ||  WIRE
               Q1  |---o--||---o
                  <|      ||
   RX            |       1uF
   o-------------o
                 |
                .-.
             1k | |R2
                | |
                '-'
                 |
                ===
                GND

Q1 acts as an envelope detector and provides a logic level signal to RX.
TX is high-Z when receiving.  When transmitting, RX sees what got sent
and the 'slave' device can release the line if it sees an error.

TX needs to be driven without a DC component, which can probably be done
by manchester encoding bytes before sending them via the UART (start/stop
are already balanced).  It might be necessary after a quiescent period
to drive 0xAA (or 0x55) to frame the message and 'balance' the line.

--
Ben Jackson
<EraseMEbenspam_OUTspamTakeThisOuTben.com>
http://www.ben.com/

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