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'[OT]:Common ground.'
2000\06\09@190259 by Andrew Seddon

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Please forgive me but I am entirely self taught and there are some big gaps
in my knowledge.

Could somebody please explain to me why a common ground is required when
communicating between say two pics via a serial line?? Is it just to make
sure the ground reference levels are the same?

2000\06\09@203115 by Dale Botkin

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On Sat, 8 Apr 2000, Andrew Seddon wrote:

> Please forgive me but I am entirely self taught and there are some big gaps
> in my knowledge.

Mine, too...  everyone's gaps are in different areas, though, which is
what makes a list like this so great!

> Could somebody please explain to me why a common ground is required when
> communicating between say two pics via a serial line?? Is it just to make
> sure the ground reference levels are the same?

You're right, of course -- The serial input signal is referenced to the
processor's ground. If, for example, you're using two battery powered PICs
with no ground connection between them, there's no common ground
reference. One PIC can drive its xmit pin high (referenced to its own
ground), the other may see some noise -- but it won't see the signal
change in relation to its ground.  That's why you always need at least two
wires to communicate, signal and ground (assuming you're not doing it via
radio... but that's another subject).

Dale
---
The most exciting phrase to hear in science, the one that heralds new
discoveries, is not "Eureka!" (I found it!) but "That's funny ..."
               -- Isaac Asimov

2000\06\10@002039 by Steve Landas

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This particularly applies when the two devices share different grounds from
the wall. Often if they share the same circuit you can get away with using
separate signal grounds for the RS232.

Steve

2000\06\11@001812 by Chris Eddy

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Andrew;

You can break communications circuits into two main groups.. Isolated and
non-isolated.  Isolated would pertain to for instance twisted pair ethernet,
where a small coupling transformer links the signals from the driver/recievers
to the lines.  Modems also isolate with a transformer.

Non-isolated connections, of which RS232 is a prime example, rely on direct
(galvanic) connection.  All integrated circuits must be presented with a signal
which falls between their power supply rails (with some exceptions).  If the
RS232 sender on one end is at a ground potential of say 30VDC above a seperate
reciever somewhere else, then that 30V added to a 10V plus signal level may be
over the power supply rails of the reciever.  So you must draw the two circuits
together to the same relative ground potential.

Any case where you cannot bring the grounds to the same potential may require
some fancy footwork, such as isolation with optocouplers and DC/DC converters.
Or transformers, if the signal can be reconstructed, IE with audio (modem)
signals.

Not only does the common ground have implications for the proper common mode
range of interfacing, but it also impacts proper surge protection schemes, and
EMI schemes which may impact approval validations.

Keep filling those gaps.  We need you out here in the front lines.
Chris Eddy

Andrew Seddon wrote:

> Please forgive me but I am entirely self taught and there are some big gaps
> in my knowledge.
>
> Could somebody please explain to me why a common ground is required when
> communicating between say two pics via a serial line?? Is it just to make
> sure the ground reference levels are the same?

2000\06\11@090206 by Andrew Seddon

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Cheers Chris, that clears a good few things up.

Andy

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