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