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'PIC net (BASIC question)'
1998\07\14@214846 by Mark A Winters

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My apologies for the newbie question -- my background is more in software
than hardware.

With regards to this topic, I have a basic question: When is it necessary to
go to something like RS-485, rather than just connecting PICs directly?

I have an application in which I need to connect a master PIC to three
slaves, each about 15 feet away. The communication needs between the PICs is
very simple -- under 300 baud would be fine (I plan on bit-banging the
protocol).

Would direct pin-to-pin connection of the PICs work in this case, or do I
need to go to something more sophisticated?

Thanks in advance for any advice...

Mark

1998\07\14@220805 by Jason Tuendemann

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Mark

Depending on the application  and the environment it will be in it sounds
that direct connection would do your job but by using a UART RS232/485 the
signal over distances will have a lot better signal to noise ratio
especially RS485 because of the differential pair.

Jason
{Original Message removed}

1998\07\15@163950 by Thomas McGahee

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Mark,
As the lines get longer there are two major factors that enter in. One
of these is noise. The wires tend to act like antennae and pick up
electrical noise. When this noise level causes the PIC to 'see' the
wrong bit value, then you have problems. The other problem is related
to the capacitance and loading factors. The more things you drive, the
more current you have to source and sink. The longer the wire length,
the more capacitive loading there is. This capacitive loading sets a
limit to how fast a signal can change.

Using twisted pair wire decreases electrical noise but increases
capacitive loading. If the voltage level is increased the signal
to noise level is improved. A further enhancement is to use a
differential signal instead of a ground referenced signal.

15 feet is long enough that some form of signal to noise improvement
is warranted. Besides the "official" standards, you can also
implement your own 'private' method. The 'standard' methods are
time-tested and approved, and if you are making something that is
going to link up with other equipment, it is often best to stick
with the standards.

Fr. Tom McGahee


> {Original Message removed}

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