On Mon, Jan 30, 2012 at 04:56:15PM -0200, Isaac Marino Bavaresco wrote:
> Em 30/1/2012 16:12, Mark Hanchey escreveu:
> > On 1/29/2012 10:28 PM, Steve Willoughby wrote:
> >> Rather than the expense of running RS-485 transceivers on each node,
> >> would it be practical to run a plain TTL level line from one to anothe=
r,
> >> and do some kind of synchronous serial protocol over that?
> > It should be fine, especially considering the short distance, assemble=
=20
> > the length of cable you intend to use and try it out of the ground, if=
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> > it works that way it will work fine in the ground. .  Make sure to use=
=20
> > conduit for the wire so that in the future if a break happens you can=20
> > pull a new wire easily.  The main concerns are the usual buried cable=20
> > problems,  moisture, lightning, other people digging and cutting the=20
> > line. The wire is so low voltage that if it did short out the only harm=
=20
> > would be to the pic chips.
> >
> > Mark
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> Why not use a system similar to what model railroad control boards employ=
?
> Just two wires, the data is superimposed on the power wires by reversing
> polarity of both wires.
> Use a full bridge rectifier in each receiver plus an electrolytic
> capacitor to extract power. The data discrimination may use a simple
> resistor-diode-transistor circuit. Simplex communication though.
>=20
> If two-way communication is necessary, the slave boards could transmit a
> high-frequency (100s of KHz) signal superimposed on the DC. It is simple
> to generate such signal in software, so no expensive hardware is needed
> and the more complicated demodulation circuit is only needed on the
> master board.


An example of this, along with an actual line protocol, is Roman Black's
BlackNet Protocol:

http://www.romanblack.com/blacknet/blacknet.htm

Seems to have all the wanted advantages:

2 wire, with data/power on a single line
simple hardware

Take a look...

BAJ

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>=20
> Isaac
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> --=20
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Byron A. Jeff
Department Chair: IT/CS/CNET
College of Information and Mathematical Sciences
Clayton State University
http://cims.clayton.edu/bjeff
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