The problem with anti collision detection is that it=20
is the node that is talking that also checks for=20
collision (by listening and comparing the transmitted=20
data with the received data) and that a '1' is equally=20
strong as a '0' on the bus. This could mean that two=20
nodes far enough apart from each other could both be=20
talking and still not see the collision with the other=20
since they are so far apart that the other transmitter=20
isn't strong enough to affect the looped back data to=20
the receiver in the same chip, especially if there=20
exists some extra protection components (resistors and=20
or inductors) in series with the driver input/output.

This is why it is hard to make a network with nodes=20
that is spontaneously transmitting as opposed to=20
polled by a master with RS485. CAN on the other hand,=20
is designed just for this.

> Because RS-485 is an electrical specification and not a hardware or
> protocol specification, as such, it does not describe any method for
> collision detection. However, for this circuit I have added it.
>=20
> Also, I've included an "anti-chatter" circuit (basically a charge
> pump) to the "talk enable"; in the event that the PIC goes south while
> talking to the bus, the device can not tie up the bus (note: the
> interface circuit does not require the use of the anti-chatter
> circuit).
>=20
Clever, I will have that in mind for my next RS485=20
network.

Ruben

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Elektronik
Box 9124, 200 39 Malm=F6, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body