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'RS-485 networking question (NOT PIC SPECIFIC)n'
1997\01\28@042412 by Kalle Pihlajasaari

Hi Kevin,

> Is it typical to leave the receiver of an RS-485 transceiver enabled all
> the time or only when the transmitter is disabled.  In once case the
> connected micro would hear everything it sent as well as everything
> everyone else sent, in the other it would only hear others.  Downside to
> hearing yourself would be unecessary interrupts, upside might be the
> ability to detect certain collisions and electrical problems.  Anyone else
> have pros/cons?

You will not hear collisions that you make on the local driver
as it will always be 'louder' than remote devices and you will
hear your own version of the data.  Electrical faults you will
be able to detect much better if you put like 4.7 Ohm resistors
in series with the driver to the bus but connect the receiver on the
bus side of the resistors.  This will allow for detecting a lot more
fault and some multiple collision faults but cannot be done with the
8 pin trancievers as they bus the TX and RX together in the IC.

So  unless you have something to do with the data or expect to detect a
failed IC (which may work sometimes) you might as well ignore the
received data (or have the activity LED still indicate traffic for
testing and installation purposes)

Kalle Pihlajasaari
Interface Products   P O Box 15775, DOORNFONTEIN, 2028, South Africa
+ 27 (11) 402-7750   Fax: 402-7751

DonTronics, Silicon Studio and Wirz Electronics uP Product Dealer

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