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'[EE] RS485 traces (on board)'
2009\05\07@120232 by alan smith

picon face

Looking for advice...

PIC driving a RS485 transeiver, out a RJ45 thru CAT5.  The distance between the processor and the jack is maybe 4 or 5 inches.  Right now the driver is next to the jack, but the layout engineer is asking if he can move it closer to the processor.  So, whats the drawback (or advantage) of driving the differential current pair across the board vs just running the digital lines to the driver.  The driver creates a true differntial pair, so as long as it was routed as a matched pair, that actually might be a better solution than a simple pair route of the tx and rx lines.  thoughts?


     

2009\05\07@123540 by Wouter van Ooijen

face picon face
> so as long as it was routed as a matched pair,
> that actually might be a better solution than a simple pair
> route of the tx and rx lines.  thoughts?

Lines outside of the board == source of electrical interference
(depending on the surroundings). I'm no expert, but I would except an
RS485 driver to be somewhat good at blocking such dirt, so the lines at
the PIC side are somewhat more clean. So the driver should be near to
the connector, and the external side of the driver should be considered
'dirty'. But isn't this a very basic idea any PCB designer should have?

--

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu

2009\05\07@133704 by Pablo Ginhson

picon face

I use in a lot of my projects the RS485. What Wouter say is correct.

Put the driver, I use SN75176 normally, with the pull Up and Down of 1K and the end resistor 120E, near of the connector. The Pic side doesen´t matter, but don´t put very very long distance from one to the other; I mean with 5 or 7 cm from PIC to Driver will work good. At least in my projects still working so........

Regards



Pablo

http://www.ginhson.com.ar



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