RS 485 interface chip needed

[Dan Hampleman]

Can someone recommend a chip I can use to network together micros using an
RS-485 type signal where (correct me if I'm wrong) the same signal is sent
down 2 different wires only the voltages are opposed to cancel out noise. I
want to network together several PIC16F84's in a car using my own protocol.
My main concern is noise. I've heard someone makes a chip for RS-485 like
the Maxim232A is to RS-232. I'm also wondering what's the best kind of wire
to use. Thanks in advance.

Dan Hampleman
dham...@pdq.net

[Jean-Michel Rubillon]

I've been using the ADM485 from Analog Devices and/or Maxim.
The "best" cable depends on your transmission speed requirements.
I'm using Belden cables: twin twisted pairs with oveall shielding, a bit pricy
but very effective. Basically I'd recommend twisted pair cable as a minimum,
shielded if subject
to "loud" noise.
For your protocol I'd advise you use some sort of error checking code to add
some robustness to the network. Also, don't send long streams of data, use
packets of a few bytes. If an error occurs you will have smaller amounts of
data to transmit....

If you need more info or some help drop me an e-mail....

Jean-Michel Rubillon
Design Engineer - Control Techniques Worldwide R&D
I speak for myself not for my employer!

[Paul Grohe]

On Tue, 3 Nov 1998 00:07:39 -0600,
in the newsgroup sci.electronics.components,
"Dan Hampleman" <dham...@pdq.net>
from ISPNews http://ispnews.com thoughtfully posted:

> Can someone recommend a chip I can use to network together micros using an
> RS-485 type signal where (correct me if I'm wrong) the same signal is sent
> down 2 different wires only the voltages are opposed to cancel out noise. I
> want to network together several PIC16F84's in a car using my own protocol.
> My main concern is noise. I've heard someone makes a chip for RS-485 like
> the Maxim232A is to RS-232. I'm also wondering what's the best kind of wire
> to use. Thanks in advance.

Have a look here:

http://www.national.com/catalog/AnalogInterface_DataTransmissionCircuits_RS485.html

Look at the "multipoint" transceivers.

RS-485 does not use negative voltages like RS-232 - so a
"Maxim-like" device with a negative voltage charge pump is not
required - although the receiver does have to be able to
withstand common mode voltages extending below ground.

>I'm also wondering what's the best kind of wire
> to use. Thanks in advance.

See AN-916 below. Shielded twisted pair is best.

For more information, see these Application Notes:

http://www.national.com/an/AN/AN-216.pdf
http://www.national.com/an/AN/AN-409.pdf
http://www.national.com/an/AN/AN-454.pdf
http://www.national.com/an/AN/AN-759.pdf
http://www.national.com/an/AN/AN-847.pdf
http://www.national.com/an/AN/AN-903.pdf
http://www.national.com/an/AN/AN-916.pdf
http://www.national.com/an/AN/AN-917.pdf
http://www.national.com/an/AN/AN-972.pdf
http://www.national.com/an/AN/AN-979.pdf

There are quite a few - but they should give you a good basic
understanding of RS-485 and address your cabling concerns.

Cheers,
Paul Grohe

--
---------------------------------------------------------------
Paul Grohe National Semiconductor Corp.
Assoc. Analog Apps Engineer 2900 Semiconductor Drive
Standard Analog Apps Group Mail Stop C2693
mailto:gr...@galaxy.nsc.com Santa Clara, CA. 95052-8090 USA
Usenet Disclaimer: Any opinions expressed are mine, not NSC's
----------------------------------------------------------------

[FC]

I use the 75176 it is made by several manufacturer although they are not
always exactly the same. The spec calls for 32 node on the pair but I
think that Linear Tech (i think) claim up to 100 on a pair.

FC

In article <364254a6...@139.187.81.1>, gr...@galaxy.nsc.com says...