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List:       linux-arm
Subject:    Re: RS485 arm-linux driver
From:       "Jeff Sutherland" <jeffs () accelent ! com>
Date:       2002-09-18 13:09:26
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On Wednesday 18 September 2002 09:38 am, Gad Hayisraeli wrote:
> ----- Original Message -----
> From: "Russell King - ARM Linux" <linux@arm.linux.org.uk>
> To: "Gad Hayisraeli" <gad@syete.co.il>
> Cc: "Linux-Arm List" <linux-arm@lists.arm.linux.org.uk>
> Sent: Wednesday, September 18, 2002 12:50 PM
> Subject: Re: RS485 arm-linux driver
>
> > On Wed, Sep 18, 2002 at 01:40:23PM +0200, Gad Hayisraeli wrote:
> > > i want to use my mediaEngine (SA1110 based) RS232 port as RS485,
> > > connect to many RS232 ports and control them thru this single port.
> > > is this possible:
> > > 1. converting the RS232 to RS485 (in the mediaengine manual they said
> > >that the same port can be used for both)

Check the wiring diagram for the port in your manual.  Electrically RS232 and 
RS485 are completely different, but the serial data format is the same.

> > > 2. can i control many RS232 from 1 RS485 ? or i have to convert each of
> > > them to RS485 ?

RS232 is strictly one on one.  RS485 is a half-duplex 'party line' 
interconnect that can support up to 32 nodes.  With proper cabling and 
terminations it's possible to get over 1km distance between two nodes, but 
not with a 'star' wiring topology.  You need to use a 'ring' wiring method 
kind of like 10-base2 (coax) Ethernet.  Correct cable termination is critical 
to getting good distance performance as well.  As far as software goes, all 
the IOCTLs needed are already supported.  Works fine for me on Linux.  Your 
app, however, has to implement a protocol for managing all the devices, as 
only one transmitter at a time can gain access to the line.
-- 
  Jeff Sutherland, Accelent Systems, Inc.   <http://www.accelent.com>
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Kodachrome: After nearly 70 years there's still no better way to
preserve an image.

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