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From: "Jim Pennell"
Subject: Re: RS-485 termination
Date: Wed, 23 Oct 2002 06:47:13 -0700
Organization: MindSpring Enterprises
References: <3DB580A9.C3C4AC4A@scazon.com> <3DB64E6D.7E42A7A8@scazon.com>
X-Server-Date: 23 Oct 2002 13:47:16 GMT
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
> One problem I may run into is that my transmitter will
> probably be located in a central terminal room with all
> the receiver cable runs leading to the one room.....how
> do you think this will influence things? The cable length
> could be a mere 25' to one room for instance and up to 200'
> to the farthest perhaps. I would put a jumper on my
> receivers so that the farthest rcvr could be terminated.
Wait a minute... This sounds like separate lines to each reciever from
the same transmitter. That could be a problem. RS485 is meant to be ONE
long cable from the transmitter to the farthest reciever with other
recievers connected to the same cable with, at the most, short lengths from
the common cable to the individual recievers.
Long separate runs going off to different recievers, well that gets
nasty. Won't work very well. Most likely get a lot of ringing and
And, yeah, you need the two wires for the twisted pair, and another wire
as the ground. Place the 100 ohm resistor across the twisted pair wire at
the far end of the cable run.
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