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From: "Roger Lascelles"
Subject: Re: RS485 help #2
Date: Fri, 25 Oct 2002 10:58:16 +1000
X-Newsreader: Microsoft Outlook Express 5.50.4133.2400
The RS485 balanced pair and transceivers are designed for fast baud rates -
10 to 100 times faster than you require. It all gets less critical if you
slow everything down.
At slow baud rates you can slow down the rising & falling edges of your
waveform. This will have two effects : 1) EMI radiation is reduced 2)
Termination becomes less critical or can be omitted, depending on line
At the slow baud rates, you can also limit your receiver high freq response
and get noise reduction.
At low baud rates you can even omit the balance if you force a current into
the line and simply turn on the LED in an opto isolator at the receiving
On a practical note : if you have a CRO and some time, the best approach
might be to experiment and come up with a simple system. I do not believe
you will get all the answers on this newsgroup, because by its nature, a
newsgroup can only offer quick advice. Many of the engineers on this group
could do the maths with line length, baud rate, bandwidth etc and give you
answers - but they cannot be expected to put in the time for no charge. In
addition, if you do not have a grasp of transmission line basics, even
detailed advice will not have much meaning to you. If you don't want to
experiment, I recommend you read up on data transmission basics, or else pay
an engineer to to get you started.
"EBG" wrote in message
> I designated this #2 as I wanted to start fresh with a new post.....thanks
> for all the response on my previous "RS485 termination" post a few days
> Jim, especially.
> Let me ask this.....
> The fact is, I don't care whether my interface really conforms to a
> "standard"...RS232 422 or 485.
> My goal is to have a "star network" (central PIC transmitter with
> I neeed to transfer data at a mere 1200 baud (fact is, it can even go
> to up to 10 PIC receivers, which simply do some switching tasks.
> I need however, to eliminate the EMI emmisions coming from the cable by
> using a balanced connection.
> How about this.....
> Has anyone built their own balanced network using a dual optoisolator (say
> PS2501-2)at the xmtr and each of the receivers? Couldn't this be done?
> split the RS232 coming out of the pic, invert one side and drive the
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