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Subject: Re: RS485 help #2
X-Newsreader: Forte Free Agent 1.21/32.243
X-Original-Trace: 5 Nov 2002 20:18:38 +1000, 22.214.171.124
Date: Tue, 05 Nov 2002 09:18:42 GMT
NNTP-Posting-Date: Tue, 05 Nov 2002 20:18:42 EST
Organization: Comindico Australia - reports relating to abuse should be sent to firstname.lastname@example.org
On Thu, 24 Oct 2002 09:09:25 -0500, "EBG" wrote:
>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 ago.
>Let me ask this.....
>The fact is, I don't care whether my interface really conforms to a specific
>"standard"...RS232 422 or 485.
>My goal is to have a "star network" (central PIC transmitter with branches).
>I neeed to transfer data at a mere 1200 baud (fact is, it can even go lower)
>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 a
>PS2501-2)at the xmtr and each of the receivers? Couldn't this be done? Just
>split the RS232 coming out of the pic, invert one side and drive the
As Frank Bremelman said in his post - the PIC doesn't do RS232 - but if
it has an on-chip UART, then the i/p & o/p pin signals are the 0-5V levels.
Generally a separate RS232 chip is used to connect the UART pins to the
RS232 comms lines to provide the +12V / -12V RS232 signal levels.
As for using opto's - as another poster has pointed out, you will need
to have a separate floating power supply to drive all the opto LEDs -
I'm presently designing a system that uses an optocoupler on the transmit
side to drive an output transistor (rated to 2 amps) which pulls the Comms
signal line (which can be connected to up to 32 opto-isolated receivers - mine
is in a single loop configuration - not a STAR set-up) low - which activates
all the receiver LEDs (the anodes are connected to the separate isolated
+12V Comms supply) at the same time (thus the 2 amp rating on the send
transistor). I'm using digital on/off signalling as I don't need to worry
about EMI emissions as there is only the one two wire signal cable used - so
turning the Tx & Rx LEDS on/off at 150Kbit speeds is very useful.
However for your needs, I suggest you consider the possibility of using FSK
signalling to do your lower speed 1200 baud comms - say 38.4Khz for a high /
19.2Khz for a low - and transmit & receive the FSK signal 'linearly' across
the Tx / Rx LEDs (more as a linear sine wave than as a digital signal) - this
will dynamically reduce any EMI that may be induced. Use a NE567 to
recover the FSK signal back to digital at each receiver point.
In my system, my comms signal is half-duplex, so I have both transmit &
receive circuits on the same 2 wire comms line - for your set-up, one Tx
opto / output transistor combination should be able to drive up to 32 (or
even more) receive opto LEDs - either in a single net config (serial) or in
a STAR config (parallel). An output drive transistor rated at around 2 amps
or more is recommended to be able to drive ALL of the receive opto LEDs
similtaneously without degrading the signal.
Your suggestion of converting to 'balanced' RS232 (or RS422 / 485) lines
may not be necessary if you use FSK signalling at your 1200 baud speed.
Anyway, I hope my comments are useful.
(Retired - sometimes electronics tinkerer from OZ).
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