I have a requirement for monitoring multiple RS232 lines (19600bps)
connected between some instruments and a PLC in a factory environment.
I have to ensure full isolation between all the channels, I must not add
any significant load to the RS232 line and I must stick as closely as
possible to the standard RS232 logic level thresholds

 

My first though is to power an RS232 line driver (MAX232 etc.) from an
isolated dc-dc converter and then isolate the TTL logic from the micro
using opto-couplers.  Obviously I would need a separate supply (or at
least separate isolated outputs) and separate RS232 line drivers for
each channel.  I only need to receive data, no data will be transmitted
over the isolated interface.  so I don't think I can get away with
kludges like driving an opto-coupler directly.

 

Firstly, is this is a reasonable solution?  Are there alternatives?
Assuming it is, I have looked at using the potted PCB mounted DC-DC
converters that are available from several manufacturers, but they are
rather expensive (>5UKP each).  Since the current requirements are so
low (driver and opto-coupler will be under 10mA), efficiency is not an
issue and I've been looking for a cheaper solution.  I found the
following application note:

 

http://www.exar.com/Files/Documents/sipex/ApplicationNotes/ANI-11_Isolat
edRS232_120406_B.pdf

 

This shows an ingenious method of using the switched capacitor supply
within a line driver to drive a small transformer (which are $1.60 each
in the low quantities I will be needing).  Now since I don't need  to
regenerate the RS232 line voltages this extra driver would be redundant
in my design, but I was considering driving one of these these small
transformers from a 50% PWM output from the micro, with a suitable
transistor driver.  Would there be any potential issues driving multiple
transformers from a single driver, or should I have a separate driver
for each transformer?  My main concern is if any high voltage transients
appear on one line, they could be more easily coupled into the others if
the primaries of all the transformers were coupled together.  A low
driver output impedance would help to suppress this I guess.  Thoughts?


 

Cheers

 

Mike


=======================================================================
This e-mail is intended for the person it is addressed to only. The
information contained in it may be confidential and/or protected by
law. If you are not the intended recipient of this message, you must
not make any use of this information, or copy or show it to any
person. Please contact us immediately to tell us that you have
received this e-mail, and return the original to us. Any use,
forwarding, printing or copying of this message is strictly prohibited.
No part of this message can be considered a request for goods or
services.
=======================================================================
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist