Michael Rigby-Jones wrote:
> 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

How much load is "not significant"?  Is there any power at all available on
the driving side?  If these are truly RS-232 lines, then they should be
capable of a few mA at least.

You can make opto couplers work at a few mA.  One trick is to use two of
them, each with a floating phototransistor output.  A high input level
lights one LED and a low the other.  Then arrange the two phototransistors
in a totem pole on your side.  That will be good enough to drive a CMOS
logic input, and you can go from there.


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