RussellMc wrote:
>
> A special case may exist if input s are say [+50 V | float]  rather
> than +50/ground. In such a case if inputs are taken to emitters of NPN
> optos and collectors all joined, then a sense resistor from the
> combined collectors to a voltage > 50V (say 60V) will cause all
> combined collectors to be at 60 V (transistor off OR inout float) and
> 50V (transistor on and input = +50V.
>
> Whether it is acceptable for a floating input to have 60V applied in
> this was is moot in the absence of a proper problem definition.
>
>
>   
I'm confused..
there is 100V to 1kV isolation on typical optocouplers.

The LED bit is driven via resistor from 50V line. It can be balanced 
pairs, isolated pairs or common ground.

Transistor outputs: I've only ever seen NPN commonly. Some vague memory 
of triac/thyristor?  Though I suppose PNP  or FET is possible.
 
The base can be "floating".

At more than 5V reverse  you could have zener action on  BE  junction 
and  current  via  BC. But at PIC input and drive levels I think an 
output transistor matrix will work. Examination of a datasheet or device 
would tell.  many NPN have zener volts around 6V to 7V, which is why a 
1N4148 diode across base from ground on RS232 line driving a NPN via 1k 
to 10K is a good idea, in the event you actually meet a serial port with 
ANY -ve swing or indeed -8.6 (typical pumped interface converter from 
+5V)  to -12V (original  drivers with a real -12V supply).
 

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