RussellMc wrote:
>
>
> I drive the LED from the PIC with XY drive and tak the output from ALL
> transistors arranged with either C or E commoned (see text ) and E or
> C connected one per opto to an input.
>
>   
Yes.. an X-Y addressed switch with a single common
That won't work with phone lines though as they are "balanced". If you 
have a single common, your shift register idea is better.
On "line" cards for Central Office or small office Voice mail we used 
total isolation, no assumptions. No interconnect between pairs except on 
C.O. end only on transformer centre tap. We also did some Active Hybrid 
(ICs and -48V line drive, no transformer!). Typically the hybrid 
transformer(s) at exchange end had AC centre point fed by -48 & 0V.

On subscriber end cards (PABX or voicemail) no earth at all.

> My meythod has the following "advantages"
>
> - Load resistance on input can be high - does not need to assume
> ability to draw LED driving current.
>
> - Does not need a resistor per input or pair of resistors etc - uses
> an OPTO per input but R's etc are common to all circuitry.
>
> - Sample input voltages relative to reference of choice - eg voltage
> may be +50/0 or +50 float etc.
>
>   
But we don't know what sort of 50V lines it is. If it's common "ground" 
then the shift register or CD4051 approach is better than 90 opto-isolators.
If it's phone lines then the reverese opto isn't suitable at 
subscriber/PABX, but could be used at C.O. end.

you can use optoisolator to detect subscriber line powered without 
exceeding current draw to cause off hook sense at exchange.
>> The LED bit is driven via resistor from 50V line. It can be balanced
>> pairs, isolated pairs or common ground.
>>     
>
> Yes. Not in my scenario. Good solution if tolerable. Note that at 50V,
> dissipation is 50 mW per mA of opto current. And, if say this was a
> phone line that current would be far too high to be acceptable. OP
> needs to specify.
>
>
>   
Well, at low currents the transfer gain falls. So maybe 20uA on 
transistors needs 100uA of LED.

Normally LEDs are driven in mA range, but 100uA or maybe even 50uA 
depending on isolator is possible.
lower drive might be problematic with leakage currents on X-Y matrix 
even if workable for a single sensor.
>
>   

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