Thanks for the suggestions? Just one query.

Why have you mapped Off to +5v and -50v to 0v? I was thinking 5v for 
-50v and 0v for off?

Im still a little confused what is meant by positive earth and -50v?. 
The circuit board on which this is going to be mounted has a -50v 
continuous supply which Im going to use for general power.  Will this 
cause a problem being negative?

Andrew


On 31/05/10 21:21, Michael Watterson wrote:
> Andrew Wood wrote:
>    
>> 50v Positive Earth when operated, open when not operated. Needs to be
>> checked at a frequency of 13Hz (every 80millisec)
>>
>>
>>      
> Ok, so you can level translate to +5V for off and 0V for -50V in by two
> resistors and 2 x cheap 1N4148 diodes for each of 90 signals.
> One resistor is 10K pullup to +5v  with clamp diode from ground to
> pin.on Shift register parallel in, 2nd diode anode to input pin also
> and cathode via 100k resistor to the -50v or off signal.
>
> OFF will = +5V
> -50V in will = 0V
>
> use 12 x 8 bit shift registers with clock pins all parallel driven by a
> PIC pin
> parallel all "load" pins. drive by a PIC pin
> daisy chain SO and SI  (11 connections between 12 chips) and final SO to
> PIC input pin.
>
> Software:
> do forever
>   toggle load pin to read all 90 inputs to Shift registers  (<  2uS)
>    for 90 times:  (takes
>       toggle clock pin and then read last SO pin.  (takes<   2uS  per
> bit!)
>    end for
>    send data to host via serial or USB*  (even at 4,800 baud is<  50ms
> for 96 bits) (or do this every 8 inputs)
> wait till 80ms is up  (do nothing for 30ms if 4800baud, or 55ms if 9600
> baud!)
> loop
> (*Doing this on 18F2550 using JAL USB serial library is simpler than
> serial in Assembler)
>
>
>    

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