One way to do it is to use an output with a pulse/pattern,
connected through the sensing device (switch, photo sensor...)
connecting it back to the input. If the switch is closed the
pulse/pattern should be available on the input. If the
pulse/pattern isn't on the input you know that the switch isn't
closed, isn't working or that the circuit isn't working, which
should be a safe state (not dangerous state - motors or relays
off).

To improve safety, the sensing device should invert the signal
(photo sensor) so that a simple short circuit on the input and
output wires also can be detected.

To improve safety even more, the input should be doubled (two
sensors) and both should be working in parallell to be able to
set the device in the dangerous state.

What kind of device is it for?

Ruben


> Hi,
>
> I am writing a safety-related software and need to detect if an input
> pin gets stuck at 1 (or 0). Could any one tell me how to do it in the
> software?
>
> Thanks alot!
>
> Tony
>
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==============================
Ruben Jvnsson
AB Liros Electronic
Box 9124, 200 39 Malmv, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
==============================

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