In article ,
Tim Wescott wrote:

On Wed, 09 Dec 2009 09:06:23 -0500, Joseph Gwinn wrote:

In article ,
cavelamb wrote:

[snip]

I would like to see limit switches at each end just for safety sake.
That would take up two bits of input per vent. But if the device has
comfortable "over-run" areas, it might not be needed.

I agree that limit switches are needed, but they should physically
interrupt power to the motor, and in no way depend on software.

The problem is that once these limits are hit, the computer can no
longer recover control, and a manual reset is needed.

The usual solution is to have two sets of limit switches, inner and
outer.

The inner limit switches are sensed by the computer, telling it to stop
moving in that direction.

The outer set physically interrupts power to the motor, to prevent
damage should the computer fail to do the right thing.

Joe Gwinn

With a DC system it's easy to set up limit switches that'll prevent a
motor from driving any further into a stop, but will let it drive out.

And even though I write embedded code, too, I still prefer to have
mechanical safety devices -- for when someone else starts maintaining my
bug-free code, of course.

Yep. I would insist, not just prefer, unless it could be shown that
fetching up against a stop was harmless.

I wasn't trying to design it, but wanted to make sure people didn't
trust software too much.

Joe Gwinn