home and limit switch in series

[Doug Fortune <pentam-at-home.com>]

Darrell Gehlsen wrote:

> If the system would hit the limit switch, then back out slow until the
> switch tripped the other way with the option of looking for an index, that
> would allow you to use the same switch for homing and limit.
> Darrell

As the parallel port only has 5 incoming signals, I would think
a decent arrangement would be:

 - wire the home switch and limit (ie the other end of that axis' travel)
     switch "in series normally closed" for each axis.    That gives 5 axes
     limits, or 4 axes and a special function (described below).

- When a HL (Home/Limit)  line breaks open, EMC will know if it was currently
     travelling "towards Home"  or "towards Limit" and act accordingly.

- The problem is when the machine is shut off while in contact with
     (at least) one of the  HL  switches.  When it starts up, it won't know
     which direction to go to get "out of limit".

Special Function (SF):

To handle the above case, beyond the Home (only) switch is a normally closed
switch wired in series to similar switches on all axes (ie for 3 axes, you'd
have 3 switches, one behind each Home switch on every axis, all in series).

Upon startup, if any of the HL switches are open, EMC would try to
figure out how to get home, and so, moving only a single axis at a time
slowly in the direction towards Home, if it:
  - breaks the SF line, then reverse until the HL switch for that axis
      closes (now you are Home)
  - closes the HL line, (it must be at limit), and so rapids back to Home.

and a similar sequence on each other axis to Home them too.   It would also
be convenient for EMC to calibrate  "the distance"  from Limit to Home on
each axis upon initial configuration, so it can rapid Home the most rapidly!

Additionally, 1 or more  emergency stop (normally closed) switches can
be inserted into the SF loop, to provide an emergency stop functionality,
thus maximizing the usage of minimal number of parallel port input lines.

Hope this makes sence.

Doug Fortune