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« on: October 19, 2017, 02:39:39 AM »
Hi John,
I think there is some confusion about limits and homes. Limit switches are at the end of each axis and the principle reason for them is to stop the machine
from banging into the endstop and wrecking itself. Home switches are to reference your machine at the start of your session.
It is quite common to use the limit switches as home switches temporarily at the beginning of the session while you reference your machine. During that time
the limit switches have to be ignored and treated differently. Once homing (referencing) is complete then they go back to being limit switches.
To be perfectly honest I think this idea of 'double duty switches' just to save a few bucks and/or a few Mach inputs is a disaster waiting to happen.
The common way to wire your limit switches is to have all six switches in series with normally closed contacts. If any one of the switches opens the machine
Estops. The machine can't tell which switch opened just that one has. You can then hit <limit override> and back off the offending axis , remembering
that while you do so Mach can't stop you from making a mistake, if you go the wrong way you may make the situation worse. So while this method has its
disadvantages it does only require one Mach input pin.
If you wish to do the 'double duty' home switch thing....hit <ref axis> and that axis will drive in the nominated direction at the nominated speed until the limit
switch string goes open circuit, then it will back off until the switch closes and reset the machine co-ords commonly to zero but you can nominate any number,
the 'home offset', that you like. It does not mean that it drives to that location. So while homing the one input pin is treated differently and you have no limit
protection at all. This has caused innumerable crashes over the years.
I prefer to have three homes switches in addition to the six limit switches. I also prefer that each home switch have a dedicated input pin. Given that
a parallel port only has 5 input pins you are going to run out. You need a second port, now you can have not only another 5 inputs but you can assign
pins 2-9 as inputs also rather than their more normal use as outputs so the second port gives you an extra 13 inputs. Highly recommended. With all these
inputs you don't have to compromise, the limit switches ALWAYS are limits and HOMES are only ever home switches. Further than that it means you home
switches can be really nice microswitches which give repeatable and accurate homing which is a real boon to your machining operations.
If you are prepared to stump up the bucks for a PCI to parallel card and another breakout board you will make a big advance it what you can achieve and reduce
the chances of confusion which result in crashes.
Craig
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