Re: [CAD_CAM_EDM_DRO] limit switches

On Thu, 23 Oct 2003 10:52:16 -0000, you wrote:

>Hi from Australia,
>
> I am building a milling machine with gecko drives
> and CNC PRO. So far I have everything up and running but I am having
>lots of trouble finding a difinitive limit switch wiring diagram.
>Being a poor dumb hammer user i wouldn't know what a pull up resistor
>was if you stapled it to a trout and beat me about the head and upper
>body with it. I would be really really really thankful to any one
>who could help me solve this problem. I love this discussion group
>and have learnt a lot from following the posts
>
> THANX>>>

OK, Let's see if I can help a bit on the theory.

The logic family (how it is internally put together in the chip
itself) generally used is called TTL (Transistor-Transistor Logic).
It responds to logic levels, which are labeled one and zero. For this
family, a zero is any voltage at the input of the chip less than
roughly 0.6 volts. A logic one is any voltage greater than about 2.6
volts.

Bottom line so far... Whatever is sensing the limit has to produce a
voltage either less than 0.6 or greater than 2.6 volts to work
reliably. Since this kind of chip is probably in your computer's
parallel port, or the breakout box that goes to the parallel port...
these numbers are important.

The easiest way to connect a limit switch is with the switch connected
to the circuit ground and the other end connected to the sense input.
With the switch closed, the input will be ground, which is zero volts,
which is less than the 0.6, so that part is fine.

However, with the switch open, the line to the input will float.
It's not connected to any particular voltage at all. Now while the
open input will be treated as a logic one (which is good)... It's
about the worst logic one you can possibly get (right at the lower
limit). So that long lead starts to act like an antenna and picks up
anything that it can... Motor noise, radio station, last week's
pizza... you name it.

However, if you take a resistor, and run it to 5 volts on one side,
and that switch line to the other, the voltage will 5 volts with the
switch open (and zero volts with it closed). This gives you a very
high class (i.e. good) logic one at the input with the switch open,
and reduces the effect of any noise you might pick up.

That resistor is called a pullup resistor. A typical value will be
1000 ohms, 1/4 or 1/2 watt. Even Tandy has them.....

Hope that this makes sense.

Harvey

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