Re: [CAD_CAM_EDM_DRO] Re: Encoder index and homing

On Tuesday 01 February 2005 02:52 pm, Tom Hubin wrote:

> The second resistor is used to pull up the unused Set input. One end of
> the resistor to +5v and the other end to flip flop Set input. Some folks
> don't use a resistor and just wire +5v to the Set input. My early days
> of EE was designing to meet mil spec and we tied NOTHING directly to
> power except power pins. Unusued TTL inputs were pulled up via
> resistors.

There's a historical reason for this. The original TTL parts were spec'd for
an _absolute maximum_ voltage of 5.5V on any input pin, but would tolerate
up to 7V on the power supply pin. If a +5V regulator is used (cheap and
plentiful), then it's not ordinarily a matter for much concern.

<...>

> How it works: Travel toward the limit switch until the limit switch
> closes. The flip flop Q output will immediately go low when the limit
> switch closes. Then travel away from the limit switch. On the first
> rising edge of the encoder index after the limit switch opens, the flip
> flop Q output will go high as the encoder index rising edge clocks the
> flip flop.

Nifty!

> When you first power up you should home twice since the flip flop may
> power up either high or low. This will guarantee a valid home.

This could also be handled by a fairly simple powerup reset circuit.

> You should NOT be within one index pulse of the end of the travel range that
> is opposite the limit switch.

Why?

> Now this is not the ideal solution since you would probably prefer a
> normally closed switch. That way the default for a disconnected limit
> switch is safer. I did this for somebody several months ago. I'll have
> to dig up my notes to see how I did that with a N/C limit switch. I may
> just have used the other half of the dual flip flop as an inverter.

I'll be looking forward to it.

OTOH, if you're doing this for more than one axis, it shouldn't be any big
deal to add a gate package as well.