Re: Agilent Encoder Pull-up resistor

It does make sense to me to have differential
encoder feedback on a drive with only single
ended step and direction inputs.

My reasoning is that many applications will have
the driver inside a control box where the step and
direction wiring is kept as far away as possible
from the noisy motor outputs and power wiring
and this wiring can be relatively short

On the other hand, the encoder inputs are typically
much longer cables and are often run parallel to
the high current motor drive signals. Shielding just
isn't sufficient in some cases to prevent the 20KHz
PWM noise from coupling into the encoder inputs, and
20KHz is well within the pass band of any low pass
filter that will still allow the needed signals to
be received.

Regards,
Steve Stallings
www.PMDX.com


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
<mariss92705@...> wrote:

>
> It makes a lot of sense to use differential drivers/recievers if

MHz

> or more signals must be sent.
>
> These signals cannot be effectively low-pass filtered to remove

noise

> because often the signal period is less than the noise pulse.
> Filtering would then remove the signal as well as the noise.
>
> Low and moderate cost optical encoders use photo-transistors. Photo-
> transistor frequency respose begins to roll-off above 50kHz at

best.

> This makes the minimum usable encoder logic 0/1 time 10uS.
>
> Noise is generally pulses narrower than 1uS, so low-pass filtering

is

> very effective in removing the noise while preserving the signal.
>
> To it makes absolutely no sense to use differential encoders for
> reasons of noise reduction. Answer for yourself this question: Why

do

> drives that use differential encoders also not use differential

step

> and direction inputs?
>
> Step pulses can have pulse widths at or under 1uS. These pulses
> cannot be low-pass filtered for the reasons mentioned. If

differental

> inputs are used anywhere, they should be used first on the
> step/direction interface.
>
> I'd love to hear the reasoning for that.
>
> Mariss
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Peter Homann" <groups@>
> wrote:
> >
> > Jon,
> >
> > That's just plain rude and offensive!
> >
> > The advantages of using differential signals are very clear. It's
> the
> > reason why it's standard practice in a lot of the industrial

control

> > industry.
> >
> >
> >
> > Note to moderators: I didn't think this short of "Bashing" was
> allowed on
> > this group.
> >
> >
> >
> > Jon Elson wrote:
> > > afogassa wrote:
> > >
> > >>
> > >> Rutex new drives needs differential encoders if the distance

from

> > >>pcb to the motor is over 1 foot or so, rutex has a pcb that
> > >>converters the signal from incremental encoders to differential
> it's
> > >>about 20Us$ or so.
> > >> After installing the converter don't use pullup resistors.
> > >>I'm using rutex drives and it's very good, little hard to tune

it

> > >>with software, I like the old ones with pots to set it up.
> > >>
> > >>
> > >>
> > > Oh, wonderful! They must be doing something really "SPECIAL"

in

> their
> > > drives
> > > to require this. I have several installations with 30 FEET of
> encoder
> > > cable that
> > > work perfectly with single-ended encoders. So, I'm glad I

don't

> use
> > > Rutex drives.
> > >
> > > Jon
> > >
> > >
> > > Addresses:
> > > FAQ: http://www.ktmarketing.com/faq.html
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