Re: solution for optoisolator 3.3 or 5 volt

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "dcassyc1" <denisc1@...> wrote:

>
> Hello,
> I'm building a bi directional optical isolator for a parallel port
> and want it to work on 3.3 volt and 5 volt computers. If I run the
> computer parallel port side leds and photodetectors off a 3.3 volt
> isolated supply will it work OK on computer which have 5 volt
> parallel ports?
> cheers,
> Denis
>

Depending on the opto you choose and parallel port drive capability it
may or may not work.

First problem is that some ports can't turn on an opto that takes 15
ma. You will need to buffer the signals. It a problem with current
not voltage.

Second will be speed. If your intent is to pass isolated step & dir,
conventional Opto's have extremely poor bandwidth. Pulses from PP can
be very narrow. You have to drive the opto emitter hard to get
anything past 5 or 6 KHZ. You can forget anything over 10KHZ with
normal opto. You will need to look at high speed optos (6N136) for
any step and Dir.

If your motor drives have opto isolation already there is no need for
optos on the outputs of the PP. Driving an opto with an opto is
silly and causes additional bandwidth reductions

You need separate power sources (different grounds) on each side of
the optical boundary.

While voltage translation is easy with opto's and the protection and
noise barrier is important you have to work within the constraints of
frequency and drive currents.

TOM CAUDLE
www.CandCNC.com