Re: opto isolator??

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Brian Fairey" <bfairey@...>
wrote:

>
> I want to connect my driver boards to my PC via the printerport. I am
> told I need an optoisolator to prevent damage to my PC. Can I buy one
> or do I have to build one? and how?
> Brian Fairey
> Waterloo
> Canada.
>

There are so many flavors of breakouts as to cause confusion.

You only need OUTPUT isolation (Step & Dir signals) in rare cases and
never with higher end drives that have opto isolation (like the
Geckos. You have to make sure the Optoisolation provided for step &
dir is fast enough. Standard Optos have a cutoff freq below 10,000HZ
so don't work for high speed signals. In addition you have to have
isolated supplies to get true optoisolation (no common ground). Most
BOB's have "buffered" outputs that give a level of protection to the
parallel port but more importantly provide the level translation from
3V logic to 5 needed for a lot of motor drives. The parallel port on
most newer PC's is pretty wimpy and is made to drive any MOS type high
impedance load (like in a printer).

On better BOB's you find the INPUTS (like for the Home and Limit
switches, etc) are opto isolated. Once again to get the advantages of
both protection and noise reduction, you need a separate supply with a
separate ground, driving the outside part of the world. Some BOB's
have the proper power supplies built on to card, others require you
buy it as a separate part or figure out how to get it there yourself.

Better BOB's will be expandable (you can add features) or come with
the features on the card. Useful features are drivers for Relays and
/or High current relay outputs, Diagnostic LED's, second port
expansion for added I/O or plugging in devices (like MPG's or Hand
Controllers). Some offer more options like pre-cut cases and direct
connection (one cable) to some popular drives (Xylotex, Gecko, etc).


Tom Caudle
ww.CandCNC.com