Re: [CAD_CAM_EDM_DRO] Re: Parallel Interface-Breakout Selection

You need the relays to control the spindle, coolant, drives etc.
For instance on my Bridgeport I have 1 relay built into the emergency
stop circuit so you can't turn on the drives until the computer turns
this relay on. I have another relay controlling the coolant and yet
another controlling the spindle contactor.

One problem with using the parallel port for controlling the emergency
stop is that you cannot guarantee what state the pins will be in when
the computer starts up. If you are unlucky you can end up with the
computer turning the drives and maybe the spindle on before you start
your control software. Obviously this can be dangerous. Mach2 has the
ability to output a 12kHz signal from one pin when everything is OK. A
simple charge pump circuit can then be used to recognise this signal and
turn on the machine. This feature is not essential but it does add extra
safety.

One of the big problems with CNC machines is electrical noise. It is
very common for switch inputs to pick up interference from adjacent
power cabling, motors etc. This interference can cause all sorts of
problems. These problems tend to be intermittent and can be really
difficult to trace. Opto isolators combat electrical noise in two ways.
First they provide electrical isolation from the machine and second they
are insensitive to short noise spikes. Optos also provide a safety
feature. If something goes very wrong with your wiring you will only fry
the opto osolators rather than your expensive computer.

Buffered outputs as provided by both the PMDX and Campbell designs
boards can also save the computer in the case of an electrical fault.

For ease of use I would say the PMDX board has the edge with it's built
in relays, closely followed by the Campbell Designs board.


Les

efoda5446 wrote:

> i'm not exactly sure i understand the whole relay bit when it comes to
> auxilary switching outside of just wiring up your drives. why would
> you need that? the problem i have looking at the features and
> comparing is none of them say,"this feature does this so you can do
> this, etc." like, what the heck is a charge pump (i guess, if i don't
> understand it i probably don't need it, though ;) and what's the
> benefit of opto isolated outputs?
>
> i think choosing one of these will probably work just fine but are
> there any other boards to look at, that anyone could suggest, or are
> these pretty much the options.
>
> thanks your guys' input has been great!
>