Re: [CAD_CAM_EDM_DRO] Re: Caliper to PC interface, Part II: Data Format ?????? Help ?

----- Original Message -----
From: <ccs@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Saturday, October 20, 2001 7:09 AM
Subject: Re: [CAD_CAM_EDM_DRO] Re: Caliper to PC interface, Part II: Data
Format ?????? Help ?


>
> > Yes, but... on the old PS2 parallel ports, the data port could be
> > configured for EITHER input or output. On the new PCs, the data port is
> > output only. One has to use the status port pins for inputs. The Bi
> > directional parallel port was an oddity that is no longer produced, as
far
> > as I can tell. Maybe
> > certain parallel port cards have bi directional ports on them.
>
> Acutally, most modern parallel ports do have bidirectional capability
> through one or both of two special modes (ECP and EPP).

I've tested bidirectionality in ECP and EPP modes and have not got it to
work. Do you have firsthand knowledge that they are bidirectional ? I've
set up the control register for bidirectional operation but I can't get the
pull up circuitry to disengage.

One mode is
> easy to work with, the other is a real bidirectional data bus that can
> get a bit more complicated. Anyway, if you do a web search on
> 'parallel port FAQ', 'ECP FAQ', 'EPP FAQ' I think you will find the
> info you need.
>
> There are what makes parallel port disk drives, video capture, network
> adapters, etc possible. I know these devices are all going USB, but I
> think many computers still support them for legacy reasons.

Agreed. Some of those modes are DMA driven, which would really be nice.

> You might want to use a tristate buffer device so the calipers can
> only drive the port once you have configured it in input mode. And
> even then, some current limit resistors might be a good idea.

I've played with that a bit.

> Or if you don't need to read the devices too frequently, simply
> multiplexing them may be the best bet.

Timing is an issue. The calipers output data every 300ish mSec, to the beat
of their own drum. There is no practical way to set when a caliper outputs
its data. All three could send at once or they cold be spread out evenly.

At most you would have to wait
> two sample periods to get each sample. I don't know if your software
> has the capability to read from three mutualy-asynchronous clocked
> serial streams at once anyway?

Right now it doesn't, but it will. Right now I am using a simple, but
effective polling algorithm. It uses about 1% of the available CPU time.
In the future I will use a DMA scheme or interrupt driven data capture or
multiple threads, each polling for one caliper. Any of them could work, the
approach is just different.


> Chris
>
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