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

Hi, make sure you have a bidirectional cable for the parallel port. some
cables were output only cables! cul brian f.

----- Original Message -----
From: <ccs@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Saturday, October 20, 2001 10:45 AM
Subject: Re: [CAD_CAM_EDM_DRO] Re: Caliper to PC interface, Part II: Data
Format ?????? Help ?


>
> > > 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.
>
> They really are bidirectional. The term 'pull up' usually refers to a
> passive device - a resistor, which is over-ridden by an open-collector
> (open drain) switching device. I'd try putting the port in input
> mode, then pulling the line down through an ammeter and seeing if the
> current stays within the source specification for a parallel port.
> Also see if the port then reads that as a zero. This may require
> nixing my current limit series resistor suggestion.
>
> > Agreed. Some of those modes are DMA driven, which would really be nice.
>
> This is not an application where DMA is at all suitable. DMA is for
> transferring blocks of data - you need to get a word and think about
> it. DMA would only be usefull if you wanted to make an oscilloscope
> like 'recording' of the output which you could later analyze to
> extract the bit transisitions of data and clock. But recording in
> this fashion would be woefully inefficient as your non-synchronized
> sampling rate must be well above the data rate.
>
> > > 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.
>
> Yes, so the worst case is that it would take 1.8 seconds to see a
> valid sequence on all three devices. Select first device, wait until
> you see a new sequence start, select second device, wait until
> sequence start, etc. For some applications this might be okay - you
> haven't said what you want to do with it. The 300 mSec individual
> rate is fine for a DRO or for logging dimensions of finished parts,
> but it is no where near fast enough for any kind of servo loop.
>
> > 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.
>
> I don't think you actually need multiple threads - you can just
> process each input word for the three calipers in sequence, using
> seperate variables to track the state of each. But maybe you are more
> familiar with multithreading than I.
>
> I don't know that you can get multiple interrupts off any of the
> parallel port schemes, so you would have to add interrupt multiplexing
> logic... poling with on-the-fly data extraction is probably the best
> solution.
>
> Chris
>
>
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