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From: gcouger@olesun.okstate.edu (Gordon Couger)
Subject: Re: Problem with PW driver from PC
Message-ID: <Cv3nzE.400@osuunx.ucc.okstate.edu>
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Organization: Biosystems and Agricultural Engineering at Oklahoma State Univ.
References: <33iegl$q7r@io.salford.ac.uk>
Date: Thu, 25 Aug 1994 16:47:38 GMT
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In article <33iegl$q7r@io.salford.ac.uk>,
J.W.D.Boucher <j.w.d.boucher@aeromech.salford.ac.uk> wrote:
>At the moment Im trying to drive some small precision position servos 
>using a pulse width modulated signal from a PC.  Im using a Digital 
>output card  ( FPC - 024; 8255 i/o card - if thats relevant ) which holds 
>an output bit high for a given number of clock cycles, and then goes low. 
> It all works great except that randomly each of the servos being driven 
>by the program coughs ( turns quickly one way or the other by about 10 
>degrees and then returns to where it was supposed to be ).  The time 
>between coughs seems to be of the order of 1 minute and the best idea 
>I've had is that there might be some kind of refresh routine that I dont 
>know about in the PC.
>
The villan is probably timer interupts that cause the computer to
be off doing somthing else. Because dos is not reenterent many interupts
must disable other interupts to insure that things don't get MUNGed
during the interupt. To see if this is the problem disable the interupts
and see if the problem goes away. There may be some non maskable interupts
that exicute routinly but I don't think so. I know that disk rights
total take over the computer. 

The only solution if disabling the interupt doesn't work that I can
think of is to use a PIC or 68HC11 to control the servo and let the
msdos box just tell it when to change and let the little computer
just maintain the position until told to change

Good luck
Gordon

                             Gordon Couger                            
                             Biosystems & Agricultural Engineering              
                             Oklahoma State University                         
                             114 Ag Hall, Stillwater, OK  74074                 
 gcouger@olesun.agen.okstate.edu 405-744-9763 day 624-2855 evenings 
    I do not speak for my employer
