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From: cncst@unix.cis.pitt.edu (Christophe N. Christoff)
Newsgroups: comp.lang.pascal
Subject: About Emptying COM1 port
Message-ID: <34753@unix.cis.pitt.edu>
Date: 4 Sep 90 17:11:16 GMT
Reply-To: cncst@unix.cis.pitt.edu (Christophe N. Christoff)
Organization: Univ. of Pittsburgh, Comp & Info Services
Lines: 56

I asked the question about how to empty COM1 before, and the 
problem I put there was too simple for a correct anwser.  Now
I'll tell the problem in details.

I used PD program "COMM_TP4.TPU" in SIMTEL to read from COM1.
if everything is okey, the program works well.  But most of time,
i.e., every morning I turn on the machine, the program will not
work until I empty COM1 before I load the program.  Once the
program begins to work, another warm boot will cause no problem,
but a cold boot will cause the same problem until I reempty the
COM1 port. That is also the case I have to empty the port each
morning since it is a cold boot, of course.  I tried to find the
problem and finally found out that each cold boot causes some
characters left in the port.  So I tried to empty it from within
the program, but from that PD program, I cannot figure out how to 
do it. And I cannot find out one program to do it without
human interruption, I mean without pressing any key.

I tried every method I can think off my head, and there is no luck.
By the way, one simple way to clear that is at the DOS prompt level,
I typed:

COPY AUTOEXEC.BAT COM1 > NUL

DOS responds:
Write fault error writing device COM1
Abort, Retry or Ignore

I just choose Abort.  This will clear COM1 for me and enable my 
program to run.  But the question is that nobody like this ugly message
and further, we have to type "A" which has nothing to do with data
collection program.

I tried to pipe an "A" to that COPY command, but in this case,
PIPELINE command does not work!!!  I learnt the pipe commands
from folks around net.


echo y | del subdir

where subdir is a subdirectory name and it is not empty.

I used 

echo a | copy autoexec.bat com1

DOS simply ignore all this pipeline!!

Any idea why does pipeline not work? and why a cold boot place something
in the COM1 port? 

I get rid of every resident program and the problem still exists:

I use:
IBM DOS v3.2
IBM AT with EGA monitor.