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Re: XON/XOFF communications problem on serial port

From: Bob Proulx
Subject: Re: XON/XOFF communications problem on serial port
Date: Fri, 17 Oct 2003 19:27:17 -0600
User-agent: Mutt/1.3.28i

Andrej Trobentar wrote:
> I don't know if this is the right email to send my question to, but I have
> tried on a couple of linux mailinglists and noone seems to know the
> answer. To my question...

And bug-shellutils because?

> I have an VT 510 terminal connected to a linux PC (three wire cable MMJ
> <-> DB9).

I really hate hearing that you only have three wires and are not using
any hardware flow control.

> I have set the port to use XON/XOFF communications and tell it
> to operate at 115200bps.

Trying to use 115.2kbs without hardware flow control!  My first
reaction is complete dismay.  My second suggestion is to try no faster
than 4800bps without flow control.  That is as fast as my vt100 can
handle.  Perhaps your vt510 can handle a faster rate.  :-)

> When I send like 100 lines to the terminal with the echo command I
> can see errors on the terminal (the error characters are like
> question marks, but turned upside down).

The buffer is being overrun with data faster than it can be processed
by the terminal.  Errors in the data are being created.  In your
character set those corrupted characters are being turned into other
characters.  Or perhaps your terminal is displaying the overrun
error.  Others tend to call that "line noise".

> If I connect the terminal with the same cable to a Windows 2000 PC
> everything works fine - no errors on the terminal. What could cause
> such behavior?

How are you sure that you are creating the same test case with those
two systems?  Windows surely has no way to produce a 100 lines of
characters exactly the same as your linux pc.  I suspect that it is
not able to produce output fast enough to overrun your terminal
buffer.  I suspect that your linux pc is faster and more likely
statistically to do that.

> I would apriciate if you could help me or point me to the right address.

I strongly recommend hardware flow control.  You really, really, want
to have more wires in your cable.  Trying to deal with the problems of
software flow control is very painful and you are unlikely ever to be
satisfied with the result.

If you do insist upon using software flow control then you should slow
down to a slower rate such that your terminal is able to keep up.  At
some point you will not have any buffer overrun problems.

There really is not a good address for your question.  I have not paid
too much attention lately to the newsgroups but I would guess that the
news group comp.os.linux.misc would be a reasonable place to broker
such questions if you were wishing to discuss this problem.  Beware
that the noise ratio in the newsgroups has gotten extremely high.


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