Re: XON/XOFF communication problem

From: Floyd Davidson (floyd_at_barrow.com)
Date: 11/06/03 

Date: Wed, 05 Nov 2003 14:37:46 -0900

"Andrej T." <andrej@rikom.si> wrote:
>Floyd Davidson wrote:
>> The cable schematic looks fine. What kind of cable is it physically?
>> And how long is it? At 115.2kbps you can't expect it to work over
>> any significant length. More than a few feet probably won't work.
>
>The cable is 1m long so I guess this shouldn't be the

Heh, at 1m it's hard to imagine it wouldn't work with wet
string... :-)

>problem. And it is an UTP cable, the same that I'm using when

Is this a cable you made?

Twisted pair is a _bad_ idea for RS-232 cabling, unless it is
done just exactly right. For example, the TxD and RxD is *not*
a balanced pair and using one pair of a UTP cable for those two
leads will result in crosstalk between them. The TxD and RxD
leads should each be paired with some lead that will *not* be
changing, such as DCD, RING, DTR, or whatever. The same goes
for CTS/RTS, except since that operates (generally) at a much
lower rate there won't be nearly the same problem.

I'm not sure if a 1m cable would be long enough to exhibit this
or not, but it might be. On the other hand, I'm certain that
at 38.4Kbps it wouldn't be a problem.

>connecting the VT 510 terminal to an Etherlite (which btw. works
>fine, except that the schematic is different).

How is the schematic different???

>> What happens at speed higher than 19.2Kbps? Do you get the same
>> kind of errors?
>
>Yes, over 19.2Kbps I get the same errors then with XON/XOFF
>communication. I will test the cable again with the script that
>opens the port just once and will write the expiriances in the
>next post.
>
>> You did change the gettydefs to enable hardware flow control,
>> right? That is the -CRTSCTS entry.
>
>Before running the script I have set the port with the command :
>"stty -F /dev/ttyS0 -ixon -ixoff crtscts". But as I said before
>I will test the cable again to be sure!

Hmmm... you may have to get that into the same process that
opens the port.

  (
   stty -ixon -ixoff crtscts
   while read input < filename
   do
     echo $input
   done
  ) < /dev/ttyS0 > /dev/ttyS0

>But it's still a mystery to me why the XON/XOFF communication
>isn't working... How can I locate the author of the XON/XOFF
>communication code?

I have no idea!

>Is there a standard that says that XON/XOFF
>communications is only working at certain speeds?

Nope.

>I know this is
>an old protocol, but I would like to know if this is possible in
>Linux or not.

I'm amazed that you can't get anything working. I don't
remember ever trying using XON/XOFF, but I have setup Linux
systems to do computer to computer serial port comm at 115.2Kbps
and didn't have a problem. It's been awhile though.

I may just give it a try, just to verify for you that it can (or
can't be done). It kinda fits into another project... I just
wrote 5 different "terminal programs" in 5 different ways, to
demonstrate how various types of serial IO is done (async,
canonical, two processes, two threads, etc. etc). I've only been
testing them by talking to a modem at 57.6Kbps. One purported
"valuable" use of this type of program is to use the terminal
program on a separate computer, connected to the development
system by a null modem, to aid in testing serial comm programs
under development. Hence I really should verify that the
programs work on my hardware at the highest rate that I can get
the serial ports to run at.

I'll see if I can find a null modem and get back... 

-- 
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

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