rts/cts - a tutorial on flow control

[Phil Ngai]

In article <879 at sdcsvax.UUCP> brian at sdcsvax.UUCP (Brian Kantor) writes:
>XON-XOFF (aka DC1/DC3) are examples of reserved characters.  When they
>occur in the data stream, they are interpreted.  If its desired that
>they be treated as data (for example, as a command key for EMACS, or as

You're not supposed to use XOFF/XON as data. Many people would claim
that EMACS is broken for using XOFF for a search command. Some have
remapped it. Certainly terminals like the VT100 and the VT220 like to
use XOFF/XON for flow control. I have seen people send padding characters
to avoid using XOFF/XON, since I seem to have overworked the word "gross"
I won't give my opinion about such methods.

>But consider.  Now you have two Un*x systems talking to each other with
>UUCP, and the network gets congested.  UUCP needs an eight-bit
>transparent path, so it doesn't really handle XON-XOFF.  How is the
>network to tell the sending computer that it should stop?

If you want to complain about not having an eight-bit transparent path,
try beating on the people who run X.25 networks. See how far you get.
Our friends in Europe decided the right way to use uucp over X.25 networks
was to invent a new "f" protocol (which is in the 4.3 BSD uucp) which
restricts itself to 7 bit characters. Binary data can be sent, the
protocol maps the byte into a valid character range and prefixes a special
byte. I believe this is what kermit does to use 7 bit data paths also.

>similar.  And how about a Diablo terminal, that wants ETX/ACK instead of
>XON/XOFF?

I believe those are valid characters. By the way, if you are interested
in funky character sets, you should look at the programmers manual for
the VT220 to see how they handle the extra characters needed to function
in a European market. I'll give you a hint: they don't use XOFF as data.
One neat thing (which is sort of irrelevant) is the availability of
software configurable character sets. You can actually down load the fonts.
And you don't need an eight bit data path for that either.


-- 
 What do you do the day after a peak experience?

 Phil Ngai (408) 749-5720
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