Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!olivea!apple!agate!ucbvax!hpindwa.UUCP!jkl
From: jkl@hpindwa.UUCP (Jon Lewis)
Newsgroups: comp.protocols.ibm
Subject: Re: 3270 ATTN key -- what does it generate?
Message-ID: <9012190925.AA26026@lilac.berkeley.edu>
Date: 18 Dec 90 20:30:13 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: Jon Lewis <hpda!hpcupt1!hpindwa!jkl@ucbvax.Berkeley.EDU>
Organization: The Internet
Lines: 27

I'll add some info as to what the ATTN key does when pressed on a real 3270.
I can't speak about tn3270, as I know nothing about that product.

Keyboard                 3274                          VTAM

ATTN key pressed
                         DFC Signal ---->>>>>

                                                       When convenient,
                                                       send change direction
                                                       (CDI).  This turns
                                                       the line around, and
                                                       allows the 3270 to
                                                       enter data.

Note, that it is up to the application/presentation services layer on the
IBM side to handle the DFC (Data Flow Control) command and turn the line
around.  For LU2(3270), when a signal is received, the application
should complete any pending I/O, e.g. if it is the middle of a chain, and
turn the line around as soon as possible.

Hope this helps, if not, and you want some more clarification, drop me
some email.

Jon Karl Lewis
Information Networks Division - Hewlett Packard Company
jkl@hpindzl.cup.hp.com