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From: jkl@hpindwa.cup.hp.com (Jon Lewis)
Newsgroups: comp.protocols.ibm
Subject: Re: 3270 ATTN key -- what does it generate?
Message-ID: <39290006@hpindwa.cup.hp.com>
Date: 18 Dec 90 20:30:13 GMT
References: <9012162110.AA17045@lilac.berkeley.edu>
Organization: Hewlett-Packard, Cupertino CA
Lines: 29

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