TN3270(1) BSD Reference Manual TN3270(1)
tn3270 - full-screen remote login to IBM VM/CMS
tn3270 [-d] [-n filename] [-t commandname] [sysname [port]]
Tn3270 permits a full-screen, full-duplex connection from a UNIX machine
to an IBM (or compatible) machine. Tn3270 gives the appearance of being
logged in to the remote machine from an IBM 3270 terminal. Of course,
you must have an account on the machine to which you connect in order to
log in. Tn3270 looks to the user in many respects like the Yale ASCII
Terminal Communication System II. Tn3270 is actually a modification of
the Arpanet TELNET user interface (see telnet(1)) which will, in certain
circumstances, interpret and generate raw 3270 control streams.
The flags to tn3270 are as follows:
-d Turn on socket-level tracing (for super-user only) -nfilename
Specify a file to receive network trace data output (from
commands "toggle netdata" and "toggle options", see
telnet(1)); the default is for output to be directed to the
standard error file. -tcommandname Specify a UNIX command to
process IBM 4994 style transparent mode data received from
the remote IBM machine.
sysname The name of the remote system. If the remote name is NOT
specified, the user will be prompted for a command (see be-
port The port to connect to on the remote system. Normally,
tn3270 attempts to connect to the standard TELNET port (port
23) on the remote machine.
When tn3270 first connects to the remote system, it will negotiate to go
into 3270 mode. Part of this negotiation involves telling the remote
system what model 3270 it is emulating. In all cases, tn3270 emulates a
3278 terminal. To decide which specific model, tn3270 looks at the num-
ber of lines and columns on the actual terminal (as defined in the TERM
environment variable; see termcap(5)). The terminal (or window in which
tn3270 is running, on multiple window systems) must have at least 80
columns and 24 lines, or tn3270 will not go into emulation mode. If the
terminal does have at least 80 columns and at least 24 lines, the follow-
ing table describes the emulation:
27*132 3278 model 5
43*80 3278 model 4
32*80 3278 model 3
24*80 3278 model 2.
Emulation of the 3270 terminal is done in the UNIX process. This emula-
tion involves mapping 3270-style commands from the host into appropriate
sequences to control the user's terminal screen. Tn3270 uses curses(3)
and the /usr/share/misc/termcap file to do this. The emulation also in-
volves simulating the special 3270 keyboard keys (program function keys,
etc.) by mapping sequences of keystrokes from the ASCII keyboard into
appropriate 3270 control strings. This mapping is terminal dependent and
is specified in a description file, /usr/share/misc/map3270, (see
map3270(5)) or in an environment variable MAP3270 (and, if necessary,
MAP3270A, MAP3270B, and so on - see mset(1)). Any special function keys
on the ASCII keyboard are used whenever possible. If an entry for the
user's terminal is not found, tn3270 looks for an entry for the terminal
type unknown. If this is not found, tn3270 uses a default keyboard map-
ping (see map3270(5)).
The first character of each special keyboard mapping sequence is either
an ASCII escape (ESC), a control character, or an ASCII delete (DEL). If
the user types an unrecognized function key sequence, tn3270 sends an
ASCII bell (BEL), or a visual bell if defined in the user's termcap en-
try, to the user's terminal and nothing is sent to the IBM host.
If tn3270 is invoked without specifying a remote host system name, it en-
ters local command mode, indicated by the prompt ``tn3270> ''. In this
mode, tn3270 accepts and executes all the commands of telnet(1), plus
one additional command:
transcom Specify UNIX command for IBM 4994 style transparent mode
Tn3270 command mode may also be entered, after connecting to a host, by
typing a special escape sequence. If tn3270 has succeeded in negotiating
3270 mode with the remote host, the escape sequence will be as defined by
the map3270 (see map3270(5)) entry for the user's terminal type (typi-
cally control-C); otherwise the escape sequence will initially be set to
the single character `^]' (control right square bracket).
While in command mode, any host login session is still alive but tem-
porarily suspended. The host login session may be resumed by entering an
empty line (press the RETURN key) in response to the command prompt. A
session may be terminated by logging off the foreign host, or by typing
``quit'' or ``close'' while in local command mode.
The IBM 4994 style transparent mode command is invoked when tn3270 re-
ceives IBM 4994 style transparent output from the remote host. Output
and input pipes are created for communication between the two processes.
The pipes are closed when a 3270 clear command is received from the re-
mote hosts, signaling the end of transparent mode output. Transparent
mode is necessary for sending ASCII control characters over the 3270 ter-
minal connection; ASCII graphics terminal support is accomplished this
way. Developers of transcom commands should note that the transcom stdin
pipe end will be in CBREAK mode, with ECHO and CRMOD turned off.
Tn3270 checks the following environment variables: TERM, MAP3270,
MAP3270[A...]. Information on these can be found in mset(1). Tn3270 also
checks SHELL, KEYBD and API3270.
mset(1), telnet(1), curses(3), termcap(3), termcap(5), map3270(5),
"Yale ASCII Terminal Communication", System II Program
Description/Operator's Manual, IBM SB30-1911.
The tn3270 command appeared in 4.3BSD.
Tn3270 is slow and uses system resources prodigiously.
Not all 3270 functions are supported, nor all Yale enhancements.
Error conditions (attempting to enter data in a protected field, for ex-
ample) should cause a message to be sent to the user's terminal instead
of just ringing a bell.
4.3 Berkeley Distribution April 18, 1994 3