TN3270 1 "December 21, 1988"

Table of contents


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 \s-UNIX\s0 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:

Turn on socket-level tracing (for super-user only)
-n filename
Specify a file to receive network trace data output (from commands "toggle netdata" and "toggle options", see telnet(1c)); the default is for output to be directed to the standard error file.
-t commandname
Specify a \s-UNIX\s0 command to process IBM 4994 style transparent mode data received from the remote IBM machine.
The name of the remote system. If the remote name is NOT specified, the user will be prompted for a command (see below).
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 number 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 following table describes the emulation:

         minimum size             emulated
        (rows*columns)            terminal
        --------------          ------------
            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 \s-UNIX\s0 process. This emulation involves mapping 3270-style commands from the host into appropriate sequences to control the user's terminal screen. Tn3270 uses curses(3x) and the /etc/termcap file to do this. The emulation also involves 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, /etc/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 mapping (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 entry, 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 enters 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:

Specify \s-UNIX\s0 command for IBM 4994 style transparent mode processing.

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 (typically 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 temporarily 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.




Greg Minshall


The IBM 4994 style transparent mode command is invoked when tn3270 receives 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 remote hosts, signalling the end of transparent mode output. Transparent mode is necessary for sending ASCII control characters over the 3270 terminal 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.


mset(1), telnet(1), curses(3x), termcap(3x), termcap(5), map3270(5), Yale ASCII Terminal Communication System II Program Description/Operator's Manual (IBM SB30-1911)


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 example) should cause a message to be sent to the user's terminal instead of just ringing a bell.