Emulation of the 3270 terminal is done in the Unix 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 (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 the following commands:
open connect to a remote host
close close the current connection
quit exit tn3270
z suspend tn3270
status print connection status
? print help information
Other common telnet commands are not available in tn3270. Tn3270 command mode may also be entered, after connecting to a host, by typing a special escape character (typically control-C).
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.
Not all 3270 functions are supported, nor all Yale enhancements.