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3270: The Attack of the IBM Terminals
All the terminals discussed earlier in this chapter that are handled by telnet are basically souped-up Teletypes, with data passed character by character between the terminal and the host. This kind of terminal interaction can be called Teletype-ish.IBM developed an entirely different model for its 3270-series display terminals. The principle is that the computer’s in charge. The model works more like filling in paper forms. The computer draws what it wants on-screen, marks which parts of the screen you can type on, and then unlocks the keyboard so that you can fill in whichever blanks they want. Whenever you press Enter, the terminal locks the keyboard, transmits the changed parts of the screen to the computer, and awaits additional instructions from headquarters.To be fair, this method is a perfectly reasonable way to build terminals intended for dedicated data-entry and data-retrieval applications. The terminal on the desks at your bank or the electric company are probably 3270s — or more likely these days, cheap PCs emulating 3270s. Because the 3270 terminal protocol squeezes a great deal more on a phone line than Teletype-ish, having all the 3270s in an office sharing the same single phone line, with reasonable performance is quite common.The Internet is a big place, and plenty of IBM mainframes run applications on the Internet. Some of them are quite useful. Some large library catalogs that haven’t moved to the Web yet speak 3270-ish. Usually, if you telnet to a system that wants a 3270, it converts from the Teletype-ish that telnet speaks to 3270-ish so that you can use it anyway. Some 3270 systems speak only 3270-ish, however, and if you telnet to them, they connect and disconnect without saying anything in between.
A variant of telnet that speaks 3270-ish is called tn3270. If a system keeps disconnecting, try typing the command tn3270 instead. (Large amounts of UPPERCASE LETTERS and references to the IBM operating systems VM or OS/390 or z/OS are also tip-offs that you’re talking to a 3270.) Even if a 3270 system allows regular telnet , you get a snappier response if you use tn3270 instead.
