The original IBM 3270 terminal, mid 1970s. Although it bears a superficial resemblence to regular ASCII "start/stop" terminals, the similarity ends just below the surface. The 3270 is much more tightly coupled to the mainframe than a regular terminal would be. Up to 32 (?) 3270 terminals are connected via coaxial cable to a 3274 control unit, which in turn is connected (perhaps along with many other 3274s) to a mainframe I/O channel. Rather than just sending characters back forth, a 3270 terminal session sends structured "3270 datastreams" consisting of control blocks and so forth.
Users are generally presented with forms to fill out. The user moves about the form with arrow, tab, and backtab keys, filling in and correcting the various fields, and then presses the Enter key when ready to submit the form to the mainframe. The mainframe receives a series of data elements tagged to identify which field they belong to. When the mainframe is not ready for input, it literally "locks" the keyboard.
At some point after the introduction of 3270 terminals, 3270 protocol converters began to appear, which allowed ordinary ASCII terminals to access 3270 applications by masquerading as 3274 control units to the mainframe and converting between 3270 datastreams and the escape sequences of the particular ASCII terminal. Examples include the IBM Series/1, the IBM 7171, the IBM 4994, the IBM 3174, plus various non-IBM products including, most recently, generic terminal servers and UNIX host computers or workstations offering tn3270 connections.
Photo: University of Heidelberg.