3270

3270 denotes IBM's family of mainframe display terminals and the data-communication protocol used with them. Introduced in the early 1970s for IBM System/360 and later System/370, 3270 devices were designed around a block-oriented data stream that updates the entire screen as a unit rather than character-by-character I/O. The standard display was an 80-column by 24-row text screen, though some models supported 132-column layout or larger panels. User input is processed through a keyboard with function keys (PF keys) and an attention identifier (AID) for commands such as Enter, Erase, and Clear.

3270 terminals communicate with IBM mainframes using a set of display controllers and adapters, such as the 3274/3276 multiplexer/controller family and the 3278/3279 display terminals. These devices manage session state, screen formatting, and data streams, and they can operate in local mode or as networked controllers behind a host computer. The 3270 data stream encodes screen content, attributes, and field security (protected fields) and supports multiple simultaneous sessions on some controllers.

3270 PC and PC/3270 were later IBM and third-party products that integrated 3270 display capability into personal computers, using an adapter or software emulator to connect to a mainframe.

In modern computing, 3270 data streams survive primarily through emulation. TN3270 and TN3270E over TCP/IP mimic the 3270 protocol over the Internet or corporate networks, enabling access to host systems from modern workstations and mobile devices. Popular 3270 emulators include x3270 and IBM Personal Communications; many mainframe applications still rely on 3270 interfaces.