3270

The IBM 3270 is a class of block oriented computer terminal (sometimes called display devices) introduced by IBM in 1971 normally used to communicate with IBM mainframes. The 3270 was the successor to the IBM 2260 display terminal. Due to the text colour on the original models, these terminals are informally known as green screen terminals. Unlike a character-oriented terminal, the 3270 minimizes the number of I/O interrupts required by transferring large blocks of data known as data streams, and uses a high speed proprietary communications interface, using coaxial cable.

Although IBM no longer manufactures 3270 terminals, the IBM 3270 protocol is still commonly used via terminal emulation to access mainframe-based applications. Accordingly, such applications are sometimes referred to as green screen applications. The use of 3270 is slowly diminishing as more and more mainframe applications acquire Web interfaces, although some Web applications merely use the technique of "screen scraping" to capture old screens and transfer the data to modern front-ends.

The 3270 series was designed to connect with mainframe computers, often at a remote location, using the technology then available in the early 1970s. Two of the major design goals of 3270s are minimizing the amount of data transmitted, and minimizing the frequency of interrupts to the mainframe.

3270 devices are clustered, with one or more displays or printers connected to a control unit (the 3275 and 3276 included an integrated control unit). Originally devices were connected to the control unit over coaxial cable; later token ring, twisted pair, or Ethernet connections were available. A local control unit attaches directly to the channel of a nearby mainframe. A remote control unit is connected to a communications line by a modem. Remote 3270 controllers are frequently multi-dropped, with multiple control units on a line.

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