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Concept
IBM 3270
Summary
The IBM 3270 is a family of block oriented display and printer computer terminals introduced by IBM in 1971 and normally used to communicate with IBM mainframes. The 3270 was the successor to the IBM 2260 display terminal. Due to the text color 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.
IBM no longer manufactures 3270 terminals, but the IBM 3270 protocol is still commonly used via TN3270 clients, 3270 terminal emulation or web interfaces to access mainframe-based applications, which are sometimes referred to as green screen applications.
The 3270 series was designed to connect with mainframe computers, often at a remote location, using the technology then available in the early 1970s. The main goal of the system was to maximize the number of terminals that could be used on a single mainframe. To do this, the 3270 was designed to minimize the amount of data transmitted, and minimize the frequency of interrupts to the mainframe. By ensuring the CPU is not interrupted at every keystroke, a 1970s-era IBM 3033 mainframe fitted with only 16 MB of main memory was able to support up to 17,500 3270 terminals under CICS.
Most 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.
IBM 3270 devices are connected to a 3299 multiplexer or to the cluster controller, e.g.
Official source
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