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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. == Principles == 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. In a data stream, both text and control (or formatting functions) are interspersed allowing an entire screen to be "painted" as a single output operation. The concept of "formatting" in these devices allows the screen to be divided into clusters of contiguous character cells for which numerous "attributes" (colour, highlighting, character set, protection from modification) can be set. An attribute occupies a physical location on the screen that also determines the beginning and end of a "field" (separately addressable sub section of the screen). Using a technique known as "read modified", a single transmission back to the mainframe can contain the changes from any number of formatted fields that have been modified, but without sending any unmodified fields or static data. This technique enhances the terminal throughput of the CPU, and minimizes the data transmitted. Some users familiar with character interrupt-driven terminal interfaces find this technique unusual. There is also a "read buffer" capability that transfers the entire content of the 3270-screen buffer including field attributes. This is mainly used for debugging purposes to preserve the application program screen contents while replacing it, temporarily, with debugging information. Early 3270s offered three types of keyboards. The ''typewriter keyboard'' came in both a 66 key version, with no programmed function (PF) keys, and a 78 key version with twelve. Both versions had two ''program attention'' (PA) keys. The ''data entry keyboard'' had five PF keys and two PA keys. The ''operator console keyboard'' had twelve PF keys and two PA keys.〔 Later 3270s had twenty-four PF keys and three PA keys. When one of these keys is pressed, it will cause its control unit to generate an I/O interrupt to the host computer and present a special code identifying which key was pressed. Application program functions such as termination, page-up, page-down, or help can be invoked by a single key press, thereby reducing the load on very busy processors. In this way, the CPU is not interrupted at every keystroke, a scheme that allowed an early 3033 mainframe with only 16 MB to support up to 17,500 3270 terminals under CICS. On the other hand, vi-like behaviour was not possible. (But end-user responsiveness was arguably more predictable with 3270, something users appreciated.) For the same reason, a porting of Lotus 1-2-3 to mainframes with 3279 screens did not meet with success because its programmers were not able to properly adapt the spreadsheet's user interface to a "screen at a time" rather than "character at a time" device. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「IBM 3270」の詳細全文を読む スポンサード リンク
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