IBM 3270

terminals made by IBM (known as "Display Devices") normally used to communicate with IBM mainframes. The 3270 attempts to minimise the number of I/O interrupts required, by accepting large blocks of data known as datastreams

Principles

In a datastream, 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.

Further, using a technique known as 'Read Modified' the changes from any number of formatted fields that have been modified can be read as a single input without transferring any other data, another technique to enhance the terminal throughput of the CPU. Modern users however, sometimes find this system extremely bizarre, since it is very different then any user interface encountered in the consumer market.

The 3270 has twelve, and later twenty-four, special Programmed Function Keys, or PF keys, and three PA (or Program Attention keys). When one of these keys is pressed, it will cause its control unit (generally a 3274 or 3174) to generate an I/O interrupt 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-push, thereby reducing the load on very busy processors.

In this way, the CPU is not bothered at every keystroke, a scheme which allowed a 3033 with 16 MB to support up to 17500 3270 terminals under CICS. On the other hand, vi-like behaviour was not possible. For the same reason, a porting of Lotus 1-2-3 to mainframes with 3279 screens did not meet success because of this lack of interactivity.

Models

• 3277 model 1 : 16x40 terminal
• 3277 model 2 : 24x80 terminal, the biggest success of all
• 3277 model 3 : 32x80 terminal
• 3277 GA : a 3277 with a RS232C I/O, often used to drive a Tektronix 4013 or 4015 graphic screen (1024x768, monochrome)
• 3278 models 3,4,5 : next-generation, with accented characters and dead keys in countries that needed them
  • model 2 : 24x80
  • model 3 : 32x80
  • model 5 : up to 42x132 (switchable)
• 3278 PS : programmable characters; able to display monochrome graphics
• 3279 : color terminal, 4-color (text) or 7-color (graphics) version,

A version of the IBM PC called the 3270 PC, released in October 1983, included 3270 terminal emulation. The software program TN3270 is a modified version of Telnet which acts as a 3270 terminal emulator and can be used to connect to an IBM mainframe over a network. Later, a PC/G (graphics) and a PC/GX (extended graphics) succeeded). Unfortunately these machines needed to be rebooted to switch between terminal mode and local (DOS) mode.

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