The IBM 3270 is a class of terminals made by IBM since 1972 (known as "Display Devices") normally used to communicate with IBM mainframes. Unlike common serial ASCII terminals, the 3270 minimizes the number of I/O interrupts required by accepting large blocks of data known as datastreams, and uses a high speed proprietary communications interface. IBM stopped manufacturing terminals (and punch cards for that matter) many years ago, but the IBM 3270 protocol is still commonly used via emulation to access some mainframe-based applications. Use of 3270 is slowly diminishing over time as more and more mainframe applications acquire Web interfaces, but in some situations (such as call centers) the "green screen" 3270 interface is still the most productive and efficient.

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. Some users familiar with character interrupt-driven terminal interfaces find this technique unusual. However, most Web interfaces operate in much the same way and often for the same reasons.

The first 3270s had no PF keys. Later 3270s has twelve, and later twenty-four, special Programmed Function Keys, or PF keys, and three PA (or Program Attention) keys placed in one or two rows at the top of the keyboard. When one of these keys is pressed, it will cause its control unit (historically, usually, an IBM 3274 or 3174, but nowadays the onboard mainframe equivalent) 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 an early 3033 mainframe with only 16 MB to support up to 17500 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 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. In contrast, IBM's OfficeVision office productivity software enjoyed great success with 3270 interaction because of its design understanding, and for many years the PROFS calendar was the most commonly displayed screen on office terminals around the world.

As mentioned above, the Web (and HTTP) is more similar to 3270 interaction because the terminal (browser) is given more responsibility for managing presentation and user input, minimizing host interaction while still facilitating server-based information retrieval and processing. In fact, not too many years ago 3270 terminals were considered "smart" (or "programmable" or "intelligent") rather than "dumb."

Models

• 3277 model 1 : 16×40 terminal
• 3277 model 2 : 24×80 terminal, the biggest success of all
• 3277 model 3 : 32×80 terminal
• 3277 GA : a 3277 with a RS232C I/O, often used to drive a Tektronix 4013 or 4015 graphic screen (1024×768, monochrome)
• 3278 models 3,4,5 : next-generation, with accented characters and dead keys in countries that needed them
  • model 2 : 24×80
  • model 3 : 32×80
  • model 4 : 43×80
  • model 5 : 27×132 or 24×80 (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. Later, the PC/G (graphics) and PC/GX (extended graphics) followed.

Telnet 3270

TN3270 is a slighly modified version of the Telnet protocol which allows a 3270 terminal emulator to communicate over a TCP/IP network (vs. SNA). Most standard telnet clients cannot be used as a substitute for TN3270 clients, as they use vastly different protocols and escape sequences.

Known TN3270 Clients

• IBM Host On-Demand and Personal Communications
• Attachmate EXTRA!
• QWS3270
• Rumba
• Reflection

References

• Introduction to Telnet 3270 from Cisco
• RFC 1041 - Telnet 3270 regime option
• RFC 1576 - TN3270 Current Practices
• RFC 2355 - TN3270 Enhancements

IBM | IBM hardware | Clear text protocols | Network-related software | Remote administration software

Tn3270 | IBM 3270