IBM 3270

IBM 3279 Colour Display Terminal

The IBM 3270 is a class of block oriented terminals made by IBM since 1972 (known as "display devices") normally used to communicate with IBM mainframes. As such, it 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 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, using coaxial cable.

IBM stopped manufacturing terminals many years ago, but the IBM 3270 protocol is still commonly used via terminal emulation to access some mainframe-based applications. Accordingly, such applications are sometimes referred to as green screen applications. Use of 3270 is slowly diminishing over time as more and more mainframe applications acquire Web interfaces, but some web applications 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 distance, using early 1970s technology. Two of the major design goals of 3270's are related to this: minimizing the amount of data transmitted, and minimizing the frequency of interrupts to the mainframe.

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. An attribute occupied a physical location on the screen which also determined the beginning and end of a "field" (separately addressable sub section of the screen).

Further, 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 (such as Microsoft Windows) find this technique unusual. There was also a "read buffer" capability which transferred the entire content of the 3270-screen buffer including field attributes. This was mainly used for debugging purposes to preserve the application program screen contents while replacing it, temporarily, with debugging information.

The first 3270s had no function keys. Later 3270s had 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 press, thereby reducing the load on very busy processors.

In this way, the CPU is not interrupted at every keystroke, a scheme which 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 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.

In contrast also, ICI Mond Division's Works Records System, the first known shared public spreadsheet used the 3270 successfully for what was, in effect, a high powered version of today's spreadsheets with additional functions. It remained in continual use for 27 years up until 2001 and, despite its lack of a GUI, cells could be defined anywhere on the screen (not necessarily in rows or columns) and could be instantly re-configured for length, content and formulas as required. It is interesting to note that ICI's online, fully interactive system pre-dated PC spreadsheets by quite a few years and allowed multiple users to use the spreadsheets at the same time, similar to today's Web based shared spreadsheets.

As mentioned above, the Web (and HTTP) is 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.

Applications development has in many ways returned to the 3270 approach. In the 3270 era, all application functionality was provided centrally. With the advent of the PC, the idea was to invoke central systems only when absolutely unavoidable, and to do all application processing with local software on the personal computer. Now in the Web era (and with wikis in particular), the application again is strongly centrally controlled, with only technical functionality distributed to the PC.

In the early 1990s a popular solution to link PCs with the mainframes was the Irma board. It was a piece of hardware that plugged into a PC and connected to a coaxial cable towards the mainframe. IRMA also allowed file transfers between the PC and the mainframe.

Third parties

Many manufacturers, such as Hewlett Packard, created 3270 compatible terminals, or adapted ASCII terminals such as the HP 2640 series to have a similar block-mode capability which would transmit a screen at a time, with some form validation capability. Modern applications are sometimes built upon legacy 3270 applications, using software utilities to capture (screen scraping) screens and transfer the data to web pages or GUI interfaces.

Models

The IBM 3270 display terminal subsystem consisted of displays, printers and controllers.

Displays

• 3277 model 1 : 40×12 terminal
• 3277 model 2 : 80×24 terminal, the biggest success of all
• 3277 model 3 : 80×32 terminal
• 3277 GA : a 3277 with an 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 : 80×24
  • model 3 : 80×32
  • model 4 : 80×43
  • model 5 : 132×27 or 80×24 (switchable)
• 3278 PS : programmable characters; able to display monochrome graphics
• 3279: colour terminal, 4-colour (text) or 7-colour (graphics) version
• 3290 : a large, monochrome plasma display unit, capable of displaying in various modes, including four independent 3278 model 2 terminals, or a single 160×62 terminal; it also supported partitioning.

• 3178 : lower cost terminal (1983)
• 3179 : low cost colour terminal (1984)
• 3104 : low-cost R-loop connected terminal for the IBM 8100 system

(Generally, 3277 models were upper-case only, except for the mixed EBCDIC/APL which had lower case; lower-case capability and possibility of dead keys, at first a simple RPQ (Request Price Quotation, tailored on request at extra cost) was only added in 3278 & 3279 models.)

A version of the IBM PC called the 3270 PC, released in October 1983, included 3270 terminal emulation. Later, the 3270 PC/G (graphics) and 3270 PC/GX (extended graphics) followed.

Display-Controller

• 3275 remote display with controller function (no additional displays and/or printers)
• 3276 remote display with controller function (up to a limited number of displays and/or printers)

Printers

• 3284 matrix printer
• 3286 matrix printer
• 3287 printer, including a color model
• 3288 line printer
• 3268-1 : R-loop connected stand-alone printer for the IBM 8100 system

Controllers

• 3271 remote controller
• 3272 local controller
• 3274 cluster controller (different models could be channel-attached, BSC attached or SDLC attached, and had between eight and 32 co-ax ports)
• 3174 cluster controller

Manufacture

The IBM 3270 display terminal subsystem was architected and developed by IBM's Kingston, NY, laboratory (which was later closed during IBM's difficult time in the mid-1990s). The printers were developed by the Endicott, NY, laboratory. As the subsystem expanded, the 3276 display-controller was developed by the Fujisawa, Japan, laboratory, and later the Yamato laboratory; and the 3279 color display and 3287 color printer by the Hursley, UK, laboratory. The subsystem products were manufactured in Kingston (displays and controllers), Endicott (printers), and Greenock, Scotland, UK, (most products) and shipped to users in U.S. and worldwide. 3278 terminals continued to be manufactured in Hortolandia, near Campinas, Brazil as far as late 1980s, having its internals redesigned by a local engineering team using modern CMOS technology, while retaining its external look and feel.

Telnet 3270

Telnet 3270, or TN3270 describes either the process of sending and receiving 3270 data streams using the Telnet protocol or the software that emulates a 3270 class terminal which communicates using that process. TN3270 allows a 3270 terminal emulator to communicate over a TCP/IP network instead of an SNA network. Standard telnet clients cannot be used as a substitute for TN3270 clients, as they use fundamentally different techniques for exchanging data.

References

• Partial IBM history noting the unveiling of the 3270 display system in 1971
• 3270 Information Display System - 3270 Data Stream Programmer's Reference from IBM
• Introduction to Telnet 3270 from Cisco
• RFC 1041 - Telnet 3270 regime option
• RFC 1576 - TN3270 Current Practices
• RFC 2355 - TN3270 Enhancements
• A brief history of 3270

See also

• 3270 emulator
• List of IBM products
• IBM 5250 display terminal subsystem for IBM AS/400

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This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.