IBM 3270

IBM 3270 series

Company type	Computer terminal series
Industry	Computer hardware
Founded	1971
Founder	IBM
Products	Block-oriented display and printer computer terminals
Parent	IBM

The IBM 3270 series comprises block-oriented display and printer computer terminals launched by IBM in 1971, primarily intended for communication with IBM mainframe systems. It followed the earlier IBM 2260 display terminal model. Often referred to as green screen terminals because of their original green text display, the 3270 minimizes I/O interrupts by transmitting large data blocks known as data streams through a high-speed proprietary interface utilizing coaxial cable.

Although IBM has ceased production of 3270 terminals, the IBM 3270 communication protocol remains extensively employed via TN3270, terminal emulation, or web-based interfaces to access mainframe software, commonly termed green screen applications.

Principles

Introduced in the early 1970s to facilitate remote connections to mainframes, the 3270 was engineered to optimize terminal support per mainframe by reducing data transfers and CPU interrupts. This efficiency allowed a 1970s-era IBM 3033 mainframe with only 16 MB of memory to manage up to 17,500 terminals running under CICS.

Most 3270 devices function within clusters, which connect multiple displays or printers to a central control unit—as seen in models like the 3275 and 3276. Initial connections relied on coaxial cables; later expansions supported Token Ring, twisted pair, or Ethernet. A local control unit links directly to the mainframe channel, whereas a remote control unit connects through a modem and can be configured as multi-dropped, allowing several controllers on one line.

The system uses 93-ohm RG-62 coaxial cable in point-to-point configurations, transmitting data at 2.3587 Mbit/s with modified differential Manchester encoding. Cable lengths of up to are supported, although IBM cited typical maximum distances around . Connector types evolved from standard BNC connectors to specialized dual-purpose connectors capable of handling shielded twisted-pair cables without requiring baluns.

Data streams combine text with control and formatting commands, enabling full-screen updates in a single transaction. Screens are organized into fields—groups of adjacent character cells—with attributes such as color, highlighting, character set, and write protection. These field attributes occupy designated screen positions marking field boundaries; individual characters may also have specific attributes.

Employing "read modified" functionality, only altered fields are transmitted back to the mainframe during one operation, thereby decreasing CPU usage and network traffic. A complete read buffer option allows transfer of the entire screen content including attributes, primarily for debugging purposes.

Initial keyboards were offered in three variants: a typewriter keyboard (with 66 or 78 keys plus up to twelve programmed function (PF) keys), a data entry keyboard (featuring five PF keys), and an operator console keyboard (with twelve PF keys). Each included two Program Attention (PA) keys. Later versions introduced additional keys such as an Attention key, Cursor Select key, System Request key, twenty-four PF keys, three PA keys, and a TEST REQ key. Activating these keys generates an I/O interrupt or attention signal that identifies the specific key pressed, enabling application functions like termination or help through single keystrokes—thus reducing processor overhead.

This block-oriented architecture restricted character-by-character input common in editors like vi, which posed challenges for software such as Lotus 1-2-3 adapting its user interface for screen-at-a-time interaction. Nevertheless, it delivered more consistent end-user responsiveness valued in business settings.

Applications

Following their debut, the 3270 terminals and their compatibles became the primary interface devices for IBM System/370 and its subsequent models. Both IBM and independent software developers designed interactive programs assuming the presence of 3270 terminals, incorporating ISPF panels and utility support.

The Conversational Monitor System (CMS) within VM has maintained 3270 terminal support through to z/VM. Similarly, the Time Sharing Option (TSO) available in OS/360 and successors provides both line mode and full-screen application functionality akin to ISPF.

For OS/360, Device Independent Display Operator Console Support (DIDOCS) within Multiple Console Support (MCS) accommodates 3270 devices; however, contemporary MVS variants have discontinued support for line mode consoles except for the Hardware Management Console (HMC). The SPF and Program Development Facility (ISPF/PDF) editors utilized in MVS and VM/SP heavily rely on 3270 features. Likewise, the XEDIT editors spanning VM/SP through z/VM exploit the capabilities of 3270 terminals.

Since the early 1970s, Customer Information Control System has incorporated 3270 panel support, with numerous CICS applications crafted specifically for these terminals.

The online editing and job submission system Source Program Maintenance Online II (SPM) was architected around the 3270 interface. Various iterations of Wylbur provided comprehensive full-screen applications compatible with 3270 terminals.

McGill University’s MUSIC/SP operating system integrated 3270 support featuring a full-screen text editor, a menu-driven interface, and a PANEL facility for developing 3270-based applications.

The introduction of the modified data tag facilitated efficient transformation of structured punched card input into formatted displays on 3270 terminals, enabling batch programs designed for card input to operate interactively on these devices.

IBM’s OfficeVision suite of office productivity tools achieved significant market penetration by optimizing interaction with the 3270 terminal. For many years, the calendar application in PROFS was among the most frequently accessed screens worldwide on office terminals.

A version of the WordPerfect word processor adapted for the IBM System/370 was engineered specifically to leverage the 3270 architecture.

SNA

3270 terminals can function within either an SNA or non-SNA environment. When operating under SNA, controllers are identified as PU – Physical Unit type 2.0 (or PU2.1 for APPN), generally hosting LU – Logical Unit types 1, 2, and 3 devices. Local controllers connected via channel attach are managed by VTAM, whereas remote controllers fall under the control of the NCP – Network Control Program housed in front-end processors such as the 3705, 3720, 3725, and 3745.

Third parties

Initial third-party engagement with the 3270 was led by the University of Michigan’s development of the Michigan Terminal System, which expanded hardware compatibility beyond IBM equipment. Numerous manufacturers—including GTE, Hewlett-Packard, Honeywell, Memorex, ITT Courier, McData, Harris, Alfaskop, and Teletype/AT&T—produced terminals compatible with the 3270 or modified ASCII-based terminals such as HP’s 2640 series to emulate block-mode operation featuring screen transmission and validation. The industry distinguished between 'system compatible' devices (which conformed to the 3270 data stream) and 'plug compatible' hardware (which matched IBM’s coaxial interface specifications, enabling interoperability between IBM terminals and third-party controllers). Contemporary business solutions frequently incorporate legacy 3270 systems through software tools that capture screen data for integration into web or graphical user interfaces.

In the early 1990s, the widely adopted Irma expansion card allowed personal computers to connect to mainframe controllers via coaxial cables, facilitating file transfers and emulating IBM 3270 PC protocols.

Models

The IBM 3270 terminal system comprises displays, printers, and controllers. Certain models such as the 3275 and 3277 offer optional components like the selector-pen, a pointing tool enabling field selection, support for ASCII character sets as an alternative to EBCDIC, audible alert features, and keyboard keylocks. A numeric lock on the keyboard restricted input to numbers only within numeric fields. Subsequent upgrades introduced an Operator Identification Card Reader capable of reading magnetic stripe cards, facilitating secure operator verification.

Displays

Most versions of the 3277 support uppercase input exclusively, except for those equipped with mixed EBCDIC/APL or text keyboards, which also allow lowercase characters. On later 3278 and 3279 models, lowercase letters and dead key functionality could be added through an RPQ.

The IBM 3270 PC, released in October 1983, featured built-in 3270 terminal emulation. Later iterations included the 3270 PC/G (graphics-enabled), 3270 PC/GX (enhanced graphics), 3270 Personal Computer AT, and their respective graphics-capable versions.

CUT versus DFT

3270 display units are classified based on where the data stream processing concludes: CUT (Control Unit Terminal) or DFT (Distributed Function Terminal). CUT terminals handle data stream processing within the controller by converting EBCDIC into the '3270 Character Set' for display and translating keyboard input back to EBCDIC. In contrast, DFT terminals pass most of the data stream directly to the terminal itself, which interprets the protocol independently.

DFT terminals support advanced capabilities such as Extended Attributes and Graphics, as well as multi-session functionality allowing up to five concurrent sessions. This feature is exemplified in multisession displays like the 3290 and 3194 models and is commonly implemented in second-generation 3270 terminal emulation software.

The MLT (Multiple Logical Terminals) option on the 3174 controller permits multiple sessions from CUT terminals.

CUT versus DFT

Display Station Name	CUT or DFT
3178 — all variants	CUT
3179 — Model 1	CUT
3179 — Models G1, G2	DFT
3180 — Model 1	CUT
3191 — all variants	CUT
3192 — Models C, D, F, L, W	CUT
3192 — Model G	DFT
3193 — all variants	DFT
3194 — all variants	DFT
3277 — all variants	CUT
3278 — all variants	CUT
3279 — all variants	CUT
3290 — all variants	DFT
3472 — all except model G	CUT
3472 — model G	DFT
3482 — all variants	DFT

3277

• Model 1: terminal with a display of 40×12 characters
• Model 2: terminal featuring an 80×24 character screen; this was the most widely adopted model
• GA: includes RS-232 input/output ports and was frequently paired with Tektronix 4013 or 4015 monochrome graphic displays

3278

• Models numbered from 1 to 5 introduced accented characters and dead key functionality tailored for international users:
  • Model 1: screen size of 80×12
  • Model 2: screen size of 80×24
  • Model 2A: console with reserved lines at 80×24
  • Model 3: switchable between resolutions of 80×32 or 80×24
  • Model 4: switchable between resolutions of 80×43 or 80×24
  • Model 5: switchable between resolutions of 132×27 or 80×24
• Extended Highlighting capabilities encompass blinking text, character set selection, reverse video effects, and underscoring applied at the character level
• Programmed Symbols (PS): customizable characters designed for monochrome graphic representation

Together with the color-capable 3279 display and the accompanying 3287 printer, the introduction of the Extended Display Stream (EDS) protocol by the 3278 enabled support for these advanced display features.

3279

The IBM 3279 marked IBM's debut in color terminals when it was released in 1979. The initial lineup included four distinct models, with a fifth model later introduced specifically for use as a processor console.

• Model Variants
  • 2A: 80 columns by 24 rows, base color display
  • 2B: 80x24, extended color capabilities
  • 2C: 80x24 base color (console version) featuring four reserved lines
  • 3A: 80 columns by 32 rows, base color
  • 3B: 80x32 with extended color support
  • S3G: 80x32 extended color model equipped with a programmed symbol set for graphics

Colors in base color mode were defined by combinations of protection and intensity attributes

Protection	Intensity	Color

In extended color mode, seven colors were available through a dedicated color field and character attributes

Neutral (White), Red, Blue, Green, Pink, Yellow, and Turquoise.

Before the widespread adoption of personal computers, the 3279 served extensively as an IBM mainframe terminal. It was part of the 3270 terminal family and operated using the 3270 data stream. Connectivity was provided via the 3274 controller, which could be channel-attached to an IBM mainframe or connected through SDLC (Synchronous Data Link Control) under the Systems Network Architecture (SNA) protocol as a logical unit type 2 (LU2). Entry-level models (2A and 3A) utilized red and green colors for input fields and blue and white for output. The enhanced models (2B and 3B) supported seven colors and offered an optional Programmed Symbol Set for graphic display—a feature standard on the S3G variant.

Graphics functionality on the IBM 3279—including the Programmed Symbol Set available on models 2B, 3B, and S3G—was integrated with IBM's Graphical Data Display Manager (GDDM). This development was carried out at IBM’s Hursley Development Laboratory located near Winchester, England.

3290

Announced on March 8, 1983, the 3290 Information Panel featured a 17-inch amber monochrome plasma display. It supported multiple operational modes such as four independent terminals modeled after the 3278 model 2 or a single terminal with a resolution of 160×62 characters. The screen could be divided into up to sixteen explicit partitions for logical segmentation. Graphics were enabled through the use of programmed symbols. As a Distributed Function Terminal (DFT), the 3290 required downstream load (DSL) of microcode sourced from floppy disks or hard drives.

317x

• 3178: A cost-effective terminal introduced in 1983
• 3179: Announced on March 20, 1984, this low-cost color terminal featured a compact design with a bright, smudge-resistant 14-inch CRT capable of displaying seven colors and accommodating up to 1,920 characters. It offered a customizable keyboard and provided functionality similar to that of IBM’s earlier Models S2A and S2B from the 3279 series. Shipments began in March 1984 with an initial price point of $2,295.

3180

Released on March 20, 1984, the IBM 3180 was a monochrome terminal supporting various basic and extended display modes. Its basic configuration offered a primary screen size of 24 rows by 80 columns. Extended modes allowed secondary screen sizes ranging from 24×80 up to 27×132 characters. Applications had the capability to override screen dimensions when operating in extended mode. The terminal also supported one explicit partition that could be reconfigured dynamically under application control.

3191

The IBM 3191 Display Station serves as a cost-effective monochrome CRT terminal. Models A and B utilize 12-inch CRTs capable of showing 1920 characters, whereas Models D, E, and L employ 14-inch CRTs that support either 1920 or 2560 characters.

3192

• Model C: 14-inch CRT with seven colors, displaying 80x24 or 80x32 characters
• Model D: green monochrome 15-inch CRT offering character formats of 80x24, 80x32, 80x44, or 132x27
• Model F: high-resolution 14-inch CRT with seven colors and various character layouts
• Model G: seven-color 14-inch CRT supporting 80x24 or 80x32 characters
• Model L: green monochrome 15-inch CRT with selector pen functionality and multiple character configurations
• Model W: black-and-white 15-inch CRT with several character format options

3193

The IBM 3193 Display Station is equipped with a high-resolution, portrait-mode monochrome 15-inch CRT designed to display letter- or A4-sized documents alongside alphanumeric content. It enables compressed image transfer from scanners, which decreases both transmission duration and storage requirements through integrated decompression.

3194

Featuring a built-in 1.44 MB 3.5" floppy disk drive, the IBM 3194 Display Station supports IND$FILE file transfers.

• Model C: color CRT measuring 12 inches, with character formats of 80x24 or 80x32
• Model D: monochrome 15-inch CRT offering multiple character resolutions up to 132x27
• Model H: color CRT of 14 inches supporting various character formats

Subsequent

• 3104: budget-friendly R-loop connected terminal designed for the IBM 8100 system
• 3472 Infowindow

Non-IBM Displays

A number of third-party manufacturers produced IBM-compatible terminals adhering to the IBM 3270 standard.

GTE

GTE developed the IS/7800 Video Display System, which is compatible with IBM’s 3277 terminals connected via a 3271 or 3272 controller. However, an incompatibility related to RA buffer sequencing caused issues when logging into VM/SE systems.

Harris

Harris offered the 8000 Series Terminal Systems compatible with IBM’s 3277 terminals on the 3271 or 3272 controllers. Subsequently, Harris released the Information Processing Systems models ranging from the 9100 to the 9200 series, including units such as the 9178, 9278, 9279-2A, 9279-3G, and the 9280.

Informer 270 376/SNA

Informer Computer Terminals manufactured a specialized version of their model 270 terminal compatible with IBM’s 3270 standard and featuring a coaxial port for connection to a 3x74 controller.

Memorex Telex

• Memorex model 1377 terminal compatible with IBM’s 3277 connects through either the Memorex model 1371 or model 1372

Other Memorex/Telex models include:

• Model 2078
• Model 2079
• Model 2080
• Model 2178
• Model 2179

Technical documentation is available for these plug-compatible devices.

Nokia/Alfaskop

• Alfaskop Display Unit model 4110
• Alfaskop Display Unit model 4112

AT&T

AT&T introduced the Dataspeed 40 terminal/controller in 1980, designed for compatibility with the IBM 3275 terminal.

Graphics models

IBM implemented two distinct graphics technologies for its terminals. The initial method utilized optional Programmed Symbol Sets (PSS) on models such as the 3278, 3279, and the later-standardized 3287; this approach constructed graphics by downloading custom symbols as needed and was standard on the advanced model 3279-S3G (also known as the 3279G).

The alternative solution employed All Points Addressable (APA) or Vector Graphics technology for more efficient image rendering. The first terminal supporting APA was the model 3179G, succeeded by the models 3192G and then the higher-resolution model 3472G.

Both graphic techniques are supported by IBM’s Graphical Data Display Manager (GDDM), which debuted in 1979, as well as SAS’s SAS/GRAPH software suite.

IBM 3279G

Introduced in 1979, the IBM 3279-S3G (commonly referred to as 3279G) succeeded the earlier 3279-3B by offering improved PSS graphics capabilities. It featured support for seven colors through programmable symbol sets that were loaded using GDDM’s Write Structured Field command.

This model incorporated six symbol sets, each containing 190 symbols, amounting to a total of 1,140 programmable characters. Among these, three sets utilized three color planes—red, blue, and green—allowing for seven-color graphic displays.

Depending on the specific version, each character cell was rendered using either a 9x12 or 9x16 dot matrix. Programming individual symbols entailed uploading 18 bytes of data per cell, which represented a significant data load compared to conventional text-only screens.

For instance, rendering a hyperbola required calculating the necessary symbols, loading them into the terminal’s memory, and then painting the screen by specifying cell coordinates and selecting the corresponding symbols.

An optional Attribute Select Keyboard enhanced user interaction by enabling operators to pick attributes, colors, and symbol sets directly, distinguishing this terminal model through improved usability.

IBM 3179G

Unveiled on June 18, 1985, the IBM 3179G was an IBM mainframe terminal supporting either 80×24 or 80×32 character displays with a palette of sixteen colors alongside advanced graphics features. It was notable for being the first terminal—besides the 3270 PC/G series—to offer All Points Addressable (APA) vector graphics. The device included a 14-inch CRT screen delivering vivid colors with a smudge-resistant surface and was priced at $2,295 in 1984.

The terminal architecture separated text and graphics into distinct screen layers: the text layer showed standard 3270 characters, while the graphics layer provided a dedicated 720×384 pixel area capable of rendering vector graphics in sixteen colors. These layers functioned independently and were transmitted via separate data channels.

Software such as IBM’s Graphical Data Display Manager (GDDM) sent vector instructions to the 3179G, which internally converted vectors into raster images. However, since the terminal did not retain graphic data locally, all graphic redraws required fresh transmissions from the host system.

As a distributed function terminal (DFT), the 3179G loaded its microcode from a cluster controller’s floppy disk or hard drive. The G10 variant came with a standard 122-key keyboard layout, while the G20 offered support for the APL programming language using the same keyboard design. Compatibility extended across IBM System/370 mainframes as well as IBM’s 4300 series, 303x, 308x, IBM 3090, and IBM 9370 systems.

IBM 3192G

Released in 1987 as a successor to the 3179G, the IBM 3192G maintained support for sixteen colors and introduced local hardcopy functionality via connection to an IBM Proprinter with graphical output capabilities. Additionally, it facilitated text-only printing through an extra logical unit (LU) dedicated to system printers.

IBM 3472G

Launched in 1989, the IBM 3472G followed the 3192G model and expanded functionality by supporting up to five simultaneous sessions—including one dedicated to graphics. Unlike its predecessor, it eliminated the need for an expansion unit when attaching peripherals such as a mouse or color plotter. Moreover, it accommodated input devices like tablet digitizers and barcode readers for enhanced data capture.

APL / APL2

Beginning with the introduction of the IBM 3277 terminal, IBM provided APL keyboards that allowed direct input of APL’s symbolic instructions. Terminals equipped with an APL character set could display these specialized symbols alongside standard 3270 characters. The graphic-capable 3179G extended this capability further by supporting an expanded set of 138 characters under the APL2 standard—including unique symbols like Diamond and Quad Null. Subsequent models featuring APL2 support included variants of the IBM 3191 (Models D, E, L), the CUT edition of the IBM 3192, and the IBM 3472. This implementation is commonly referred to as APL2 within IBM environments.

Display-Controller

• The 3275 functioned as a remote display unit with integrated controller capabilities, supporting a single printer connection.
• Launched in 1981, the IBM 3276 combined the roles of a remote controller and display terminal, enabling support for up to eight terminals including itself. It came standard with two type A coaxial ports and allowed addition of up to three adapters for extra peripherals. Connectivity to both SNA and non-SNA hosts was possible via BSC or SDLC communication protocols, reaching data rates up to 9,600 bits per second. Its terminal functionalities closely paralleled those found in the 3278 model.

Printers

• IBM’s range of printers encompassed matrix models such as the 3284, 3286, and 4224, the color-capable 3287, the high-speed line printer 3288, and the R-loop connected 3268-1 designed for the IBM 8100 system.
• In 1984, IBM introduced IPDS (Intelligent Printer Data Stream), enabling online AFP document printing with two-way communication that supported text, fonts, images, graphics, and barcodes. The IBM 4224 dot matrix printer was among the devices compatible with IPDS.
• With the emergence of laser printers from companies like HP and Canon in the PC market, users of IBM 3270 systems gained access to alternative printing options by employing protocol converters from firms such as I-data and MPI Tech. These converters emulated IBM’s 3287 printers and were later enhanced to support IPDS.
• The IBM 3482 terminal, released in 1992, incorporated a printer port capable of both host-addressed printing and local screen copying functions.
• Subsequent iterations of the 3174 controller improved the Asynchronous Emulation Adapter (AEA) to accommodate serial-interface printers alongside asynchronous RS-232 terminals.

Controllers

• 3271 remote controller
• 3272 local controller
• 3274 cluster controller (available as channel-attached or remote via BSC or SDLC connections, supporting between 8 and 32 coaxial ports)
• 3174 cluster controller

For both the 3274 and 3174 models, IBM utilized configuration support letters coupled with release numbers to denote specific hardware configurations and microcode capabilities.

By the year 1994, the 3174 Establishment Controller was capable of connecting to multiple hosts using Token Ring, Ethernet, or X.25 networks in addition to traditional channel attach or SDLC links; it supported terminal connections through twisted pair wiring, Token Ring, Ethernet, coaxial cables, as well as TN3270 sessions. Furthermore, it accommodated asynchronous ASCII terminals, printers, and plotters alongside conventional 3270 devices.

3274 controller

Introduced in 1977 as a successor to the 3271–2 series, the 3274 controllers transitioned from fixed hardware features to microcode-based functionality loaded from an internal 8" floppy drive.

The product line included remote controllers with port counts of 8, 12, 16, and 32 as well as local channel-attached units with up to 32 ports—totaling sixteen different models released over time. The 3274-1A was classified as an SNA physical Unit type 2.0 (PU2.0), requiring only one channel address for all connected devices but lacking compatibility with the earlier 3272 model. Conversely, the 3274-1B and -1D variants were designed as local non-SNA compatible units.

This controller introduced Category A coaxial protocol technology that differentiated it from Category B devices such as the IBM 3277 terminal and the IBM 3284 printer. Early Category A devices comprised models like the IBM 3278 and IBM’s first color terminal—the IBM 3279 Color Display Station.

To ensure backward compatibility, coaxial interface boards (referred to as 'panels') could be installed to support Category B equipment—allowing connection of up to sixteen terminals per panel count but fewer if fully loaded with Category A devices.

Throughout its operational lifespan, supported features included:

• Extended Data Stream
• Extended Highlighting
• Programmed Symbol Set (PSS)
• V.24 interface speeds up to 14.4 kbit/s
• V.35 interface speeds up to 56 kbit/s
• X.25 network connectivity
• DFT – Distributed Function Terminal capability
• DSL – Downstream loading for models like the 3290 and 3179G
• Support for multiplexers such as the 9901 and 3299
• Entry Assist functionality
• Dual Logic allowing two simultaneous sessions from a CUT mode display

3174 controller

In 1986, IBM launched the 3174 Subsystem Control Unit as a successor to the 3274 series. This new controller was developed to advance the 3270 product family by introducing enhanced connectivity options and additional functionalities. Differing from the 3274, the 3174 utilized smaller 5.25-inch floppy disks instead of the larger 8-inch versions. Larger floor-standing models featured up to ten adapter slots, some dedicated to channel adapters or serial interfaces. Local configurations could be set up for either local SNA or non-SNA environments, including support for PU2.1 (APPN).

The available models comprised: 01L, 01R, 02R, 03R, 51R, 52R, 53R, 81R, and 82R. Models with the "01L" designation were local channel-attached units; those marked with "R" operated remotely; and models ending with "3R" connected upstream through Token Ring networks. Floor units (0xL/R) accommodated up to 32 coaxial devices utilizing either built-in or external multiplexers such as the TMA/3299. Shelf-mounted units (5xR) offered nine coax ports expandable to sixteen, while desktop variants (8xR) provided four coaxial ports with expansion capacity up to eight.

IBM redesigned the traditional BNC coax connector into a Dual Purpose Connector (DPC), capable of supporting both the conventional 93-ohm unbalanced coaxial cables and the newer 150-ohm balanced twisted-pair wiring without requiring extra adapters.

The initial microcode release, known as Configuration Support A, maintained compatibility with existing hardware while adding features like Intelligent Printer Data Stream (IPDS), Multiple Logical Terminals, Country Extended Code Page (CECP), Response Time Monitor functionality, and Token Ring host interface support.

Configuration Support S introduced the ability for both local and remote controllers to serve as 3270 Token-Ring DSPU Gateways, managing up to 80 downstream Processing Units (PUs).

By 1989, IBM rebranded this line as the 3174 Establishment Controller and expanded the model range with floor units featuring an additional 32 coax ports: models such as 11L, 11R, 12R, 13R, 61R, 62R, 63R, 91R, and 92R were added. Configuration Support B release 1 enhanced Token-Ring gateway capabilities by increasing DSPU support to 250 devices and introduced 'Group Polling' to reduce mainframe/VTAM polling overhead.

Later iterations of Configuration Support B incorporated features like Local Format Storage for CICS screen buffering, Type Ahead input processing, Null/Space handling improvements, and ESCON channel compatibility.

Between 1990 and 1991, seven additional models were launched: 21R, 21L, 12L (which included ESCON fiber optic connectivity), 22L, 22R, 23R, and 90R. The '2xx' series represented rackmount versions equipped with four adapter slots. The model 90R functioned primarily as a Token Ring DSPU gateway featuring one coax port expandable through multiplexers.

Configuration Support C brought ISDN connectivity for downstream PCs via ISDN lines; it also integrated APPN protocols enabling seamless inclusion within APPN networks. Peer Communication allowed PCs connected via coax to access resources on Token-Ring networks.

Subsequent releases from C2 through C6 added capabilities such as Split Screen display functionality, session-to-session Copy operations, Calculator tools, AS/400 host access featuring emulation of the IBM 5250 keyboard layout, advanced APPN enhancements, and TCP/IP Telnet support that permitted simultaneous interaction with mainframe systems and TCP/IP servers using the TN3270 protocol. Additionally, IP forwarding enabled devices connected downstream of the controller to route IP traffic over Frame Relay WAN links.

In 1993, IBM introduced Ethernet Adapter support (model FC3045) alongside new models including the 14R, 24R, and 64R—representing IBM’s final hardware updates for the entire 3174 lineup.

By this stage, floor-standing and rackmount controllers supported a wide array of adapters: Channel adapters; ESCON adapters; Serial interfaces (V.24/V.35); Concurrent Communication Adapters; Coax adapters; Fiber optic “coax” adapters; Asynchronous adapters; ISDN adapters; Token-Ring adapters; Ethernet adapters; and line encryption adapters.

In 1994, IBM incorporated RPQ features into Configuration Support-C release 3 that added TN3270 client functionality to improve TCP/IP network integration.

GTE

The GTE IS/7800 Video Display Systems incorporated two controllers compatible with IBM terminals:

• 7801 (remote controller, analogous to the 3271)
• 7802 (local controller, analogous to the 3277)

Harris

The Harris 8000 Series Terminal Systems accommodated a variety of controllers, including:

• 8171 (remote controller, equivalent to the 3271)
• 8172 (local controller, equivalent to the 3277)
• 8181 (remote controller, equivalent to the 3271)
• 8182 (local controller, equivalent to the 3277)
• 9116
• 9210
• 9220

Home grown

A custom-built establishment controller known as the OEC (Open Establishment Controller) integrates an Arduino shield equipped with a BNC connector and operates via a Python script on POSIX-compliant systems. The OEC facilitates connection of 3270 displays to IBM mainframes through TN3270 or interfaces with other systems using VT100. At present, support is limited exclusively to CUT displays.

Memorex

Memorex developed two controllers for its 1377 terminal, which was compatible with the IBM 3277: the 1371 for remote connections and the 1372 for local use. Subsequently, Memorex introduced controllers compatible with IBM's 3274 and 3174 models, including:

• 2074
• 2076
• 2174
• 2274

Multiplexers

IBM's 3299 device served as a multiplexer connecting a configured 3274 controller outfitted with the 9901 multiplexer feature to up to eight terminals or printers, thereby minimizing coaxial cable usage.

Following the launch of the 3174 controller, multiplexers—either internal or external—became a standard component. The model 3174-1L featured four multiplexed ports, each capable of supporting eight devices. The internal multiplexer card used was the TMA (Terminal Multiplexer Adapter) model 9176.

Several manufacturers, such as Fibronics and Adacom, produced multiplexers designed for TTP – Telephone Twisted Pair and fiber-optic connections as alternatives to coaxial cables.

In certain situations, multiplexers were employed to increase port availability on smaller remote controllers; for instance, the IBM 3299 expanded coaxial port counts from four to eight on models like the 3174-81R/91R or from eight to sixteen on models such as the 3174-51R/61R.

Manufacture

The IBM 3270 display terminal subsystem was engineered at IBM’s Kingston facility in New York, which ceased operations in the mid-1990s. Printer development was undertaken at IBM’s Endicott laboratory in New York. As the subsystem evolved, the design of the 3276 display-controller took place initially at IBM’s Fujisawa site and later at its Yamato laboratory in Japan. The color display model 3279 and color printer model 3287 were developed at IBM Hursley in the United Kingdom. Production occurred across multiple locations: Kingston handled displays and controllers; Endicott focused on printers; Greenock in Scotland produced most components for global distribution. Notably, the 3278 terminals were manufactured in Hortolândia, Brazil into the late 1980s, where they were locally redesigned using CMOS technology while preserving their original design framework.

Telnet 3270

Telnet 3270 (tn3270) denotes both the transmission of 3270 data streams over the telnet protocol and the associated terminal emulation software that replicates a 3270 terminal through this method. This technology allows emulators of the 3270 terminal to operate over TCP/IP networks instead of traditional SNA networks and supports connections for both terminals and printers. Conventional telnet clients cannot substitute for tn3270 clients due to fundamental differences in how data is exchanged. TN3270 is widely utilized for accessing IBM mainframe applications online through VTAM.

3270 Character Encoding

IBM 3270 terminals accommodate a range of keyboards and character encodings. The US English EBCDIC set, utilized by models such as 3275, 3277, and 3284–3286, encompasses both control and printable characters. Optional variants exist for US ASCII and several European language-specific EBCDIC sets, including UK, French, German, and Italian.

On 3275 and 3277 devices that do not support lowercase text, all lowercase letters are rendered as uppercase. Control characters like NL, EM, DUP, and FM usually appear as specific symbols (5, 9, *, ;), unless printer control bits activate their control functions rather than printing them.

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Data Stream Structure

Data transmitted to a 3270 device comprises commands, optionally a Copy Control Character (CCC) or Write Control Character (WCC), device addresses for copy operations, orders, character data, and structured fields. Commands direct the 3270 control unit to execute operations such as reading or writing on targeted devices. Orders define the format of the device buffer, while structured fields carry supplementary control instructions and associated data.

For local non-SNA controllers, the command is represented by a CCW opcode instead of the initial byte in the outgoing display stream. Conversely, in other controllers, the command constitutes the first byte of the display stream after excluding protocol headers.

Command Set

The following table summarizes datastream commands alongside CCW opcodes for local non-SNA controllers (excluding those for local SNA controllers):

Command	local non-SNA	EBCDIC	ASCII	Graphic	Description
Copy	—	F7	37	7	Copies data between device buffers, such as from display to printer
Erase All Unprotected	0F	6F	3F	?	Clears all unprotected fields and resets modified data indicators
Erase/Write	05	F5	35	5	Clears device buffer then writes WCC followed by orders and data
Erase/Write Alternate	0D	7E	3D	=	Clears buffer, sets alternate size, then writes WCC with orders and data (not supported on 3271, 3272, or 3275)
No Operation	03	—	—	—	May be used to retrieve pending status information
Read Buffer	02	F2	22	2	Reads entire device buffer including attribute data
Read Modified	06	F6	36	6	Reads AID, cursor position, and modified fields except in short read mode
Read Modified All	—	6E	3E	:	Reads AID, cursor position, and all modified fields without short read restrictions
Select	0B	—	—	—	Transfers data from device to control unit with channel disconnected
Select RB	1B	—	—	—	Selects Read Buffer operation; can only chain to Read Buffer (models: 3274 D and 3174)
Select RBP	3B	—	—	—	Selects Read Buffer from Position; chains only to Read Buffer (3274 D)
Select RM	DB	—	—	—	Selects Read Modified function (3274 D)
Select RMP	2B	—	—	—	Selects Read Modified from Position (3274 D)
Select WRT	4B	—	—	—	Can only chain to Write or Write Structured Field commands (3274 D)
Sense	04	—	—	—	Retrieves error information following a unit check
Sense ID	E4	—	—	—	Retrieves identification of the control unit
Write	01	F1	31	1	Writes WCC followed by orders and data to device buffer
Write Structured Field	11	F3	—	3	Executes Write Structured Field command
For remote 3270 terminals, non-significant bits are set so that commands correspond to valid EBCDIC or ASCII characters.

Write Control Character

Commands transmitted through Write or Erase/Write operations include a command code, a Write Control Character (WCC), and optionally a buffer containing instructions or data. The WCC governs device functions such as initiating printing, defining print layout, triggering alarms, enabling keyboard input, or resetting Modified Data Tags within the buffer.

Orders

Orders are composed of an order code byte followed by zero to three bytes of variable-length data.

Commands for 3277

Command	Hexadecimal code (EBCDIC)				Explanation
	Byte 1	Byte 2	Byte 3	Byte 4
Start Field (SF)	1D	Attribute character (refer to Attributes)			Defines the beginning of a field at the current buffer location and assigns its attribute
Set Buffer Address (SBA)	11	Address byte 1	Address byte 2		Moves the current buffer pointer to the specified address (see Buffer addressing)
Insert Cursor (IC)	13				Places the cursor at the present buffer position
Program Tab (PT)	05				Advances the cursor to the first position of the next unprotected field
Repeat to Address (RA)	3C	Address byte 1	Address byte 2	Character to repeat	Populates buffer locations from current position up to but not including the specified address with the given character and advances buffer pointer accordingly
Erase Unprotected to Address (EUA)	12	Address byte 1	Address byte 2		Clears unprotected fields starting from current position up to but excluding the specified address

Attributes

The 3270 system recognizes three categories of attributes:

• Field attributes
• Extended attributes
• Character attributes

Field Attributes

The original 3277 and 3275 terminals utilized an eight-bit field attribute byte, employing five significant bits:

• Bits 0 and 1 guarantee that the attribute corresponds to a valid EBCDIC (or ASCII) character.
• Bit 2 distinguishes between unprotected fields, which accept operator input ('0'), and protected fields ('1').
• Bit 3 controls input type: '0' allows alphanumeric characters; '1' restricts input to numeric only, with automatic keyboard shift activation.
• Bits 4 and 5 together determine display characteristics and detection by selector pen:
  • '00'B indicates visible but not pen detectable.
  • '01'B means visible and pen detectable.
  • '10'B represents intensified (bright), visible, and pen detectable.
  • '11'B signifies non-visible, non-printable, and not pen detectable; used for embedding static data accessible during device reads.
• Bit 7 ("Modified Data Tag"): a '0' denotes unaltered by operator; a '1' signals that the field has been modified. This bit may be set programmatically to treat a field as changed.

Later hardware versions introduced support for base colors, enabling four distinct colors on displays and printers by combining field intensify and protection bits. For comprehensive information, consult IBM's documentation on color handling and programmed symbols.

Extended Attributes

The IBM 3278, IBM 3279, and subsequent models incorporated extended attributes to enable features such as seven distinct colors, blinking text, reverse video effects, underlining, field outlining, validation checks, and programmable symbol sets.

Character Attributes

These terminals also supported character-level attributes that could override the attributes assigned to an entire field. This capability allowed applications like the LEXX to apply unique fonts, colors, or styles to individual characters displayed on the screen.

Buffer Addressing

Displays and printers in the 3270 family utilize a buffer where each screen position corresponds to one byte. For instance, the 3277 model 2 featured a 24 by 80 character screen, totaling 1920 bytes. Buffer positions are numbered from zero up to one less than the total screen size (e.g., 0 through 1919). Most operations act upon the "current" buffer address, which automatically advances as data is written. The buffer address can also be explicitly set using the Set Buffer Address (SBA) command, typically followed by Start Field or Start Field Extended. Twelve-bit addressing accommodates displays with up to 1920 characters; larger screens require fourteen or sixteen bits.

Address encoding is performed in two bytes. For twelve-bit addresses, the higher-order bits correspond to valid EBCDIC (or ASCII) characters—for example, address zero is represented as X'4040' (space-space), while address 1919 appears as X'5D7F'. Fourteen- and sixteen-bit addresses use consecutive bits spanning both bytes.

Example

The following data stream demonstrates writing an attribute at row 24, column 1; placing protected characters '> ' at columns 2 and 3; creating an unprotected input field from columns 5 through 79; and inserting an attribute at column 80 to close the input field. This sequence is usually transmitted via an Erase/Write command, where any unspecified positions are reset to '00'x. Hexadecimal values:

  Data        Description
  D3          WCC [reset device + restore keyboard + reset MDT]
  11 5C F0    SBA Row 24 Column 1
  1D F0       SF/Attribute [protected, alphanumeric, normal intensity]
  6E 40       '> '
  1D 40       SF/Attribute [unprotected, alphanumeric, normal intensity]
              SBA omitted here—writing continues at current buffer position
  13          IC — cursor positioned at Row 24, column 5
  11 5D 7F    SBA Row 24 Column 80
  1D F0       SF/Attribute [protected, alphanumeric, normal intensity]

Extended Data Stream

Most terminals introduced after the models such as the 3275, 3277, 3284, and 3286 support an extended data stream (EDS) that provides advanced capabilities including:

• Display buffers larger than 4096 characters
• Expanded field attributes including color options
• Character-level attribute control within fields
• Configurable display geometry
• Queries for terminal characteristics
• Programmable Symbol Sets
• All Points Addressable (APA) graphics functionality

See also

• 3270 emulator
• IBM 5250 display terminal subsystem for IBM AS/400 and IBM System/3X families
• List of IBM products

External links

• Partial IBM history outlining the introduction of the 3270 display system in 1971
• Resources for programming with the 3270 data stream
• rbanffy/3270font: TrueType font recreation of the original 3270 typeface