The IBM 3270 is a family of block oriented display and printer computer terminals introduced by IBM in 1971[1] and normally used to communicate with IBM mainframes. The 3270 was the successor to the IBM 2260 display terminal.[2] Due to the text color 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.
IBM no longer manufactures 3270 terminals, but the IBM 3270 protocol is still commonly used[3] via TN3270 clients, 3270 terminal emulation or web interfaces to access mainframe-based applications, which are sometimes referred to as green screen applications.
The 3270 series was designed to connect with mainframe computers, often at a remote location, using the technology then available in the early 1970s.[1] The main goal of the system was to maximize the number of terminals that could be used on a single mainframe. To do this, the 3270 was designed to minimize the amount of data transmitted, and minimize the frequency of interrupts to the mainframe. By ensuring the CPU is not interrupted at every keystroke, a 1970s-era IBM 3033 mainframe fitted with only 16 MB of main memory was able to support up to 17,500 3270 terminals under CICS.
Most 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).[4] 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.
IBM 3270 devices are connected to a 3299 multiplexer or to the cluster controller, e.g., 3271, 3272, 3274, 3174, using RG-62, 93 ohm, coax cables in a point to point configuration with one dedicated cable per terminal. Data is sent with a bit rate of 2.3587 Mbit/s using a slightly modified differential Manchester encoding. Cable runs of up to are supported, although IBM documents routinely stated the maximum supported coax cable length was . Originally devices were equipped with BNC connectors, which later was replaced with special so-called DPC – Dual Purpose Connectors supporting the IBM Shielded twisted pair cabling system without the need for so-called red baluns.
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 fields (clusters of contiguous character cells) for which numerous field attributes, e.g., color, highlighting, character set, protection from modification, can be set. A field attribute occupies a physical location on the screen that also determines the beginning and end of a field. There are also character attributes associated with individual screen locations.
Using a technique known as read modified,[5] 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[6] 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.[7] [8] 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 an Attention key, a Cursor Select key, a System Request key, twenty-four PF keys and three PA keys. There was also a TEST REQ key. 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 an Attention ID (AID) 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.
A downside to this approach was that vi-like behavior, responding to individual keystrokes, was not possible. For the same reason, a port 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. But end-user responsiveness was arguably more predictable with 3270, something users appreciated.
Following its introduction the 3270 and compatibles were by far the most commonly used terminals on IBM System/370 and successor systems.[9] IBM and third-party software that included an interactive component took for granted the presence of 3270 terminals and provided a set of ISPF panels and supporting programs.
Conversational Monitor System (CMS) in VM has support for the 3270[10] continuing to z/VM.
Time Sharing Option (TSO) in OS/360 and successors has line mode command line support and also has facilities for full screen applications, e.g., ISPF.
Device Independent Display Operator Console Support (DIDOCS) in Multiple Console Support (MCS) for OS/360 and successors supports 3270 devices and, in fact, MCS in current versions of MVS no longer supports line mode, 2250 and 2260 devices.
The SPF and Program Development Facility (ISPF/PDF) editors for MVS and VM/SP (ISPF/PDF was available for VM, but little used) and the XEDIT editors for VM/SP through z/VM make extensive use of 3270 features.
Customer Information Control System (CICS) has support for 3270 panels. Indeed, from the early 1970s on, CICS applications were often written for the 3270.[7]
Various versions of Wylbur have support for 3270, including support for full-screen applications.
McGill University's MUSIC/SP operating system provided support for 3270 terminals and applications, including a full-screen text editor, a menu system, and a PANEL facility to create 3270 full-screen applications.
The modified data tag is well suited to converting formatted, structured punched card input onto the 3270 display device. With the appropriate programming, any batch program that uses formatted, structured card input can be layered onto a 3270 terminal.
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.
A version of the WordPerfect word processor ported to System/370 was designed for the 3270 architecture.
3270 devices can be a part of an SNA – System Network Architecture network or non-SNA network. If the controllers are SNA connected, they appear to SNA as PU – Physical Unit type 2.0 (PU2.1 for APPN) nodes typically with LU – Logical Unit type 1, 2, and 3 devices connected. Local, channel attached, controllers are controlled by VTAM – Virtual Telecommunications Access Method. Remote controllers are controlled by the NCP – Network Control Program in the Front End Processor i.e. 3705, 3720, 3725, 3745, and VTAM.
One of the first groups to write and provide operating system support for the 3270 and its early predecessors was the University of Michigan, who created the Michigan Terminal System in order for the hardware to be useful outside of the manufacturer. MTS was the default OS at Michigan for many years, and was still used at Michigan well into the 1990s.Many manufacturers, such as GTE, Hewlett-Packard, Honeywell/Incoterm Div, Memorex, ITT Courier, McData, Harris, Alfaskop and Teletype/AT&T created 3270 compatible terminals, or adapted ASCII terminals such as the HP 2640 series to have a similar block-mode capability that would transmit a screen at a time, with some form validation capability. The industry distinguished between 'System compatible controllers' and 'Plug compatibility controllers', where 'System compatibility' meant that the 3rd party system was compatible with the 3270 data stream terminated in the unit, but not as 'Plug compatibility' equipment, also were compatible at the coax level thereby allowing IBM terminals to be connected to a 3rd party controller or vice versa. 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.
In the early 1990s a popular solution to link PCs with the mainframes was the Irma board, an expansion card that plugged into a PC and connected to the controller through a coaxial cable. 3270 simulators for IRMA and similar adapters typically provide file transfers between the PC and the mainframe using the same protocol as the IBM 3270 PC.
The IBM 3270 display terminal subsystem consists of displays, printers and controllers.Optional features for the 3275 and 3277 are the selector-pen, ASCII rather than EBCDIC character set, an audible alarm, and a keylock for the keyboard. A keyboard numeric lock was available and will lock the keyboard if the operator attempts to enter non-numeric data into a field defined as numeric.[11] Later an Operator Identification Card Reader was added which could read information encoded on a magnetic stripe card.
Generally, 3277 models allow only upper-case input, except for the mixed EBCDIC/APL or text keyboards, which have lower case. Lower-case capability and dead keys were available as an RPQ (Request Price Quotation); these were added to the later 3278 & 3279 models.
A version of the IBM PC called the 3270 PC, released in October 1983, includes 3270 terminal emulation. Later, the 3270 PC/G (graphics), 3270 PC/GX (extended graphics), 3270 Personal Computer AT, 3270 PC AT/G (graphics) and 3270 PC AT/GX (extended graphics) followed.
There are two types of 3270 displays in respect to where the 3270 data stream terminates. For CUT (Control Unit Terminal) displays, the stream terminates in the display controller, the controller instructs the display to move the cursor, position a character, etc. EBCDIC is translated by the controller into '3270 Character Set', and keyboard scan-codes from the terminal, read by the controller through a poll, is translated by the controller into EBCDIC. For DFT (Distributed Function Terminal) type displays, most of the 3270 data stream is forwarded to the display by the controller. The display interprets the 3270 protocol itself.
In addition to passing the 3270 data stream directly to the terminal, allowing for features like EAB — Extended Attributes, Graphics, etc., DFT also enabled multi sessions (up to 5 simultaneous), featured in the 3290 and 3194 multisession displays. This feature was also widely used in 2nd generation 3270 terminal emulation software.
The MLT — Multiple Logical Terminals feature of the 3174 controller also enabled multiple sessions from a CUT type terminal.
| Display Station Name | CUT vs. DFT | |
|---|---|---|
| 3178 — all models | CUT | |
| 3179 — Model 1 | CUT | |
| 3179 — Models G1, G2 | DFT | |
| 3180 — Model 1 | CUT | |
| 3191 — all models | CUT | |
| 3192 — Models C, D, F, L, W | CUT | |
| 3192 — Model G | DFT | |
| 3193 — all models | DFT | |
| 3194 — all models | DFT | |
| 3277 — all models | CUT | |
| 3278 — all models | CUT | |
| 3279 — all models | CUT | |
| 3290 — all models | DFT | |
| 3472 — except model G | CUT | |
| 3472 — model G | DFT | |
| 3482 — all models | DFT |