Green screen – all about terminal emulation

The IBM 3270 terminal

The IBM 3270 Information Display System (and related terminals) was one of the most successful and popular terminals ever made.

First announced by IBM in 1971 it could be connected to the company’s System/370 or System/360 Model 25  mainframe. The 3270 became extremely popular during the 1970’s and later on with the release of the IBM 3090 mainframe in 1985.

There are several key differences to the IBM 3270 terminal when compared to other character cell terminals that are common to UNIX users. The 3270 didn’t use the standard ASCII character set that is used by most computer systems today including Windows and Linux, instead making use of the EBCDIC (Extended Binary Coded Decimal Interchange Code) character set, a legacy from the days when Mainframe systems where programmed via punched cards. EBCDIC was used by all the IBM peripherals at the time – Mainframes system still support EBCDIC, to give backwards compatibility.

The other notable difference between the IBM3270 and most other character cell terminals is that the 3270 uses block mode communication instead of character mode. Block mode has several advantages over character mode communication, especially on the slower connections and slower Mainframes available when released.

With a block mode connection the host will completely draw the screen, populating it with fields in which the required data may be entered. The 3270 terminal will buffer all the data inputted by the user until the user hits the enter key, upon which it will  send all the entered data to the host in one go. Compare this to Character communication where every single character that is typed is sent to the host and then the host echo’s that character back to the terminal screen. This can cause a delay in each character being displayed on the screen, particularly over a slow connection and  it also increases CPU load on the host system.

Block mode was advantageous in that it freed up CPU time on the Mainframe by allowing it to process all the data in one hit, reducing the time spent managing IO.

Block mode also improved the user experience when entering data. Because all data is buffered locally the user can enter that data much faster and see the information on the screen in real time even over a slow remote connection. The down side is that this did create a delay on receiving the next screen after data transmission.

A modern day example of this concept is when you enter information into a web form: The server sends you the initial page and you enter the requested information, however nothing is actually sent to the server until you submit that form. Once the form is submitted, all the information you entered is then sent to the server and only then does the server send you the next screen.

The IBM 3270 terminal started its life with a simple keyboard with no function keys and a monochrome green display. It was that characteristic green display which gave rise to the slang ‘green screen’ which came to refer to not only the 3270 but dumb terminals in general. And this blog, of course.

IBM made several enhancements to the 3270 with the release of several new terminal models, adding 12 function keys and a final model with a total of 24 function keys and 3 PA (‘program attention’) keys.

Enhancements were also made to the screen layout with the release of several models from with improved display dimensions of 12×40 to 24×80, 32×80 and 43×80 characters. Later models allowed for switching between 27×132 & 24×80 modes. Wikipedia provides a nice list of the various models and their display characteristics.

Two of the more notable enhancements to the 3270 were the introduction in 1979 of  the 3279 color display (pictured) and the 3287 color printer. In the same year IBM also introduced the 3270 Kanji Display Terminal supporting more then 11,000 Chinese and Japanese characters.

All these terminals were connected via the IBM’s SNA (Systems Network Architecture) protocol using a coax cable connected to a terminal control unit 3174 or 3274. The 3274 terminal control unit could connect  up to 32 terminals.

The SNA protocol was very popular way of connecting to mainframe systems right through the 1970’s to 1990’s. In the early 90’s the demand for connecting the PC to the host using a terminal emulator was growing. The popular solution to this problem was to use a hardware emulation card or IRMA board that connected your PC to the SNA network using the same coax that was connected to the 3270 terminal. This method of emulating the 3270 terminal was expensive and as non SNA PC networks come on line it meant duplication of networks, one for PC’s and one for Terminals.

As Ethernet networks became more popular for PC the need to connect users to the host via Ethernet increased. The initial solution to this was an SNA Gateways like Microsoft’s SNA Gateway (Microsoft Host Integration Server). This would allow a user to connect their PC loaded with a terminal emulator to the host over an Ethernet connection via the SNA Gateway.

The main problem with SNA Gateways was that each PC required SNA Gateway client software to be installed locally. Several SNA Gateways were released by different companies such as IBM, SCO and Microsoft, just to name a few. The issue was that each implementation was slightly different and the end user required both a client that matched they gateway and  a terminal emulator that supported that specific SNA Client. This generally meant that once you purchased an SNA Gateway you were locked in to a particular emulation product. Microsoft’s SNA Gateway was widely used, and, while it came with a simple 3270 emulator users that required some of more advance features of the terminal would opt for one of the variety of 3rd party emulators available that supported the Microsoft gateway.

Eventually SNA connectivity lost favor for the TN3270 protocol, the 3270’s version of Telnet. The TN3270 differs from the standard telnet used by other terminal emulations such as the DEC VT, and requires an emulation product that has a telnet capable 3270 emulation.

With most PC networks already using TCP/IP for their Windows networking and the need for expensive emulation cards on a dedicated network removed, IBM3270 emulators also became easier to deploy and manage. The number of IBM3270 emulation products in the market also increased, and making it more cost effective to use emulation in place of the actual terminal.

This lead to the decline in popularity of the physical terminal in the office space, instead emulators like Attachmate Extra, Wall Data’s Rumba, WRQ’s Reflection or Turbosoft’s TTWin filled the gap.

However the physical terminals continued to find a home in some areas sch as manufacturing industries where either users didn’t have the need for the extra range of functions that a PC’ could provide, or alternately, the workspaces were subject to difficult conditions (such as very dusty or harsh environments)  where terminals were much better suited to the kind of day to day abuse they would encounter.

By the end of the 90’s many 3270 terminals had been replaced with emulator software or their host systems had been retired. 3270 emulators still enjoy wide usage today, however in some cases they appear in a more modern guise. For example, embedded behind a web page where the emulator is in the background using a technique called screen scraping. This is where the data sent from the host is captured from the emulator and represented to the user in a different form, in this case, over the web. This is usually done through emulation software that provides an API or OCX control.

Bonus extra: All sorts of hardware and software solutions were thrown up in an effort to bring the PC and the 3270 terminal together over the years.  Witness the IBM 5271, the mutant love child of an IBM XT PC and a 3270 terminal. The 122 key keyboard pictured in the link contains 3270 specific keys and the normal keys you’d expect on a 101 key pc keyboard, all mixed into one. The machine was powered by an 8088 processor and ran at a blazing 4.77 MHz,  just like the original IBM XT.

• The IBM 5271