3270 - A Brief History

Posted by Documentation Fri, 12 Sep 2008 13:38:00 GMT

Recently I was surfing the Web and found something about my company, HOB, at Wikipedia. In the German Wikipedia I found something about 3270 which I believe does not cover the technical background. And also, I think there is something wrong: The article says Attachmate produced 3270 terminals in the past. I remember Attachmate produced 3270 coax cards, but never real terminals.

My name is Klaus Brandstaetter, I was born in 1954, and since 1981 I am CEO of HOB Germany, which was deeply involved in the 3270 market. As I know much about 3270 I decided to write something about 3270. As 3270 was a billion dollar market in the 70's and 80's, and 3270 is still in use today, there is much to say. I will concentrate on the important facts only, but still this will be a big article.

IBM with the /370 mainframe system was the dominant IT company in the 70's and 80's. At IBM, each product they sold had a four digit number, so the terminal they developed for the /370 mainframe got the number 3277. The first display had a very small screen displaying only 12 rows with 40 characters. The terminal had a white cabinet made from sheet metal. This was called Model one, and there was also the Model two which had the same cabinet but a bigger screen of 15 inches displaying 24 rows with 80 characters each. The characters were displayed in green on black background. Some say WYSIWYG, in the case of 3270, stands for "What You See is what IBM Gave You." Each 3277 terminal was connected over a coaxial cable to a control unit. The transmission speed of this coaxial cable was at first something around one megabit per second. Later, with the next generation, 2.3 megabits per second were achieved. A terminal did have some logic, but no CPU; each terminal, of course, had a screen buffer (memory) where the characters to be displayed were stored.

If each terminal would have been connected to the mainframe directly, each keystroke would have generated an interrupt on the mainframe. At that time, there was only limited computer power, so it was not possible to process a lot of interrupts in a given time. So IBM chose to connect each terminal to a so-called control-unit. The IBM part number of the first control unit was 3272, later 3274 and 3174 followed. The first control units had a CPU, but as memory was very expensive the CPU used the screen buffer of the terminals as memory. Commands which access the memory on the terminals were exchanged over the coaxial cable between control unit and terminal. The terminals that worked that way were also called CUT, Control Unit Terminal. Each control unit could handle up to 32 terminals, later a control unit could handle up to 128 terminals. The control unit was connected to the mainframe (channel) with a so-called Bus/Tag cable. This cable had eight parallel wires for the data and another eight parallel wires for the /370 channel command. A control-unit could also be used remotely: typically used were fullduplex lines with a bandwidth of 9,600 bits per second. Can you imagine 32 users today working over a single telecommunications line with just 9,600 Bits per second? But in the 80's this was common and the users did not complain. It is also worth mentioning that 3270 terminals, especially the 3277 one, could also be connected to an IBM /3 computer, which was less powerful that the /370 mainframe. The IBM /3 was the ancestor of what later would become the /34, /38, /36, for a long time AS/400 and what is now called the i-Series. With the IBM /3 computers, 3277 terminals were used with coaxial cable, but the successors got 5250 terminals with a more sophisticated protocol, connected over twinax cable, which is also called twisted pair.

For the 3270 terminal to work, the software in the mainframe had to use the 3270 protocol. Typically in the 3270 protocol the EBCDIC character set was used. A terminal could display up to 192 different characters, the remaining byte encodings were orders and attributes. The application in the mainframe sent either a complete screen or a part of it, mixed with orders. Such orders included setting the cursor, moving to other locations on the screen and orders to fill in attributes.

Such attributes separated the screen into different fields, a part of them editable by the user, others were protected. When the mainframe application sent data to the screen, the user could edit the page displayed and navigate around on the screen. At all this was independent of the host, the mainframe. When all the page editing was done, the user had to press special keys, either Enter, PFx (Program Function) or PAx (Program Attention), and only then was all edited text sent to the mainframe, to the receiving application.

In this way, computers with, compared to today, relatively small computer power could handle the traffic of many terminals, in some installations thousands or even tens of thousands.

This type of processing is similar to what we got with HTML / WEB 1.0, and is called transactional processing.

When we look back at the history of 3270, the first generation of terminals was 3277, 3275 and 3272 as control unit. After that, the next generation consisted of the terminals 3278, 3276 and 3274 as control unit. With the 3278 there was the model 2, displaying 24 x 80 characters. Additional new models were added: model 3 with 32 x 80 characters, model 4 with 43 x 80 characters and model 5 with 27 x 132 characters. Model 5 displaying 132 characters in a line was useful to view print output, since printers at that time usually printed lines 132 characters wide. All models had an additional line to display status information. All terminals had the capability to switch to the model 2 mode, displaying only 24 x 80 characters. There followed the color terminal 3279, which could display the characters in up to seven different colors. A special and very expensive terminal, the 3279G, could also handle graphics. Graphics were built by loading special characters with pixels in the desired colors and then addressing these characters in the data stream sent to the terminal. It should also be mentioned that as a base functionality, most of the IBM terminals had an additional, built-in character set called APL characters. APL characters also could be entered from the keyboard. APL stands for A Programming Language, and APL characters were made from the Greek Alphabet.

Besides the graphics addressable through loaded characters, IBM later added vector graphics.

In the 3270 family of terminals, IBM had many different models, for example 3277, 3278, 3178, 3180, 3179, and so on. The different generations of control units were called 3272, 3274 and at last 3174. There were also terminals with a small control unit built-in, like 3275 and 3276. Many different printers were sold, some of them with the number 3286 or 3287. The printers were connected to the control unit over coaxial cable just as the terminals. For the software, IBM tried to have printers that were programmed similarly to a terminal, sending more than one line to the printer in a chunk of the data stream.

IBM sold millions of the 3270 terminals, mainly from the 3278 model and later models. But there was also competition from other manufacturers, such as Memorex, Telex, HOB, Lee Data, Ericson, Nokia, MDS, ADI and Fujitsu. Memorex and Telex later merged into one company. Altogether there were about 15 different vendors producing 3270 terminals. Most just copied IBM's models, some had unique add-ons. Only HOB succeeded in manufacturing a 3270-compatible terminal with graphics support. Some of the vendors also built their own control unit and used a different cabling schema. The company McData was very successful building 3270-type control units.

The market for 3270 terminals was lucrative until the middle of the 90's. By this time standard PCs had already taken over the business with hardware and software emulating 3270 terminals.

Look at this market now: IBM sold the first IBM PC in August 1981 after just 9 months of development time. IBM then, for a certain time, dominated the PC market. Soon after IBM brought out the first PC, the company IDEA developed and sold a so-called Irma coax card, fitting into the IBM PC with the standard bus. This also included software running in MS DOS. Hardware and software both connected to an IBM (or compatible) control unit, and the user could do what he normally would have done with a hardware terminal. This solution also included file-transfer in both directions between the IBM mainframe and the PC.

After that, IBM also developed such a coax card, and other manufacturers did the same. The emulation software from IBM was called Personal Communications or in short, PersCom. Let us name some of the other manufacturers and their products:

Attachemate with Extra!

Wall Data with Rumba

WRQ with Reflection

HOB with HOBLink 3270, later HOBLink Terminal Edition

At the end of the 90's, 3270 emulation was a big market and about one hundred companies had developed such software.

There also are 3270 emulations written in Java, HoD = Host-on-Demand from IBM or HOBLink J-Term (Java Terminal).

Software in use where the 3270 client is connected to: IMS Information Management System CICS Customer Information and Control System TSO Time Sharing Option and many others.

Software that generates 3270 data stream with graphics includes IBM GDDM (Graphical Data Display Manager) and software from SAS.

In the beginning, the 3270 data stream was sent over simple protocols such as channel-attached or a family of protocols called BSC (binary synchronous communication). Around 1970, IBM invented VTAM and SNA, System Network Architecture. SNA dominated the network protocols in use for a long time. Nowadays SNA is only used inside of the IBM mainframes, all real network connections are done over TCP/IP. For 3270 emulations this protocol (a type of telnet) is called TN3270 or TN3270E, defined in several RFCs.

Prominent people of the IT industry had made bets that the last mainframe would be switched off December 31st, 1999. As we know, this did not happen, and mainframes are still in use by many big organizations. And the 3270 protocol is also still in use.

Some time after the year 2000, an IBM marketing campaign was saying that every day there are still more 3270 transactions than there are transactions over the World-Wide-Web.

12.08.08 Klaus Brandstätter

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