Editing IBM 3270 (section)

==Principles==
The 3270 series was designed to connect with mainframe computers, often at a remote location, using the technology then available in the early 1970s.<ref name=I.7/> 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 303X#IBM 3033|IBM 3033]] mainframe fitted with only 16&nbsp;MB of [[main memory]] was able to support up to 17,500 3270 terminals under [[CICS]].

[[File:IBM 3270 cluster diagram.jpg|thumb|left|Sample IBM 3270 cluster with one control unit connected to a printer and two displays]] 
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).<ref>{{cite magazine  |magazine=[[Computerworld]]  |date=May 13, 1985  |page=103 |title=3270 clusters
   |url=https://books.google.com/books?id=WKyJOEl_vlsC}}</ref> 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 [[Bus and Tag|channel]] of a nearby mainframe. A ''remote'' control unit is connected to a communications line by a [[modem]]. Remote 3270 controllers are frequently ''[[Telecommunications link#Multipoint|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 93{{nbsp}}ohm RG-62 coaxial 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 {{Cvt|1500|m|ft}} are supported, although IBM documents routinely stated the maximum supported coax cable length was {{Cvt|2000|ft|m}}.{{sfn|3270 An Introduction|pp=2-2,8-1}}{{sfn|3270 Maintenance Handbook|pp=1–3}} Originally devices were equipped with [[BNC connector]]s, which were later replaced with special ''dual-purpose connectors'' (''DPCs'') supporting the IBM shielded twisted-pair cabling system without the need for red [[balun]]s.

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,{{efn|When the Structured Field and Attribute Processing Option is available|name=extended}} highlighting, character set,{{efn|name=extended}} and protection from modification, can be set.<ref>{{FOLDOC|IBM+3270}}</ref> A field attribute occupies a physical location on the screen that also determines the beginning and end of a field. There are also{{efn|name=extended}} character attributes associated with individual screen locations.

Using a technique known as "read modified",<ref>{{cite web
   |title=Formatted screen input
   |website=[[IBM]]
   |date=April 27, 2004
 |url=https://www.ibm.com/support/knowledgecenter/en/SSGMCP_4.2.0/com.ibm.cics.ts.applicationprogramming.doc/topics/dfhp31g.html
   |quote=.. reads a 3270 screen with a ''read modified'' command}}</ref> 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 [[Central processing unit|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<ref>{{cite web
   |title=Reading from a 3270 terminal
   |website=[[IBM]]
   |url=https://www.ibm.com/support/knowledgecenter/en/SSGMCP_5.4.0/applications/designing/dfhp3b8.html
   |quote=There are two ... , read buffer and read modified.}}</ref> 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 [[function key|programmed function (PF) keys]], and a 78 key version with twelve. Both versions had two ''Program Attention'' (PA) keys.<ref name="horswill">{{cite web
   |url=https://www.ibm.com/support/knowledgecenter/en/SSGMGV_3.1.0/com.ibm.cics.ts31.doc/dfhp3/dfhp3b5.htm
   |title=Input from a 3270 terminal
   |website=[[IBM]]
   |quote=PA (Program Attention) key}}</ref><ref>{{cite book
   |title=Designing and Programming CICS Applications
   |url=https://books.google.com/books?isbn=1565926765  |isbn=1-56592-676-5
   |author=John Horswill  |date=2000
   | publisher="O'Reilly Media, Inc." |quote=DFHPA1 is Program Attention (PA) Key 1}}</ref> The ''data entry keyboard'' had five PF keys and two PA keys. The ''operator console keyboard'' had twelve PF keys and two PA keys.<ref name=CompDesc />{{rp|p.19}} Later 3270s had an Attention{{efn|SNA only, causes an ''expedited flow'' attention signal even if the keyboard is locked.}} key, a Cursor  Select{{efn|Pressing Cursoe Select has the same effect as clicking a Selector Pen at the current cursor location.}} 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]]{{efn|For local non-SNA controllers. If attached to an [[Systems Network Architecture|SNA]] controller, it initiates a Read Modied or Short AID data transfer. For BSC controllers it causes a positive response to the next poll.}} 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 (text editor)|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.