IBM 3270

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The IBM 3270 is a class of block oriented computer terminal (sometimes called display devices) made by IBM originally introduced in 1971[1] normally used to communicate with IBM mainframes. The 3270 was the successor to the IBM 2260 display terminal. Due to the text colour 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.

Although IBM no longer manufactures 3270 terminals, the IBM 3270 protocol is still commonly used via terminal emulation to access some mainframe-based applications. Accordingly, such applications are sometimes referred to as green screen applications. Use of 3270 is slowly diminishing over time as more and more mainframe applications acquire Web interfaces, but some web applications use the technique of "screen scraping" to capture old screens and transfer the data to modern front-ends.

IBM 3277 Display Terminal model 2, the "original" 3270 (1972)
IBM 3279 Colour Display Terminal (1979)

Principles[edit]

The 3270 series was designed to connect with mainframe computers, often at a remote location, using the technology then available in the early 1970s. Two of the major design goals of 3270s are minimizing the amount of data transmitted, and minimizing the frequency of interrupts to the mainframe.

Sample IBM 3270 cluster with one control unit connected to a printer and two displays

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). 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.

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 clusters of contiguous character cells for which numerous "attributes" (colour, highlighting, character set, protection from modification) can be set. An attribute occupied a physical location on the screen that also determined the beginning and end of a "field" (separately addressable sub section of the screen).

Using a technique known as "read modified", 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 was also a "read buffer" capability that transferred the entire content of the 3270-screen buffer including field attributes. This was 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. The data entry keyboard had five PF keys and two PA keys. The operator console keyboard twelve PF keys and two PA keys.[2]:p.19 Later 3270s had twenty-four PF keys and three PA keys. 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 a special code 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.

In this way, the CPU is not interrupted at every keystroke, a scheme that allowed an early 3033 mainframe with only 16 MB to support up to 17,500 3270 terminals under CICS. On the other hand, vi-like behaviour was not possible. (But end-user responsiveness was arguably more predictable with 3270, something users appreciated.) For the same reason, a porting of Lotus 1-2-3 to mainframes with 3279 screens did not meet 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.

Applications[edit]

Following its introduction the 3270 and compatibles were by far the most commonly used terminals on IBM System/360 and successor systems. 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.

The Program Development Facility (PDF) and XEDIT editors for MVS and VM/370 respectively made extensive use of 3270 features.

The modified data tag was well suited to converting formatted, structured punched card input onto the 3270 display device. With the appropriate programming any batch program that used formatted,structured card input could 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.

Imperial Chemical Industries (ICI) Mond Division's Works Records System, the first known shared public spreadsheet, used the 3270 successfully for what was, in effect, a high powered version of today's spreadsheets with additional functions. It remained in continual use for 27 years up until 2001 and, despite its lack of a GUI, cells could be defined anywhere on the screen (not necessarily in rows or columns) and could be instantly re-configured for length, content and formulas as required. It is interesting to note that ICI's online, fully interactive system pre-dated PC spreadsheets by quite a few years and allowed multiple users to use the spreadsheets at the same time, similar to today's Web based shared spreadsheets.

A version of the WordPerfect word processor ported to System/370 was designed for the 3270 architecture.

3270 and The Web (and HTTP) are similar in that both follow a thin client client-server architecture whereby they, the clients, are given primary responsibility for managing presentation and user input. This minimizes host interactions while still facilitating server-based information retrieval and processing.

With the arrival of the web, application development has in many ways returned to the 3270 approach. In the 3270 era, all application functionality was provided centrally. With the advent of the PC, the idea was to invoke central systems only when absolutely unavoidable, and to do all application processing with local software on the personal computer. Now in the web era (and with wikis in particular), the application again is strongly centrally controlled, with only technical functionality distributed to the PC.

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. IRMA also allowed file transfers between the PC and the mainframe.

Third parties[edit]

Many manufacturers, such as Hewlett Packard, Memorex, ITT Courier 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. 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.

Models[edit]

The IBM 3270 display terminal subsystem consisted of displays, printers and controllers. Optional features for the 3275 and 3277 were the selector-pen or light pen, ASCII rather than EBCDIC character set, an audible alarm, and a keylock for the keyboard. A keyboard numeric lock was available would lock the keyboard if the operator attempted to enter non-numeric data into a field defined as numeric.[2] Later an Operator Identification Card Reader was added which could read information encoded on a magnetic stripe card.

Displays[edit]

  • 3277 model 1 : 40×12 terminal
  • 3277 model 2 : 80×24 terminal, the biggest success of all
  • 3277 model 3 : 80×32 terminal
  • 3277 GA : a 3277 with an RS232C I/O, often used to drive a Tektronix 4013 or 4015 graphic screen (monochrome)
  • 3278 models 1-5 : next-generation, with accented characters and dead keys in countries that needed them
    • model 1 : 80x12
    • model 2 : 80×24
    • model 3 : 80×32 or 80x24 (switchable)
    • model 4 : 80×43 or 80x24 (switchable)
    • model 5 : 132×27 or 80×24 (switchable)
  • 3278 PS : programmable characters; able to display monochrome graphics
  • 3279: colour terminal, 4-colour (text) or 7-colour (graphics) version
  • 3290 : a large, amber monochrome plasma display unit, capable of displaying in various modes, including four independent 3278 model 2 terminals, or a single 160×62 terminal; it also supported partitioning.[3]
  • 3178 : lower cost terminal (1983)
  • 3179 : low cost colour terminal (1984)
  • 3104 : low-cost R-loop connected terminal for the IBM 8100 system
  • 3472 Infowindow

(Generally, 3277 models were upper-case only, except for the mixed EBCDIC/APL or text keyboards, which had lower case. Lower-case capability and possibility of dead keys, at first a simple RPQ (Request Price Quotation, tailored on request at extra cost) was only added in 3278 & 3279 models.)

A version of the IBM PC called the 3270 PC, released in October 1983, included 3270 terminal emulation. Later, the 3270 PC/G (graphics) and 3270 PC/GX (extended graphics) followed.

Display-Controller[edit]

  • 3275 remote display with controller function (no additional displays up to one printer)
  • 3276 remote display with controller function (up to a limited number of displays or printers)

Printers[edit]

  • 3284 matrix printer
  • 3286 matrix printer
  • 3287 printer, including a colour model
  • 3288 line printer
  • 3268-1 : R-loop connected stand-alone printer for the IBM 8100 system

Controllers[edit]

  • 3271 remote controller
  • 3272 local controller
  • 3274 cluster controller (different models could be channel-attached or remote via BSC or SDLC communication lines, and had between eight and 32 co-ax ports)
  • 3174 cluster controller

By 1994 the "3174 Establishment Controller" supported features such as attachment to multiple hosts via token ring, ethernet, or X.25 in addition to the standard channel attach or SDLC, and terminal attachment via twisted pair, token ring or ethernet in addition to co-ax. They also supported attachment of asynchronous ASCII terminals, printers, and plotters alongside 3270 devices.[4]

Manufacture[edit]

The IBM 3270 display terminal subsystem was designed and developed by IBM's Kingston, NY, laboratory (which later closed during IBM's difficult time in the mid-1990s). The printers were developed by the Endicott, NY, laboratory. As the subsystem expanded, the 3276 display-controller was developed by the Fujisawa, Japan, laboratory, and later the Yamato laboratory; and the 3279 colour display and 3287 colour printer by the Hursley, UK, laboratory. The subsystem products were manufactured in Kingston (displays and controllers), Endicott (printers), and Greenock, Scotland, UK, (most products) and shipped to users in U.S. and worldwide. 3278 terminals continued to be manufactured in Hortolandia, near Campinas, Brazil as far as late 1980s, having its internals redesigned by a local engineering team using modern CMOS technology, while retaining its external look and feel.[citation needed]

Telnet 3270[edit]

Telnet 3270, or tn3270 describes both the process of sending and receiving 3270 data streams using the Telnet protocol and the software that emulates a 3270 class terminal that communicates using that process. tn3270 allows a 3270 terminal emulator to communicate over a TCP/IP network instead of an SNA network. Standard telnet clients cannot be used as a substitute for tn3270 clients, as they use fundamentally different techniques for exchanging data.

Technical Information[edit]

3270 character set[edit]

The following table shows the 3275/3277/3284/3286 character set for US English EBCDIC. Lower case characters display or print as uppercase. NL, EM, DUP, and FM control characters display and print as 5, 9, *, and ; characters, respectively, except by the printer when WCC or CCC bits 2 and 3 = '00'b, in which case NL and EM serve their control function and do not print.[2]:p.15 Optional characters were available for US ASCII, and UK, French, German, and Italian EBCDIC.

3270 Character Set (US EBCDIC)
_0 _1 _2 _3 _4 _5 _6 _7 _8 _9 _A _B _C _D _E _F
 
0_
 
NUL
0000
0
SOH
0001
1
STX
0002
2
ETX
0003
3


4
PT
0009
5


6


7


8


9


10


11


12


13


14


15
 
1_
 
DLE
0010
16
SBA
0011
17
EUA
0012
18
IC
0013
19


20
NL
0085
21


22


23


24
EM
0019
25


26


27
DUP
001C
28
SF
001D
29
FM
001E
30
ITB
001F
31
 
2_
 


32


33


34


35


36


37
ETB
0017
38
ESC
001B
39


40


41


42


43


44
ENQ
0005
45


46


47
 
3_
 


48


49
SYN
0016
50


51


52


53


54
EOT
0004
55


56


57


58


59
RA
0014
60
NAK
0015
61


62
SUB
001A
63
 
4_
 
SP
0020
64


65


66


67


68


69


70


71


72


73
¢
002E
74
.
002E
75
<
003C
76
(
0028
77
+
002B
78
|
007C
79
 
5_
 
&
0026
80


81


82


83


84


85


86


87


88


89
!
0021
90
$
0024
91
*
002A
92
)
0029
93
;
003B
94
¬
00AC
95
 
6_
 
-
002D
96
/
002F
97


98


99


100


101


102


103


104


105
¦
00A6
106
,
002C
107
%
0025
108
_
005F
109
>
003E
110
?
003F
111
 
7_
 


112


113


114


115


116


117


118


119


120


121
:
003A
122
#
0023
123
@
0040
124
'
0027
125
=
003D
126
"
0022
127
 
8_
 


128
a
0061
129
b
0062
130
c
0063
131
d
0064
132
e
0065
133
f
0066
134
g
0067
135
h
0068
136
i
0069
137


138


139


140


141


142


143
 
9_
 


144
j
006A
145
k
006B
146
l
006C
147
m
006D
148
n
006E
149
o
006F
150
p
0070
151
q
0071
152
r
0072
153


154


155


156


157


158


159
 
A_
 


160


161
s
0073
162
t
0074
163
u
0075
164
v
0076
165
w
0077
166
x
0078
167
y
0079
168
z
007A
169


170


171


172


173


174


175
 
B_
 


176


177


178


179


180


181


182


183


184


185


186


187


188


189


190


191
 
C_
 


192
A
0041
193
B
0042
194
C
0043
195
D
0044
196
E
0045
197
F
0046
198
G
0047
199
H
0048
200
I
0049
201


202


203


204


205


206


207
 
D_
 


208
J
004A
209
K
004B
210
L
004C
211
M
004D
212
N
004E
213
O
004F
214
P
0050
215
Q
0051
216
R
0052
217


218


219


220


221


222


223
 
E_
 


226


227
S
0053
226
T
0054
227
U
0055
228
V
0056
229
W
0057
230
X
0058
231
Y
0059
232
Z
005A
233


234


235


236


237


238


239
 
F_
 
0
0030
240
1
0031
241
2
0032
242
3
0033
243
4
0034
244
5
0035
245
6
0036
246
7
0037
247
8
0038
248
9
0039
249


250


251


252


253


254


255
_0 _1 _2 _3 _4 _5 _6 _7 _8 _9 _A _B _C _D _E _F

Data stream[edit]

Data sent to the 3270 consists of commands and orders. Commands instruct the 3270 control unit to perform some action on a specified device, such a read or write. Orders are sent as part of the data stream to control the format of the device buffer.

The following description applies to the 3271, 3272, and 3275 control units. Later models of 3270 have additional capabilities.

Commands[edit]

Command hexadecimal
Code (local)
Function[2]
Write 01 Writes data to the device
Erase/Write 05 Erases device buffer and writes data
Read Buffer 02 Reads entire device buffer, for example always 1920 bytes on a 3277-2
Read Modified 06 Reads only modified data from device
Copy N/A (Remote only) Copies data from one device buffer to another, for example from a display to a printer attached to the same control unit
Select 0B Transfers data from device to control unit
Erase All Unprotected 0F Erases all unprotected data and resets modified data tags
No Operation 03 May be used to retrieve pending status
Sense 04 Retrieves error information after unit check
For remote 3270s non-significant bits are set so that the command forms a valid EBCDIC (or ASCII) character.

Write Control Character[edit]

The data sent by Write or Erase/Write consists of the command code itself followed by a Write Control Character (WCC) optionally followed by a buffer containing orders or data (or both). The WCC controls the operation of the device. Bits may start printer operation and specify a print format. Other bit settings will sound the audible alarm if installed, unlock the keyboard to allow operator entry, or reset all the Modified Data Tags in the device buffer.

Orders[edit]

Orders consist of the order code byte followed by zero to three bytes of variable information.

Order hexadecimal
Code (EBCDIC)
Byte 1
Byte 2 Byte 3 Byte 4 Description[2]
Start Field (SF) 1D Attribute character
(see Attributes)
Indicates the start of a field at the current buffer position and provides its attribute
Set Buffer Address (SBA) 11 Address byte 1 Address byte 2 Specifies a buffer address to become the current buffer position
(see Buffer addressing)
Insert Cursor (IC) 13 Positions the cursor at the current buffer position
Program Tab (PT) 05 Advances the current buffer address to the first position of the next unprotected field
Repeat to Address (RA) 3C Address byte 2 Address byte 2 Character to repeat Stores the character in byte 4 in all buffer locations starting at the current buffer position up to but not including the specified address
Erase Unprotected to Address (EUA) 12 Address byte 1 Address byte 2 Fills all unprotected fields with nulls starting at the current buffer position up to but not including the specified address

Attributes[edit]

The original 3277 and 3275 displays used an 8 bit attribute byte of which five bits were used.[2]:p.18

  • Bits 0 and 1 are set so that the attribute will always be a valid EBCDIC (or ASCII) character.
  • Bit 2 was zero to indicate that the associated field is unprotected (operator could enter data) or one for protected.
  • Bit 3 was zero to indicate the this field, if unprotected, could accept alphanumeric input. One indicates that only numeric input was accepted, and would automatically shift to numeric for some keyboards.
  • Bit 4 and 5 operate in tandem.
    • '00'B indicate that the field is displayed on the screen and is not selector-pen detectable.
    • '01'B indicates displayable and selector-pen detectable.
    • '10'B indicates that the field is intensified (bright), displayable, and selector-pen detectable.
    • '11'B indicates that the field is nondisplay, nonprintable, and not pen detectable. This last could be used in conjunction with the modified data tag to imbed static data on the screen that would be read each time data was read from the device.
  • Bit 7 is the "Modified Data Tag", zero indicates that the associated field has not been modified by the operator and one indicates that it has been modified. As noted above this bit could be set programmatically to cause the field to treated as modified.

Later models included base colour support for four colours. "In base color mode, the protection and intensity bits are used in combination to select among four colors: normally white, red, blue, and green; the protection bits retain their protection functions as well as determining color."[5] Still later models used extended attributes to add support for seven colours, Blinking, reverse video, underscoring, Field outlining, Field validation, and programmed symbols.[5]

Buffer addressing[edit]

3270 displays and printers had a buffer containing one byte for every screen position. For example, a 3277 model 2 featured a screen size of 24 rows of 80 columns for a buffer size of 1920 bytes. Bytes were addressed from zero to the screen size minus one, in this example 1919. "There is a fixed relationship between each ... buffer storage location and its position on the display screen."[2]:p.13 Most orders started operation at the "current" buffer address, and executing an order or writing data would update this address. The buffer address could be set directly using the Set Buffer Address (SBA) order, often followed by Start Field. For a device with a 1920 character display a twelve bit address was sufficient. Later 3270s with larger screen sizes used fourteen or sixteen bits.

Addresses were encoded in orders in two bytes. For twelve bit addresses the high order two bits of each byte were normally set to form valid EBCDIC (or ASCII) characters. For example, address 0 was coded as '4040'x, or space-space, address 1919 was coded as '5D7F'x, or ')"'.[2]:pp.75-90 Programmers hand coding panels usually kept the table of addresses from the 3270 Component Description or the 3270 Reference Card handy.

Example[edit]

The following data stream writes an attribute in row 24, column 1, writes the (protected) characters '> ' in row 24, columns 2 and 3, and creates an unprotected field on row 24 from columns 5-79. Because the buffer wraps around an attribute is placed on row 24, column 80 to terminate the input field. This data stream would normally be written using an Erase/Write command which would set undefined positions on the screen to '00'x. Values are given in hexadecimal.

  Data              Description
  D3                WCC [reset device + restore (unlock) keyboard + reset MDT]
  11 5C F0          SBA Row 24 Column 1
  1D F0             SF/Attribute
                    [protected, alphanumeric, display normal intensity, not pen-detectable, MDT off]
  6E 40             '> '
  1D 40             SF/Attribute
                    [unprotected, alphanumeric, display normal intensity, not pen-detectable, MDT off]
                    SBA is not required here since this is being written at the current buffer position 
  13                IC - cursor displays at current position: Row 24, column 5
  11 5D 7F          SBA Row 24 Column 80
  1D F0             SF/Attribute
                    [protected, alphanumeric, display normal intensity, not pen-detectable, MDT off]

Notes[edit]

  1. ^ IBM Corporation. "DPD Chronology 1971". IBM Archives. Retrieved 11 February 2013. 
  2. ^ a b c d e f g h IBM Corporation (1972). IBM 3270 Information Display System Component Description. 
  3. ^ "BMS Partition Support". IBM. Retrieved 21 May 2009. 
  4. ^ IBM Corporation (1994). 3174 Establishment Controller/Networking Server Installation Guide. 
  5. ^ a b IBM Corporation. "3270 field attributes". Retrieved 7 July 2012. 

See also[edit]

References[edit]

External links[edit]


This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.