3270 Terminal Overview
The 3270 terminal family consists
of a controller, CRTs and printers. The controller can be connected through a modem
(3174, 3271, 3274, or 3276) or directly to the computer's channel (3174, 3272, or 3274).
Each CRT and printer is connected to the controller by a coax cable to a separate device
adapter inside of the controller.
The controller interfaces
the terminals to the network. The device adapter performs the
device emulation. There are many different models of the 3270 terminals and printers.
In general, the differences between the terminals are their introduction date, screen
size, and attribute support.
All JBM products emulate
modem-attached 3174 (Model C) controllers.
3270 Models
The 3270 CRT supports a variety of screen (buffer) sizes for CRTs and printers. These sizes are:
| Model |
Rows |
Columns |
Buffer |
| One |
12 |
40 |
480 |
| Two |
24 |
80 |
1920 |
| Three |
32 |
80 |
2560 |
| Four |
43 |
80 |
3440 |
| Five |
27 |
132 |
3564 |
Screen Formatting
Bisync and SNA/SDLC data streams consist of control codes and data formatting codes.
The control codes used for the Bisync or SDLC protocol are different. The data formatting codes
are similar for both protocols. The individual terminal device determines what data formatting
commands are supported.
| Field Attributes |
These attributes require a
position in the screen buffer |
| |
Screen Formatting |
Unprotected
Protected
Numeric Only
Alphabetic Only |
| |
Video Presentation |
Intensified (High) Intensity
Non display |
| |
Base Color |
Green (normal intensity and unprotected)
Red (Intensified and unprotected)
Blue (normal intensity and protected)
White (Intensified and protected) |
| |
Tab Stops |
|
| |
Changed Fields (see Read Commands) |
| |
|
| Extended Attributes |
These attributes were introduced with
the 3279 color terminal and reside in the screen position used by the Field Attribute. All of
these attributes are a subset of Structured Fields |
| |
Structured Fields (the screen
buffer is divided into separate partitions) |
| |
Video Presentation |
Blink
Reverse Video
Underscore |
| |
Extended Color |
Blue, Red, Pink, Green, Turquoise, Yellow |
| |
Programmed Symbols (special
character set used for graphics) |
Read Commands
The 3270 uses a bit in the attribute
to determine which field is sent to the host when an Attention Identifier (AID) key is entered.
When the operator (or host application) makes an entry into an unprotected field, the
Modified Data Tag (MDT) is set. When the ENTER key is pressed, the terminal scans the
screen buffer to determine which fields have the MDT set. The modified fields are then
assembled into a block for transmission to the host.
The host can use Read Commands to
have all or part of the terminal's screen buffer sent to the host.
Transmissions
The 3270 terminal uses an AID key
to cause a transmission to the network. The AID code is included in the transmission.
Some of the AIDs also send the contents of the screen buffer. Other AIDs only send the
AID character. The 3270 AID keys are: Enter, Program Function (PF), Program Access (PA),
Clear, Cancel, Test Request, System Request, and Attention.
Status Line
The 3270 terminal uses a Status Line
(Operator Information Area) to inform the operator of the terminal's status and network activity.
Error messages are also displayed. With SNA/SDLC protocol, the Logical Unit connection status
is displayed.
Printing
The 3270 printer can support two
different types of printer formatting commands:
- 3270 attributes are used to
format a screen image in the controller and then print the screen image;
- Special SNA character commands
called SCS (SNA Character String) are used to format the print.
Status and Sense Information
The 3270 terminal uses Status and
Sense (S/S) bytes to report error conditions to the host application. The S/S Bytes provide
detailed information of the error condition encountered.
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