IBM’s 3270 technology allows a user to communicate interactively with a host computer. Previously, communication was achieved using an expensive 3270 controller with several display terminals attached. The BARR/3270 package, which emulates a 3270 controller with up to four 3270 sessions on an IBM PC, PS/2, or compatible, provides a low-cost alternative. This chapter presents background on 3270 technology.
The IBM 3270 Information Display System is IBM’s solution for interactive communication with a mainframe. 3270 architecture specifies a set of rules for communication between a mainframe and a controller. Users interact with display terminals, printers, plotters, and other devices attached to the controller.
Mainframe and controller communicate using a 3270 data stream that contains data as well as commands and display orders used for formatting a screen of input fields. Typically, the mainframe may send a data stream to display an order entry screen with several input fields to the controller, which processes the input data stream and paints the screen on a display terminal. The user fills in the input fields and sends the screen to the controller by pressing an attention identifier key (AID). The controller reads the screen, creates a 3270 data stream, and sends it to the mainframe for processing. A diagram of the data flow between the mainframe and the controller is shown in the following figure. The rest of Chapter 1 describes the function of each component in the diagram, starting at the mainframe end.
The host is a mainframe computer that runs the 3270 application and provides the storage for related data. The 3270 application typically runs in a time-sharing environment where several users can be connected to the same application on the host. The host communicates with a 3270 controller by establishing a session between itself and the controller. This session serves as a communication channel between the controller and the mainframe. The program at the host that establishes and manages this channel is known as the System Sessions Control Program (SSCP). Frequently, the SSCP is also referred to as the Systems Operator. The host end of the communication channel is often called the PLU (Primary Logical Unit) and the controller end the SLU (Secondary Logical Unit). VTAM (Virtual Telecommunications Control Program) is the most widely used SSCP.
A communications link is a mechanism for transferring information between two points. Two commonly used communications links are SDLC and Token Ring. The SDLC communications link uses modems to transfer data over telephone lines to remote locations. The Token Ring communications link is used for high-speed communications over Local Area Networks (LANs).
The host computer uses communications controllers to talk to remote terminals over communication links. Management of the communication and the flow of information on these links is performed by the Network Control Program (NCP) in the Communications Controller.
A cluster controller connects several display terminals and printers to the host. The controller manages the flow of data between each terminal and the host. At one end, the cluster controller is either locally attached to the host or remotely attached by a communications link. At the other end, the cluster controller is attached to display terminals or printers by COAX cables (the familiar cable used by cable TV) or a Token Ring network (only supported by the IBM 3174 Establishment Controller).
Two protocols are used for connecting display terminals and printers to the cluster controller: CUT and DFT. If the CUT protocol is implemented, the controller processes an incoming 3270 data stream and also provides storage for the 3270 buffer. The display terminal acts as a dumb terminal and can have only one logical session. The original 3270 terminals operated in the CUT mode.
When the connection protocol is DFT, the display terminal processes the 3270 data stream and provides the buffer storage. With this protocol, the display terminal acts as a "smart" terminal and can maintain several (usually four) logical sessions. BARR/3270 uses the DFT protocol.
The 3270 terminal enables a user to communicate interactively with the controller. Terminal models vary in screen sizes, colors, and highlighting features supported. 3278 terminals are monochrome; 3279 models are color. Models numbered with an A extension do not support extended highlighting, while B extensions do. The screen size defined for a 3270 terminal is indicated by its model number:
Model 2: 24 lines by 80 columns
Model 3: 32 lines by 80 columns
Model 4: 43 lines by 80 columns
Model 5: 27 lines by 132 columns
The terminal model type BARR/3270 uses depends on the screen size, color, and extended highlighting features supported by the mainframe application.
Printer models differ in print functions, typeface styles and fonts, margin controls, and other features supported. Two types of 3287 printers are widely used:
DSC or LU3 printer: This original 3270 printer allows basic print control functions, processes the 3270 data stream like a display terminal, and prints the buffer.
SCS or LU1 printer: This printer has more advanced printer functions and formatting commands than LU3. BARR/3270 uses the LU1 printer type.
A 3270 printer can be connected directly to the controller or locally attached to a display terminal. In a locally attached configuration, either host or operator can initiate printing.
BARR/3270 on a PC emulates:
A 3274 controller with connected display terminals and printers.
Display terminal models 2, 3, 4, and 5 with screen sizes 24x80, 32x80, 43x80, and 27x132, respectively, along with extended attributes in monochrome or color.
3287 printers in either the DSC or SCS modes. Print files can be directed to a printer attached to the PC or to a disk file. Host-initiated local print functions are supported.
IBM’s EHLLAPI (Enhanced High Level Language Application Program Interface).
IND$FILE file transfer protocol between PC and host.
The 3270 software communicates with the host through the BARR/GATE LAN gateway, a dial-up line, or a dedicated line. BARR/3270 software supports both SDLC and Token Ring communication protocols when connected directly to the host. When connected through the BARR/GATE gateway, NETBIOS, NOVELL/IPX, and Token Ring 802.2 communication protocols are supported.
The figures below show several possible configurations for connecting a PC with BARR/3270 to the mainframe.
SDLC Connection
Token-Ring Connection
COAX Connection
LAN Gateway Connection
Workstations
Components
PC or PS/2
DOS
320K
Drive
Screen
Token Ring Adapter
Adapter Support Program
Key
Adapter
Modem
Compatible Printers
Parallel Ports
Serial Ports
Flow Control
Printer Test File
Hardware Configuration File
BARR/3270 works on all synchronous communications links and modems. Barr products can be used for virtually all SNA communications applications—on SDLC from 1200-384,000 bps, on Token Ring from 4,000,000-16,000,000 bps, and on high-speed LAN networks.
BARR/3270 works equally well with a dial-up line, dedicated line, or LAN.
Dial-Up
Dedicated
Token Ring Network
LAN Gateway
Function
When the PC is sending, the modem converts (modulates) the digital signals to the analog signals used by the telephone system. When the PC is receiving, the modem restores (demodulates) the signal to digital form. The word modem is derived from modulate-demodulate.
Transmission
Synchronous
Asynchronous
Standards
The following are synchronous modem standards:
Standard |
Speed (bps) |
Bell 212A |
1200 |
Bell 201C |
2400 |
Bell 208AB |
4800 |
UDS 9600 |
9600 |
CCITT V.22 |
2400 |
CCITT V.29 |
9600 |
CCITT V.32 |
9600 |
Any modem you choose to use on the dial-up line must conform to the standard that the host computing center supports. If you do not yet have a modem, speak with the computing center consultant about the type of modem that is compatible with the host installation.
External
Internal
PC-SYNC modems:
Type |
Speed (bps) |
Machine |
PC-SYNC 201C |
2400 |
PC |
PC-SYNC 208AB |
4800 |
PC |
PC-SYNC 9600* |
9600 |
PC |
PC-SYNC/2 201C/208AB |
2400, 4800 |
PS/2 MCA |
PC-SYNC V.32** |
2400, 4800, 9600 |
PC |
PC-SYNC/2 V.32** |
2400, 4800, 9600 |
PS/2 MCA |
*Conforms to the CCITT V.29 and UDS 9600 modem standards.
**With the V.32 modems BARR/3270 operates in full-duplex mode, simultaneously sending and receiving data. The V.32 modems also support the V.22 standard using the AT command set or COM1-COM4.