BARR/3270 manual

1. Introduction

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.


1.1 What is 3270?

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

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.

Communications Link

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).

Communications Controllers (3745, 3725, 3720)

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.

Cluster Controllers (3274, 3174)

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).

CUT and DFT Protocols

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.

3270 Display Terminals (3278, 3279)

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:

The terminal model type BARR/3270 uses depends on the screen size, color, and extended highlighting features supported by the mainframe application.

3270 Printers (3287)

Printer models differ in print functions, typeface styles and fonts, margin controls, and other features supported. Two types of 3287 printers are widely used:

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.

1.2 BARR/3270

BARR/3270 on a PC emulates:

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.

1.3 Possible Configurations

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

1.4 3270 Workstation Hardware

Workstations A PC equipped with a BARR synchronous communications adapter or IBM Token Ring adapter and software replaces an expensive 3174 establishment controller. In fact, the PC’s higher throughput and simpler user interface uniformly exceeds the older technology.

Components A 3270 workstation consists of a PC, BARR adapter/software, modem, and printers.

Personal Computer Requirements

PC or PS/2 BARR/3270 operates on the IBM PC series, IBM PS/2 series, and other compatibles.

DOS The PC and the host computer each have operating systems, which are programs that direct the flow of data and sort out details to free the user from complexity. The primary operating system for the PC and the one that BARR/3270 requires is DOS. BARR/3270 is compatible with DOS 3.0 or later versions.

320K Minimum memory required on PC for PC DOS or MS-DOS and BARR/3270.

Drive BARR/3270 functions with one or more disk drives.

Screen BARR/3270 supports the standard display screen (25 lines of 80 characters, monochrome or color) as well as the larger screen sizes available with newer adapters.

Token Ring Adapter, Key, and Software for Token Ring Version

Token Ring Adapter From IBM or other vendor.

Adapter Support Program For IBM, this is either TOKREUI.COM or the LAN Support Program, which consists of two files placed in CONFIG.SYS—DXMA0MOD.SYSD and DXMC0MOD.SYS. These are memory-resident and only need to be loaded once. Other hardware vendors have similar programs.

Key A small device that plugs into the computer’s parallel port. The printer cable attaches to the other end of the key. Used for software protection, it must be placed on LPT1. The key is provided by Barr with the software.

BARR Adapter and Software for SDLC Version

Adapter The hardware for the SDLC version is either a Micro Channel adapter for the IBM PS/2 series or a half-card adapter for the IBM PC series computers. The adapter is connected to an external modem with an RS232/V.24 cable, or V.35 cable if using a V.35 adapter. An X.21 cable is available for the V.35 adapter.

Modem If you are using the PC-SYNC or PC-SYNC/2 internal modem, the adapter is connected to the telephone wall jack. Modem types are explained in Section 1.5.

Printers

Compatible Printers The PC running BARR/3270 is compatible with dot matrix, nonimpact, laser, and line (band) printers. Your choice of printer is based upon your particular need. Up to seven printers can be attached to the PC. The printers are connected to the PC through the parallel or serial ports. Barr offers special support for printers using the Dataproducts interface and the IBM S/370 channel interface.

Parallel Ports The parallel port transfers all eight bits of a byte simultaneously. The speed of transmission is greatly increased as each bit travels on its own wire side-by-side. The parallel adapter interface standard, also known as the Centronics Printer Interface, is the preferable way to connect high-speed printers to the PC. DOS names for parallel ports are LPT1, LPT2, and LPT3.

Serial Ports The serial port transfers all eight bits of a byte sequentially. One wire sends data, one bit at a time, from the PC to the printer; another wire sends data from the printer to the PC. Printers with a serial interface use the RS232 standard. This asynchronous serial convention also allows the printer to be remote from the PC with a long cable. DOS names for serial ports are COM1 and COM2.

Flow Control The XON/XOFF and DTR (Data Terminal Ready) flow control mechanisms are supported on the serial ports. See Appendix C for Serial Port Interface.

Printer Test File A printer test file (PRINTER.TST) is included on the BARR/3270 distribution disk(s). This file verifies that the printer is correctly connected to the PC and that they are functioning together. See Appendix F Printer Test.

Hardware Configuration File A Hardware Configuration File with the file name HARDWARE.LST is also included on the BARR/3270 distribution disk(s). This file includes switch settings and control data for various printers, plotters, modems, and other peripherals. You can either view this file or print it.

1.5 Communications Links and Modems

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.

Communications Links

BARR/3270 works equally well with a dial-up line, dedicated line, or LAN.

Dial-Up With a dial-up line (switched), the connection is established by dialing the telephone number of the central computer. Ask the communications consultant about the types of modems supported by the mainframe. The consultant will give you the telephone number to use with your modem based on its type and speed.

Dedicated With the dedicated line (non-switched, leased), the computers are directly connected and no dial-up procedure is required. The lines may be privately owned or furnished by a telephone company. BARR/3270 operates at line speeds up to 384,000 bps on a PC 386/33MHz. High speed is an advantage with large volumes of data.

Token Ring Network The Token Ring Network is IBM’s Local Area Network protocol. The mainframe is connected to the Token Ring Network by the 3174, 3725, and 3745 communication controllers.

LAN Gateway The BARR/GATE SNA gateway connects the mainframe SNA network to a LAN. LAN protocols supported are Novell IPX, NETBIOS, and Token Ring. BARR/GATE enables any PC on the LAN to use BARR/3270 without additional hardware.

Modems

Function Data transmission between the PC and the host computer is completed through telephone lines. The PC sends signals in digital form to a modem. The modem translates these digital signals to the analog signals sent by the telephone lines.

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 The modem at each end of the transmission line—the host’s modem and the remote PC’s modem—operate at the same speed [bits per second (bps)], but must also keep in step with one another. Synchronous and asynchronous transmission are the two methods of timing communications. BARR/3270 uses the synchronous protocol, Synchronous Data Link Control (SDLC).

Synchronous Synchronous transmission incorporates a clocking mechanism coded in the signal carrier that does not need start bits. Successive characters are sent without any intervening start and stop bits. The receiver stays in sync with the sender by constantly evaluating the clock periods in the received signal. The transmission message consists of many characters; therefore, the inefficiency of stopping and starting between characters is eliminated.

Asynchronous Asynchronous transmission communicates with a single character preceded by a start bit and followed by one or two stop bits. Essentially, the transmission message contains one character with no restriction on the length of time between messages (characters). Low-speed terminals use asynchronous transmission (referred to as start-stop) to communicate with interactive systems.

Standards Modems used on dial-up telephone lines communicate at various speeds and code bits of data using various modulation techniques. These modulation techniques are defined by modem 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 External modems are connected to the BARR synchronous communications adapter in the PC with the BARR cable. The IBM PC, XT, AT, and PS/2 Models 25 and 30 use the BARR/3 adapter. IBM PS/2 Models 50 and above use the BARR/6BBB Micro Channel adapter. Both adapters are packaged with either an RS232/V.24 or V.35 interface and modem cable. See the BARR/3 manual or the BARR/6BBB manual for information on these products.

Internal Barr Systems offers the PC-SYNC series of internal autodial modems integrated with the BARR data link control hardware on a single full-slot adapter. The connection between the PC and telephone line is a single RJ11 telephone cable. PC indicates PC, XT, AT, and PS/2 Models 25 and 30. PS/2 MCA indicates PS/2 models 50 and above.

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.