IBM System/3S0 Operating System

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Systems Reference Library

IBM System/3S0 Operating System

File No. S360-jU Order No. GC30-2004-6

Basic TelecDmmunicatiDns Access Method

Program Number 360S-CO-513

This publication describes the Basic Telecommunications Access Method (BTAM) available with Release 20.6/20.7 and Release 21 of the System/360 Operating System combined with the Independent Component Release containing BTAM support for the IBM 3270 Display System. BTAM provides facilities that enable an assembler-language programmer to write a teleprocessing control program that effects communications at the Read/Write level between a System/360 and a variety of computers and terminals connected to the System/360 over common- carrier or private-wire communications networks.

BTAM provides similar facilities for the local IBM BTAM 3270 Display System. BTAM employs both start-stop and binary synchronous communications (BSC) techniques, depending on the type of remote station.

Typical BTAM applications include data acquisition, message switching, and inquiry processing.

The publication explains some concepts of teleprocessing and BTAM, describes line control and message transmission techniques, and describes each of the BTAM macro instructions and facilities needed to construct a control program. The READ and WRITE macro instructions applicable for each type of remote station and line configuration are given, along with the channel programs generated for each type.

Prerequisite to use of this publication is a knowledge of System/360 assembler language and data management facilities.

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Seventh Edition (September 1972)

Appendix J of this publication lists the types of terminals that are supported by the Basic Telecommunications Access Method component of the System/360 Operating System.

Terminals which are equivalent to those explicitly supported may also function satisfactorily. The customer is responsible for establishing equivalency. IBM assumes no responsibility for the impact that any changes to the I BM-supplied products or programs may have on such terminals.

This edition, GC30-2004-6, is a revision of GC30-2004-5 and associated Technical Newsletters GN30-2551, GN30-2563, GN30-2568, GN30-2569, GN30-2570, and GN30-2571. This edition applies to OS Release 20.6/20.7 and Release 21 combined with the Independent Component Release

containing BTAM support for the IBM 3270 Display System.

Significant changes or additions to the specifications contained in this pUblication are continually being made. When using this publication in connection with the use of IBM equipment, check the latest SRL Newslet- ter for revisions or contact the local IBM branch office.

Requests for copies of IBM publications should be made to your IBM representative or to the IBM branch office serving your locality.

A form is provided at the back of this publication for reader's comments. If the form has been removed, comments may be addressed to IBM Corporation, Dept. 636, Neighborhood Road, Kingston, New York 12401.

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This publication describes the BTAM facilities and macro instructions needed to write an application program that defines, activates, and controls a System/360-based teleprocessing system or local 3270 display system or both.

Effective use of this publication does not presuppose a knowledge of

teleprocessing techniques, but i t does require a knowledge of the System/360 assembly language and of data management techniques. The publication does not contain detailed information on the terminal equipment and computers that may be used as remote or local stations in a BTAM-controlled system. For this information, see the listing of

publications below.

This publication is organized as follows:

~ Teleprocessing and BTAM Concepts

explains concepts of teleprocessing in general, and of BTAM in particular, for the reader who is not already familiar with these concepts. This chapter also defines many terms used throughout the publication.

e Defining the TP System tells how to define to the Operating System the characteristics of the communications lines and equipment comprising the teleprocessing system. It includes information on the system generation procedure.

The DCB macro instruction, among others, is given here.

o Buffer Hanagement tells how to construct buffer pools and how to obtain and

release buffers as needed to accommodate message data.

• Code Translation describes the facilities BTAM provides for accomplishing the necessary translation between the transmission code used on communications lines and the internal code of System/

360 (EBCDIC). Th~ ASMTRTAB and TRNSLATE macro instructions are explained here.

~ Activating and Deactivating the TP Sys- temtells what procedures to follow in initializing the user program prior to message transmission, and in deactivating the system upon conclusion of message transmission. The OPEN, LOP EN , and CLOSE macro instructions appear here.

o Line Control and Message Transmission briefly explains the techniques for con- trolling communications lines of various kinds (switched, nonswitched, contention, polling), and presents the READ

PREFACE and WRITE macro instructions, used in message transmission operations. Also given are the WAIT, TWAIT, and RESETPL macro instructions.

• Start-Stop Read and Write Operations lists the types of Read and Write operations applicable to each type of remote terminal, and give.s the channel program for each.

• BSC Read and Write Operations lists the types of Read and Write operations applicable to each kind of line configuration, gives the channel program for each, and indicates the types of remote stations for which each type can be used.

o Local 3270 Display System Read and Write Operations lists the types of Read and Write operations applicable to the local 3270 display system and gives the channel program for each type.

o Error Recovery Procedures and Error Recording explains the BTAM-provided facilities for diagnosing and attempting to recover from a variety of error conditions, and for indicating and recording the occurrence of these errors.

• On-Line Testing describes the facilities available for diagnosing line and equipment troubles.

• Sixteen appendixes appear at the back of the publication; these show control block, information table, macro instruction, and error message formats, and code charts.

Before using this manual, the reader should be familiar with the following publications:

OS Assembler Language, GC28-6514

IBr1 System/360 Operating System: Supervi- sor Services and Macro Instructions, GC28-6646

Os Data Management Services Guide, GC26-3746

OS Data Management Macro Instructions, GC26-3794

The BTAM user will also need the level of knowledge of information contained in the following publications that apply to the transmission control units and terminals in his equipment configuration:

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• Transmission Control units:

IBM 2701 Data Adapter Unit, Component Description, GA22-6864

IBM System/360 Component Description:

IBM 2702 Transmission Control, GA22-6846 IBM System/360 Component Description:

IBM 2703 Transmission Control, GA27-2703

• Start-Stop Terminals:

IBM 1030 Data Collection System, GA24-3018

IBM 1050 Reference Digest, GA24-3020 IBr.1 1050 System Summary, GA24-3471 IBM 1050 Data Communications System, Principles of Operation, GA24-3474 IBM 1050 Operator's Guide, GA24-3125 IBM 1060 Data Communications System, GA24-3034

IBM 2260 Display Station; IBM 2848 Dis- play Control, GA27-2700

IBM 2265 Display Station; IBM 2846 Dis- play Control, GA27-273l

IBM 2740 Communications Terminal, GA24-3403

IBM 2740/2741 Communications Terminal Operator's Guide, GA27-300l

IBM 2760 Optical Image Unit Component Description, GA27-301l

• BSC Stations:

4

IBM SRL General Information -- Binary Synchronous Communications, GA27-3004

IBM System/3 RPG-II Telecommunications Programming Reference Manual, SC21-7507 IBM 1130 Functional Characteristics, GA26-5881

IBM 1130 Synchronous Communications Adapter Subroutines, GC26-3706

IBM 2770 System Components, GA27-3013 IBM 2780 Data Transmission Terminal, Component Description, GA27-3005

IBM 2790 Data Communication System, Com- ponent Description, GA27-30l5

IBM 2972 Models 8 and 11 General Banking Terminal System, Component Description, GL27-3020

IBM 50 Magnetic Data Inscriber, Com- ponent Description, GA27-2725

IBM 3270 Information Display System, Component Description, GA27-2749

IBM 3735 Programmable Buffered Terminal Concept and Application, GA27-3043 IBM 3735 Programmer's Guide, GC30-3001

• Local 3270 Display System:

IBM 3270 Information Display System, Component Description, GA27-2749

To assemble, linkage edit and execute a BTAM program requires knowledge of the information in:

OS Linkage Editor and Loader, GC28-6538 OS System Generation, GC28-6554

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TELEPROCESSING AND BTAM CONCEPTS • . • • 11 Categories of Communications Lines • • • 11 Line and Station Configurations • 11 Duplex Vs. Half-Duplex Transmission • 14 Transmission Techniques . • 14 Transmission Codes • • • 14 Line Control. • • • • • • • • • • 14 Establishing Contact • • • 15 Switched Lines. • • ^~ 16 Terminal Lists. • . • • • • • 16 Positive and Negative Responses 16 Buffering • . • • • • • • . • • • . • • 17 Dynamic Buffering • • . • • • • • 17 Conventions Used in this Publication • • 18 BTAM MACRO INSTRUCTION FORMATS .

Positional Operands Keyword Operands • Continuation Lines Coding Aids • • • •

21

• • 21 21 21

• 22 DEFINING THE TELEPROCESSING SYSTEM • 23 Defining Communications Line Groups 23

DCB (Define Data Control Block)

Macro Instruction • • • • . • • 25 Defining and Modifying Terminal Lists • 35

DFTRMLST (Define Terminal List)

Macro Instruction • • • . . • 35 CHGNTRY (Change Terminal Entry)

Macro Instruction 37

BUFFER MANAGEMENT • • • . • • 39

Constructing Buffer Pools 39

Using the BUILD Macro Instruction • 39 Using the GETPOOL Macro Instruction 39 BTAM Construction of Buffer Pools • 39 Programmer Buffering • . • . • • 39 Dynamic Buffering • • • • . 40 Read Operations • • • • 40 Write Operations • • . • • • • • 43 Buffer Management Macro Instructions • 43

REQBUF (Request Buffer) Macro Instruction • • • • • • • • • . RELBUF (Release Buffer) Macro Instruction

CODE TRANSLATION • •

ASMTRTAB (Assemble Translation Table) Macro Instruction • • TRNSLATE Macro Instruction • ACTIVATING AND DEACTIVATING THE TELEPROCESSING SYSTEM . • • • • Program Initialization .

Opening and Closing Line Group Data Sets . • • • • • • • • .

OPEN Macro Instruction . • LOPEN Macro Instruction

CLOSE Ma€ro Instruction . • • • LINE CONTROL AND MESSAGE TRANSMISSION Line Control

43

• 44

• • 45 45

• • 45 49 49 49

· • 50

• • 51 51

• 53

· 53

CONTENTS

Contention System • • • • • • . 53 Centrally-Controlled System . • • • 53 Switched Systems • • • • • • • • • • 54 Error Detection and Message Blocking • 54 Channel Programs • • . • • • • • • • • 54 Message Transmission • . • • . • • • • • 54 User Program Analysis • • • • • • • 55 Use of Line Control Characters • • • 55 Terminal Lists • • . • • • . . • • • 56 Data Event Control Block • • • • • • 56 READ and WRITE Macro Instructions • 57 RESETPL (Reset Polling List or

Reset Line) Macro Instruction

WAIT Macro Instruction • • • 63

· • • 64

• • • 65 TWAIT Macro Instruction

Read Skip Operations • • . • • • • • 65 START-STOP READ AND WRITE OPERATIONS • • 67 IBM 1030 Data Collection System 67 Defining Terminal Lists • 67 Read Operations • • • • • 67 Write Operations . . • • • • • • 67 READ Macro Instructions • 67 WRITE Macro Instructions • • • • 67 IBM 1050 Data Communications System --

Nonswitched Lines • • 69

Defining Terminal Lists • 69 Read Operations • • • 69 Write Operations . • • . • • • 69 READ Macro Instructions . • 69 WRITE Macro Instructions • • 69 IBM 1050 Data Communications- System --

Switched Lines • • • • . • • • • • • • 71 Defining Terminal Lists • • 71 Read Operations • • • 71 Write Operations • . . . • • • • • • 71 READ Macro Instructions • • • • • • ^0" 71 WRITE Macro Instructions . • • • • . • 72 IBM 1060 Data Communication System • • • 74 Defining Terminal Lists • • • • • 74 Read Operations • • . • 74 Write Operations • . . . • • • 74

READ Macro Instructions 74

WRITE Macro Instructions • • • • 74 IBM 2260-2848 Display Complex (Remote) IBM 2265-2845 Display Complex (Remote)

Defining Terminal Lists . • • • • 76

• 76

Read Operations • • 76

Write Operations • • • . • • • READ Macro Instructions . • • • WRITE Macro Instructions .

IBM 2740 Communications Terminal

• • • 76

• 76

• 78

General Information • 80

IBM 2740 (Basic) • • • • . • 82 READ Macro Instructions • • 82 WRITE Macro Instructions . • • • • 82 IBM 2740 with Checking Feature • • 83 READ Macro Instructions • • 83 WRITE Macro Instructions • • • • • 83 IBM 2740 with Dial-up Feature • • • • • 85 Defining Terminal Lists • • 85 Read Operations • . . . • 85 5

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Write Operations • . . . • . . 85 READ Macro Instructions • • • • . . . 85 WRITE Macro Instructions • . . . • . . 85 IBM 2740 with Dial-up and Checking

Features • • . • . • • • . · 86 Defining Terminal Lists • . • • •

Read Operations

Write Operations • • • • .

· . 86

· • 86

· . 86

• . 86 READ Macro Instructions

WRITE Macro Instructions • · . . 86 IBM 2740 with Dial-up and Transmit

Control Features • • . • . · • • 88 Defining Terminal Lists

READ Macro Instructions WRITE Macro Instructions

· . . 88 . . . . • • . 88

• • • . . 88 IBM 2740 with Dial-up, Transmit

Control, and Checking Features . . • • . 90 Defining Terminal Lists

READ Macro Instructions WRITE Macro Instructions

· • 90

· . 90 IBM 2740 with Station Control Feature

Defining Terminal Lists

• . 91

· 92

· . 92

• 92

Read Operations

Write Operations • . • READ Macro Instructions WRITE Macro Instructions •

· . 92

• . 92

· • 92 IBM 2740 with Station Control and

Checking Features . • . . • • 93 Defining Terminal Lists . . 93 Read Operations • . . . • . • • • • 93 Write Operations • . . • • • • • . • 93 READ Macro Instructions • 93 WRITE Macro Instructions . . 93 IBM 2760 Optical Image Unit - General

Information • • • • • • • . • . . 95 Operation and Message Formats • • 95 Filmstrip Positioning . . • . . . 95 Modes of Operation • . • • • . . • • • 95 Response Points and Coordinates 96 Image Index Counter . . • . . 97 Message Formats . . • • • • . • 98 Signals to Operator . • • • • • • • . 99 Error Detection and Recovery . . • 99 On-Line Testing • • . • • • . . .100 Prog+amming Considerations .lOl IBM 2740 with Checking Feature and 2760 Optical Image Unit • • • . • . . .104

READ Macro Instructions • . 104 WRITE Macro Instructions. . . . . 104 IBM 2740 with Dial-up and Checking

Features and 2760 Optical Image Unit . . 107 Defining Terminal Lists . . . • . • . 107 READ Macro Instructions • • • . . • . 107 WRITE Macro Instructions. • • . .108 IBM 2741 Communications Terminal . . 111

G~neral Information • . 111 READ Macro Instructions • . 112 WRITE Macro Instructions . . 112 Disigning a Message Control Routine .113 Programming Considerations • . . . • . 113 AT&T 83B3 Selective Calling Stations . . 116 Defining Terminal Lists . • • 116

Read Operations . . 116

Write Operations. • . . .116 READ Macro Instructions . • 116 WRITE Macro Instructions • . 116 Terminal-to-Terminal Operation . • 116 WU Model 33/35 TWX Terminals • . . .118 .6

Defining Terminal Lists • . • • . . • 118 Read Operations . . • • . • . 118 Read Conversational Operation • • • 118 Write Operations . . . . .118 READ Macro Instructions . . . 118 WRITE Macro Instructions . . . • . 119 Western Union Plan lISA Outstations . • 120 Defining Terminal Lists . • 120

Read Operations • .120

Write Operations. . . . .120 READ Macro Instructions .120 WRITE Macro Instructions. . . .120 Terminal-to-Terminal Operation . • 120 World Trade Telegraph Term~nals . • 122 General Information . . . . • • .122 Telegraph Adapter Description . • • 122 Contention Resolution . • 122 Defining Terminal Lists • .123 READ Macro Instructions . • 123 WRITE Macro Instructions • • 124 Programming Considerations for use of

Auto Poll (Start-Stop) . . . .125 Channel Programs . . . .125 BSC READ AND WRITE OPERATIONS

General Information • . . . . Transmission Codes • . . . . Remote Station Compatibility and

· .127

· .127 Intermixing . . . . . .127 User Program Analysis • • • . • 127 Line and Message Control Functions • . 128 ID Verification . . . . • • • 128 Error Inform~tion Byte (EIB) Mode .128 Double Addressing (Multipoint

Lines) . • . . . • . . • . . • • . • 129 Message Formats • . . • . . • • . . . 129 Use of Line Control Characters • . . 130 Use of SOH and STX Characters . . • 130 Coordinating BSC Central and Remote Programs • . . • . . • . . . .130

Responses . • . . • .130

Responses to Message Text .130

Responses to ENQ • • 131

Responses to EOT (Switched Line

Only) • • • • . . . · .131 BSC Npnswitched Point-to-Point

Operation . • . . . . • • . .132 READ Macro Instructions . . 132 WRITE Macro Instructions • • 133 BSC Nonswitched Multipoint Operation . • 136 Defining Terminal Lists . • 136 READ Macro Instructions . . . • . . .136 WRITE Macro Instructions . . . . • • . 138 BSC Switched Point-to-Point Operation .142 Defining Terminal Lists. . .144 Defining Terminal List (SWLST) for

Expanded ID Verification . . • • • 145 READ Macro Instructions • .148 WRITE Macro Instructions . • 151 LOCAL READ AND WRITE OPERATIONS

Local IBM 3270 Display System READ Macro Instructions WRITE Macro Instructions

PROGRAMMING CONSIDERATIONS FOR REMOTE

.156 .. 1 .156.1 .156.1 .156.1 COMPUTERS . • . • . • . . . • • • .157 IBM 2780 DATA TRANSMISSION TERMINAL -- PROGRAMMING CONSIDERATIONS . • . • • • .159

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Initiating Transmission to an IBM 2780 .159 End-to-End Control Characters • • 160 Component Selection • • • • • .161 Horizontal Format Control .162 Multiple Record Transmission. • .163·

Programming Considerations • • • .165 IBM 2790 DATA COMMUNICATIONS SYSTEM • • 167 2715 Pulse Count Feature. • • .167

2715 Tables • • • 170

.173

• • 182.5

• • 182.7 Macro Instructions .

programming Notes • • • • Message Format • • •

Control Transactions . External Alarm Contact

• • 182.13 Feature • • 182.14 IBM 2770 DATA COMMUNICATION SYSTEM

PROGRAMMING CONSIDERATIONS • System Configuration • • • . • Transmission Codes • • • • • • Terminal Polling and Selection Temporary Transmission Delays Terminal Function Control • • • • Field-Control Operation

Record Formats • • • • •

IBM 2972 GENERAL BANKING TERMINAL

.183

· .183 .183

• .183

• .184 .185 .186

• .187 SYSTEM -- PROG&M4MING CONSIDERATIONS • • 189 IBM 3735 PROGRAMMABLE BUFFERED TERMINAL -- PROGRAMMING CONSIDERATIONS • • • • • 190 IBM 3270 DISPLAY SYSTEM -- PROGRAMMING CONSIDERATIONS. • • • • • • .190.1 3270 Devices Supported. • • • .190.1 3270 Capabilities Supported • • • 190.1 Remote 3270 Display System . • • • • • 190.1 Defining Communications Line Groups.190.l Defining and Modifying Terminal

Lists • • • • • • • . • • • • • 190.1 Defining Terminal Lists .190.2 Modifying Terminal Lists. .190.3 Buffer Management . • • • • .190.3 Code Translation • • • • • • .190.3 Activating and Deactivating the

Teleprocessing System • . • • • 190.4 Line Control and Message

Translation • • • • .190.4 Read Operations • • • • • • • • • 190.4 Write Operations. . .190.8 Programming Notes . • 190.9 Error Recovery Procepures and Error . Recording • • • • • • • . .190.9

Error Conditions. • • • .190.9 Exceptional Conditions. .190.9 Retry Options • • • • • • .190.9 On-Line Testing • • • • • • • • • • 190.9 Local 3270 Display System • 190.10

Defining the Local 3270 Display

System • • • . • • • • • • • • • • 190.10 Identifying Local 3270 Devices. 190.10 Grouping Local 3270 Devices 190.10 Buffer Management • • • • • • • • 190.11 Code Translation . • • • • • • • • 190.11 Activating and Deactivating the Local 3270 Display System • • • • • • • 190.12 Attention Interruptions and Read

Initial Operations • • • • • • • • 190.12

CHGNTRY Macro Instruction • 190.12 RESETPL Macro Instruction . 190.13 Read and Write Operations • • 190.13 Read Operations • • • • • • 190.14 write Operations. • • . • • 190.15 Error Recovery Procedures and Error

Recording • • • • • • • • 190.16

Retry Options • 190.16

On-Line Testing 190.16

System Generation • 190.16

Storage Estimates • • 190.16 Fixed Main Storage Requirements • 190.16 Dynamic Main Storage Requirements. 190.16 Auxiliary Storage Requirements • • 190.18 Storage Estimates for BTAM

Modules • • • • On-Line Testing

Conversion • • • .

• 190.18 . • • • 190.18 190.18 ERROR RECOVERY PROCEDURES AND ERROR

RECORDING • • • • • . • • • .191 Basic Functions • • • • • • • . .192 Additional Functions. • • • . • .192

LERB (Line Error Recording Block)

Macro Instruction . • • • .192 LERPRT (Line Error Recording

Print) Macro Instruction. • .194 Error Detection and Analysis. • • .194 User Program Analysis Procedure • • 195 BTAM ERP Error Analysis and Recovery

Actions • • • • • • • • • • • • • • • • 197 Start-Stop Error Recovery Procedures • • 198

Error Recovery Actions for Start-Stop Operations • • • • • . • • • • • • • • 204 BSC Error Recovery Procedures • • • • • 208 Local 3270 Display System Error Recovery Procedures • • • • • • • • • • • • • • 228.1 Errors Detected by the Device or Control Unit and Channel Data Check Errors • • 228.1

Error Conditions • • • • • • • • 228.1 Recovery Actions • • . • • • • • • • 228.1 Errors. Detected by the Channel (Except Channel Data Check Errors • • • • • • 228.3

Error Conditions. .228.3

Recovery Actions. • • • • .228.3 SUGGESTED RETRY OPTIONS FOR BSC READ

AND WRITE OPERATIONS • • • • • • • • • • 229 SUGGESTED RETRY OPTIONS FOR LOCAL READ

AND WRITE OPERATIONS .232.2

ON-LINE TESTING .233

On-Line Testing for Start-Stop

Communications Lines • • • • • • • • • • 233 Test Type Codes • • • • • • .234 Terminal Test Restrictions • • • • • • 236 On-Line Testing for Binary Synchronous

Communications Lines • • 237

Types of Tests . . • • • .237

Type 00 • • 237

Type 01 • • 238

Types 02-34 • • • • • . 238 BTAM Responses to Reqeust-for-Test

Messages. • • • • • • • • • • • • 238 BTAM Initiation of Request-for-Test

Messages • • • • • .239

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ONLTST (On-Line Test) Macro

Instruction • • • • • • • • • .239 Formats of RFT, Test, and Console

Messages • • • • • • • • • • • .241 Request-for-Test Messages • • 241 Test Messages • • • • • • • .242 Console Messages • • • • • • .244 On-Line Testing for Local 3270 Display System. • • • • • • • • • • • • .246

Types of Tests • • • • • • • • .246 BTAM Response to Request-for-Test

Messages • • • • • • • • • . • • • • • 246 Formats of RFT, Test, and Console

Messages • • • • • • • • • Request-for-Test Message • Test Messages

Console Messages • • • • •

• .246

• • • 246 .246.1 .246.1 APPENDIX A: FORMAT OF TERMINAL LISTS • • 247 APPENDIX B: DATA EVENT CONTROL BLOCK • • 257 Fields Defined by User. • • .263 APPENDIX C: BTAM ERROR MESSAGES AND

ABEND CODES • • • • • • 265

Assembly Errors • • • • • • . • • .265 I/O Error Message • • • • • • • . • • ~272

Line Error Recording Messages • • 273 Transmission Control unit Inoperative

Message • • • • . • • • • • • • • .274 Remote BSC Station Error Messages .275 Error Status Messages (IBM 2770) • • • 276.1 Terminal Error Status Message (IBM

2715) ~... . • • 276.1 Error Status Hessage (Remote IBM 3270) .277 BT Al'1 Abend Codes • . • • • • . • . . .278.2 APPENDIX D: SYSTEM GENERATION MACRO

INSTRUCTIONS • • • • • • • • • • .283

TELCMLI~ Macro Instruction. • .283 DATAMGT Macro Instruction . . • . • 283

8

IOCONTRL Macro Instruction. .283 IODEVICE Macro Instruction • • • • • 283 APPENDIX E: CODE CHARTS FOR BINARY

SYNCHRONOUS COMMUNICATION AND THE LOCAL 3270 DISPLAY SYSTEM • • • • APPENDIX F: WORLD TRADE TELEGRAPH TERMINALS: TRANSLATION TABLE MODIFICATIONS • • • • . • • • • . APPENDIX G: BTAM MACRO INSTRUCTION FORMAT CHARTS • . • •

APPENDIX H: BTAM CHARACTER SET AND CODE CORRESPONDENCE CHART

287

289 .291

• .295 APPENDIX I: TRANSMISSION CODE CHART • • 306 APPENDIX J: LINE AND STATION

CONFIGURATIONS SUPPORTED BY BTAM • .311 APPENDIX K: IBl-1 2980 CHARACTER SET AND

TRANSMISSION CODE CHART .313

APPENDIX L: THE TPEDIT MACRO INSTRUCTION (IBM 50 MAGNETIC DATA INSCRIBER) • • • • 319 APPENDIX M: SAMPLE 2715 TABLE LOAD MACRO ASSEMBLY • • • • • • • • • • • • • .329 APPENDIX N: SAMPLE START-STOP AND BSC

PROGRAMS • • • • • • • • • • • • • .345 APPENDIX 0: LOCAL 3270 S~1PLE

PROGRAM • • • • . • • • . • • .362.1 APPENDIX P: REMOTE 3270 SAMPLE

PROGRAM . • • • • 362.31'

INDEX

INDEX OF BTAM AND SYSTEH HACRO INSTRUCTIONS . . . • . •

• .363 .374.1

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Figure 1. Basic Elements of a

Teleprocessing System • . . • . . . • . 12 Figure 2. Line and Sta,tion

Configurations . • . . • . . • 13 Figure 3. Sample Line Addresses and Associated TCU and Station Types . 24 Figure 4. Sample Line Group Makeup . 25 Figure 5. Relative Line Numbers for Example 1 . • . . • . . • . . . Figure 6. Relative Line Numbers for Example 2 • . . • . • . • . . Figure 7. Constructing Buffer Pools Using DS and BUILD • • . . . •

Figure ^8~ Constructing ^~ufferPools Using GETMAIN and BUILD . . . .

Figure 9. Constructing Buffer Pools Using GETPOOL • . • • • . . . .

Figure 10. Constructing Buffer Pools

· 25

· 41

· 42

• 42 Automatically • ~ . . . . • . . . 42 Figure 11. Establishing Addressability for DCBs and DECBs • . . . • . . 49 Figure 12. Initializing Your Program. 50 Figure 13. Formats of OPEN and

CLOSE Macro Instructions . • . . . . • 52 Figure 14. F (Function) Character

Codes . . . . • . . . • . 95 Figure 15. A1/11 and A₂/1₂Character

Codes . . . • . . . • . . 95 Figure i6. V

&

H (Vertical

&

Horizontal) Response Point Coordinate Codes • . . . .

Figure 17. Message Formats. • 97

· 99 Figure 18. Examples of WRITE TIO and WRITE TCO Macro Instructions . . 106 Figure 19. Examples of WRITE TIO,

WRITE TVO, and WRITE TCO . • . . . 110 Figure 20. WT Telegraph Code . . • . . 122 Figure 21. Example of a BSC Message

Control Routine . . . • • . . . . .131 Figure 22. Multipoint Polling/

Addressing Operations for 2780 • .160 Figure 23. Examples of Component

Selection for 2780 • . . . • . . . 162 Figure 24. Vertical Forms Control

Escape Sequences • • • • . .

Figure 25. Example of a 2780 Format · .162 Record . • . . . • . . . 163 Figure 26. IBM 2780 Records: Number

and Length . . . . • . . . . Figure 27. Examples of Defining Transaction Groups for Data Entry Figure 27.1. 2798 GDU Guidance

.164 .177 Panels . . . • . . .180.6 Figure 28. Message Formats • . • . • 182.7 Figure 29. IBM 2213 Vertical Forms

Control Escape Sequences

Figure 30. IBM 2265 Erase and Write-at-Line Address Control Characters . . . • . . . .

• .186 .187 Figure 3270-1. Control Unit Addresses for Polling List Entries and for

Identification in Input Messages . . 190.2

FIGURES

Figure 3270-2. Control Unit Addresses for Selection List Entries • . • . • 190.2 Figure 3270-3. Device Addresses. for

Polling and Selection List Entries and for Identification in Input

Messages . • • • . • • • . • . . . . 190.2 Figure 3270-4. I/O Interface Code

for Six-Bit Structured Data • . . . • 190.3 Figure 3270-5. Sample Input Message

(showing translation from ASCII to

EBCDIC) . . . . • • • • • • . . • . . 190.4 Figure 3270-6. Sample Output Message

(showing translation from EBCDIC

to ASCII) . . . . • . . . 190.4 Figure 3270-7. Sample Local 3270

Display Systems Showing Device

Addresses . . . • . • • . . • • 190.10 Figure 3270-8. Line Groups and

Relative Line Numbers for

Example 1 . . . • . • . . • . . 190.11 Figure 3270-9. Line Groups and

Relative Line Numbers for

Example 2 . . . • . . . • . • • 190.11 Figure 3270-10. Coding Space

Estimates. . • . . . • . • . . 190.17 Figure 3270-11. Control Information

Space Estimates for Each Device

Type . • . . . . • . . . • . 190.17 Figure 3270-12. Control Block Space

for Each Line Group or Device

Group. . . 190.17 Figure 3270-13. Control Block Space

for Each Line or Device. . . 190.17 Figure 3270-14. Control Block Space

for Each READ or WRITE Macro

Instruction. . . . • . 190.17 Figure 3270-15. Channel Program Space Estimates for Each line or Device. 190.17 Figure 3270-16. Auxiliary Storage

Requirements for BTAM • . • . • • . 190.17 Figure 31. Format of Control Byte

for OPENLST and WRAPLST Entries • . • . 247 Figure 32. Open Polling or

Addressing List (OPENLST): Format

and Examples . . • . • • • • • . • . • 248 Figure 33. Wraparound Polling List

(WRAPLST): Format and Example.. .249 Figure 34. Dial List (DIALST):

Format and Examples . • . . • • .249 Figure 35. Identification List

(IDLST)= Format a~d Example . • . • . • 252 Figure 36. Open and Wraparound Auto

Poll Lists for Start-Stop (SSALST, SSAWLST) and BSC (AUTOLST,AUTOWLST):

Format • • . . . . • . . . • . . . . .253 Figure 37. BSC Dial List (BSCLST)

(for S/360-S/360): Format and

Examples • • . • . . . • • • .2.53 Figure 38. WT Terminal List

(WTTALST): Format and Examples .254 Figure 39. Calling and Answering

Lists for Expanded ID Verification -

(SWLST): Format . . . • . • • • . • . • 255 9

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Figure 40. Format of Data Event Control Block • • • • • • . • . • . . • • .264 Figure 41. IBM 50 MDI Control Codes • . 322 Figure 42. Examples of Erroneous Records

(IBM 50 MDI) • • . . . . • . . . . • • .326 Figure LJ.3a. Initial Format on Modell 3277 Display Station • • • . .362.4 Figure 43b. Initial Format on Model 2 3277 Display Station • • • • .362.4 Figure 44a. Verification Format on

Modell 3277 Display Station .362.4 Figure 44b. Verification Format on

Model 2 3277 Display Station .362.5 Figure 45a. Control Options Format

on Modell 3277 Display Station • • • 362.5 Figure 45b. Control Options Format

on Model 2 3277 Display Station • • • 362.6 Figure 46a. Ending Format on Modell

3277 Display Station • . • • • . 362.6 Figure 46b. Ending Format on Model 2

3277 Display Station • • • • . • 362.6

10

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Table 1. Keyword Operands for the BTAM Communications-Line-Group DCB Macro Instruction • • • • • . • • Table 2. Format of Data Control

27 Block (DCB) • • . • • . • . . . 32 Table 3. DCB Field Contents • 34 Table 4. Code Translation Tables

Provided by BTAM • • • • • • • • Table 5. READ and WRITE Options for Start-Stop . • • . • . • • . . Table 6. READ and WRITE Options for BSC • • • . • • • . • . • • . Table 6A. READ and WRITE Options for Local 3270 Display System . . Table 7. IBM 2260 and 2265

• 46 58

• 60 60.1 Display Line Addresses • • . . . • • • 78 Table 8. Summary of BSC Switched

Line READ and WRITE Macro and

Terminal List Options • • . • . . • 143 Table 8.1. 2715 Macro Storage Size

Estimates • . • . • • . . • • .174.1 Table 9. AS Macro ID Parameter

Decimal and Hexadecimal Equivalents . • 176 Table 9.1. 2798 GDU Keyboard Character Conversion. • • . • • . . . • • .180.6 Table 10. ASLIST Operand Values . • . 182.1 Table 11. Transaction Control

Byte Usage . . . • • Table 12. Counter Control Operations • • . • • • • . Table 13. User Program Status Analysis . • • • • . . • • • Table 14. User Program Sense Byte

182.11 . 182.14 .196 Analysis . • . • • . • • • • • • • • .196 Table 15. Should-Not-Occur Error

Conditions Posted by BTAM ERP Table 16. Status Analysis -- Start-Stop • • • . . . • . Table 17. Sense Byte Analysis Start-Stop . . • • . • • • . Table 18. Unit Exception --

• •. 197 .198

• .198 Start-Stop • . • • . • • • . • .199 Table 19. Lost Data -- Start-Stop . • • 199 Table 20. Timeout -- Start-Stop • • • • 200 Table 21. Intervention Required

Start-Stop

Table 22. Bus Out Check • .201 Start-Stop • • • . • • • • • . • • 201 Table 23. Data Check -- Start-Stop . • 202 Table 24. Overrun -- Start-Stop • . . • 203 Table 25. Command Reject --

start-Stop • . . • . • • . . • • • . • 203 Table 26. Index to BSC ERP Tables . • • 209 Table 27. Status Analysis -- BSC • • • 209 Table 28. Channel Data Check -- BSC • . 210 Table 29. Equipment Check -- BSC .210 Table 30. Command Reject -- BSC • • . • 211 Table 31. Sense Byte Analysis

BSC . . . • • • . . • Table 32. Bus Out BSC Table 33. Overrun -- BSC

• .211 .212

• .213

TABLES

Table 34. Intervention Required--

BSC • • • . • • • • • • . .213 Table 35. Data Check -- BSC • . 214 Table 36. Lost Data -- BSC . • 215 Table 37. Timeout -- BSC . • 216 Table 38. Unit Exception -- BSC • .217 Table 39. Special Return Code Action .220 Table 40. Error Post Actions • . • • . 226 Table 40A. Error Conditions (according to CSW and sense byte) • . • • • • • 228.1 Table 40B. Recovery Actions (by error condition and failing command) . • • 228.2 Table 40C. Error Conditions {according to ERPCODES field in ERPIB} • . • • • 228.3 Table 40D. Recovery Actions (by error condition and failing command) . • . 228.3 Table 41. Retry Options for Write

Operations (Nonswitched Point-to-

Point Line [BSCl]) • . . • • . • • 229 Table 42. Retry Options for Read

Operations -- Answering (Switched

Point-to-Point [BSC2]) • • . • . • • . 230 Table 43. Retry Options for Read

Operations -- Calling (Switched

Point-to-Point [BSC2]) • • • • . . • • 230 Table 44. Retry Options for Write

Operations (Switched Point-to- Point Line [BSC2]) • • • • Table 45. Retry Options for Read Operations (Nonswitched

• • • 231 Multipoint Lines [BSC3]) • • • • • • • 232 Table 46. Retry Options for Write

Operations (Nonswitched

Multipoint Line [BSC3]) • • • • 232.1 Table 46A. Retry Options for Local

3270 Read and Write Operations Table 47. Summary of BSC On-Line Test Options (except for remote 3270

test options) • . • • • • • . • . Table 47A. Summary of BSC On-Line Test Options for Remote 3270

.232.2 .245 Devices • • . • • • . . • . • . • • • 246.1 Table 47B. Summary of On-Line Test

Options for Local 3270 Devices Table 47C. Suggested actions according to remote 3270 error status message . . • • . Table 48. World Trade Telegraph Codes ITA2 and ZSC3

Table 49. Format of Error Description Word • . . •

.246.2 .278

• • 290 . .324

lOA

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This chapter explains some fundamental aspects of computer-based data communica- tions systems (often called teleprocessing systems) of the kind accommodated by the IBM System/360 Basic Telecommunications Access Method (BTAM), and explains some basic terminology used throughout the pub- lication. As this discussion is intended to explain teleprocessing (TP) systems for the BTAM user, it does not attempt to encompass all kinds of TP systems. Thus, while some of the terms defined apply to all or most communications systems, other terms are limited to communications pro- gramming usage, or specifically to BTAM.

Moreover, concepts and terminology are pre- sented from the programmer's viewpoint, rather than from the engineer's.

Viewed in its

~ost

elementary aspect, a teleprocessing system consists of (1) a central computer and associated transmis- sion control equipment, (2) remote sta- tions, and (3) the electrical circuits

(called communication lines or data links) that connect the remote stations to the central computer (See Figure 1). For the purpose of this discussion, the central computer equipment comprises the central processing unit (CPU) and the equipment by which the CPU is connected to the communi- cations lines. The generic name of this equipment is transmission control unit

(TCU).

The equipment constituting a remote sta- tion can be either a terminal or another computer. A terminal consists of a control unit and one or more input and output

devices, each of which is called a com- ponent of that terminal. Each input device and each output device is considered a separate component.

Remote stations in a BTAM-controlled teleprocessing system are usually separated from the central computer by a .distance sufficient to require common carrier facil- ities and transmission techniques to accom- plish communication between central comput- er and remote stations. (Communi-

cations common carriers are companies that furnish communications services to the public.) However, it is the method of con- nection to the central computer, rather than the distance from the computer, that determines whether a station is classed as remote. A station is considered remote if it is connected to the central computer through a transmission control unit (TCU).

(A station connected directly to a computer data channel is termed a local station.)

TELEPROCESSING AND BTAM CONCEPTS

Except for the local 3210 display system, the System/360 Operating System version of BTA! supports only remote stations, which must be connected to the central computer by means of an IBM 2701 bata Adapter Unit or an IBM 2102 or 2103

Transmission Control. Local 3210 display systems are connected directly to a

selector, multiplexer, or block multiplexer channel of the central computer.

An operator's console is an input/output device whose function is to control the operati=ons of the computer.

The console and its terminal control unit make up a terminal that can communic- ate with the operating system and with pro- blem programs but cannot communicate with other terminals. If the operating system includes the Multiple Console Support (MCS) option, BTAM can communicate with those operator's consoles that are connected to the central computer through a 2101, 2702, or 2703 transmission control unit.

CATEGORIES OF COMMUNICATIONS LINES

Communications lines can be categorized by several sets of attributes" some of which are discussed below. Some attributes have significance for the user's BTAM program, others need only

be

specified at system generation time, similar to the way in which toe programmer specifies the attri- butes of local I/O devices.

LINE AND STATION CONFIGURATIONS

A communications line can be classified according to whether i t connects two or more than two stations, and whether or not the electrical connection between the cen- tral computer and the station is con- tinuously established. Figure 2 illus- trates a teleprocessing system comprising several types of line and station configu- rations, the elements of which are

explained below.

A nonswitched line is one that con- tinuously links the stations associated with it, regardless of the amount of time it is in use for message traffic. This kind of line is usually furnished by a com- mon carrier on a contractual basis, between specified locations for a continuous

period" or regularly recurring periods, for the exclusive use of one customer.

Teleproce·ssing and BTAM Concepts 11

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A nonswitched line

is called point-to-

point if it connects the computer to a single remote station; or multipoint, if several remote stations are connected to the line.

A switched line is one in which an electrical connection between the central computer and a remote station is estab-

lis~ed

by dialing, similar to the manner in which ordinary telephone calls

~re

made.

As in the public telephone network, the actual communication path for a given transmission is not fixed, but is automati- cally selected from a variety of possible paths by common carrier switching

equipment.

Each remote station on a switched line is continuously connected to the common carrier switching cent.er (exchange) by an access line in the same way as a telephone.

A telephone number is associated with the

ac~ess

line. Similarly, each transmission control unit at the central computer is connected to the exchange by access lines.

usually, a

TCU

has several access lines, each witn its own telephone number; mul- tiple access lines permit simultaneous com- munication with several remote stations.

Each connection of an access line at the TCU is called a switched line termination, or line appearance.

,---

Common carriers usually charge for switched lines on a time-used rather than a contractual basis.

A switched line is always considered point-to-point, as communication occurs with only one remote station on a line dur- ing any call. Switched line connections are established

^by~anual

dialing. or by automatic dialing under program control.

The dialing operation may be performed at the central computer or at the remote sta- tion" and the called station answers manu- ally or automatically. Not all of these options are available for all types of line configurations and remote stations. Within the limitations imposed by equipment or programming, the user chooses between them on the basis of the requirements of his application. For example, if the applica- tion involves collection of batched data from a number of remote stations after normal working hours, it would be appropri- ate to have each station equipped with an automatic answering facility and the cen- tral computer equipped with the calling fa- cility. This would allow the program auto- matically to call the unattended stations and receive the batched data.

4: Inpvt

Output --....,~~ Terminal

CPU I

I IMvltiplexer

Transmi"iOnl~::::

_Control

r

^____

---~~~~~---l ---'

Computer IChannel

I

L ___ _

Unit (fCU)

Central Compvter

Figure

1.

Basic Elements of a Teleprocessing System

12 OS

BTAM

SRL

Terminal

Remote Stations

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Centr,l Computer

CPU

Gi c o c

..l:

U

"1

I I I I I I

I I I I

I

,

I I , I

L ___

^'p~fr---r'~

lCU

Nonswitched multipoint

Nonswitched multipoint Nonswitched multipoint

lCU

j---)m----

I

r

I I

Switched point-to-point Remote station

1m

Data set (modem)

o

Common-carrier exchange _A_ Access line

Figure 2. Line and Station Configurations

Teleprocessing and BTAM Concepts 13

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DUPLEXVS. HALF-DUPLEX TRANSMISSION

The term duplex is applied to a communications line that can accomodate data transmission in both directions at once. Half- duplex lines permit transmission in only one direction at a time. In a BTAM- controlied teleprocessing system, data transmission is always in half-duplex mode;

messages are never transmitted in both directions at once.

TRANSMISS~ON TECHNIQUES

Transmission technique is the way in which data characters are represented on the communications line. The two techniques used by computers and terminals supported by BTAM are start-stop and binary syn!chronous.

Detailed explanations of these techniques are not given here, as the programmer need not concern himself with them except to specify to BTAM which technique is used. ,Binary synchronous communication

(BSC) ^is.~sedfor high-speed data transmission between the central computer and another remote computer or high-speed terminal. start-stop transmission (also called asynchronous transmission) is used for data transmission at lower speeds between the central computer and remote terminals of various types.

TRANSMISSION CODES

Data can be represented on a communications line by any of several transmission codes.

The code used on a given line is determined by the kind of station or the class of stations connected to the line. Some stations allow a choice of transmission codes. The BTAM programmer must be aware of the code used on a line since he must sometimes specify, in the form of bit patterns, certain data characters to be transmitted by BTAM. At the back of this manual are charts giving the specific bit patterns of the characters contained in the character sets of the various transmission codes or station types.

LINE CONTROL

Just as a computing system, with its variety of peripheral input/output equipment, requires some means to coordinate the func- tioning of the various parts, the variety 14 OS BTAM SRL

of I/O equipment comprising a teleprocessing system requires a discipline to effectively manage the flow of message traffic. A significant difference should be noted, however. In a conventional computing system, the various I/O devices are at the service of the programmer; the requirements of his program and the characteristics of the data to be processed largely determine which input and output devices are to be activated and when.

Moreover, the I/O de·vices are within reach of the computer operator; he can intervene when a device malfunctions to correct the condition or assign a different device. In a teleprocessing system, on the other hand, the central computer receives data at ran- dom from remote stations, and the operator at the central computer cannot exercis~ any direct control over remote stations. He cannot, for example, correct a malfunction- ing device at a remote station.

A further distinction between a computing system and a teleprocessing system lies in the handling of errors in data. With current techniques for transmitting data over long distances, errors are frequently introduced into message data by unavoidable transient line conditions such as crosstalk and lightning strikes. Transmission errors occur much less often in a computing system. A discipline for a teleprocessing system must accommodate the facility to detect transmission errors and, when possible, to correct them (as by retransmitting the message containing the errors). If the error is irrecoverable, its occurrence must be signaled to the user program so that appropriate action can be taken.

The scheme of operating procedures and signals by which a teleprocessing system is controlled is called line control (for binary synchronous communicationsl the term data link control is often used). A line control scheme must consider the functional characteristics and capabilities of the equipment and communication lines composing the system, as well as the operational requirements of the system. Some specific factors that line control must consider are: How is contact to be established between a sending and a receiving station?

How is a message to be directed to a specific station on a multistation line? What if two stations try to send at the same time? What should be done if a station fails to respond to a message?

Line control can be classified in two ways. The first way is by the transmission technique (start-stop or binary synchronous) that is used for the line under con- sideration. With each of these techniques is associated a set of control characters and rules for their use to effect the needed function$. Some of the control

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characters are used for both start-stop and BSC transmission, while others are peculiar to one or the other of the transmission techniques. The specific line control characters are explained under the discus- sions of these techniques in the Line Con- trol and Message Transmission chapter.

The second way in which line control can be classified is by the communication line configuration with which i t is used. For example, line control for a switched line differs from that for a nonswitched line.

While the general capabilities and functions of a given line control scheme are identified in terms of transmission technique and line configuration, individual variations in capability and function arise from differences in the kind of stations to be controlled, and by the presence or

absence in the stations of certain ^fea~

tures. For example, a given line control scheme may include the control characters needed to indicate occurrence of a transmission error and to request automatic retransmission, but some types of station equipment that use that line control scheme may not oe capable of error checking or automatic retransmission. Generally speak- ing, all stations connected to a given line must be designed to use the same line control scheme, and where a certain capability is provided by some stations but not by others, the capability cannot be used.

It is not necessary for the BTAM programmer to specify the line control scheme to be used for a given line; this information is provided implicitly at system generation time, and at assembly time in the DCB macro instruction for the line group of which the given line is a member.

The programmer must, however, have a general understanding of line control concepts in order to correctly structure that por- tion of his program that performs message transmission. Also, the programmer must know the meanings of each of the line control characters, as he must regularly insert certain of them into output areas and arrange his program to look for them in input areas.

Line control functions can be considered in two categories: the functions needed to establish contact between central computer and remote stations, and those needed to produce orderly flow of message traffic.

ESTABLISHING CONTACT

Contact may be established in several ways"

depending in part upon the line configuration involved.

In some line control schemes one of the stations on a point-to-point nonswitched line can "bid" for use of the line so that i t can send a message to the other station.

Occasionally both stations may simul- taneously bid for use of the line. When this happens, the stations are said to con- tend with each other; a system in which this situation can occur is called a contention system. The line control scheme- for a contention system must provide some means for resolving contention, that is, determining which of the contending stations is to be given the opportunity to send its message. Once one station is given control, the other is blocked from sending. A contention system is more frequently used for a point-to-point line configuration (i.e., involving only two stations) than for a multipoint configuration.

BTAM currently provides contention line control only for a point-to-point line.

The alternative to a contention system is a system in which a control station

(i.e.~ the central computer) periodically contacts each of the remote stations in turn and allows i t to send any input messages i t has ready. ("Ready" means that the terminal operator is prepared to enter data from a keyboard, or that some medium such as cards or paper tape has been placed in an input device so that the data can be transmitted automatically when the control station activates that device.) In this kind of system, each remote station has a unique identifier consisting typically of one or two characters, which, when sent over the line by the control station, causes that remote station, and no other, to respond. In a BTAM-controlled teleprocessing system only the control station, that is, the central computer, activates stations in this manner. The process of contacting in turn each, of several stations on a line to determine if any has input ready is called polling, and the station identifiers are called polling characters.

Often, the first polling character identifies the station and the second identifies a particular component from which data is solicited. A system in which stations ar~

polled is called a polling system (in con- trast to a contention system).

Although the term polling taken in its conceptual sense implies a nonswitched line to which is attached several stations, each of which is solicited in turn, the actual function of polling (that is, sending a station identifier) sometimes applies as well to a point-to-point nonswitched line or to a switched line. In the case of a switched line, the central computer may dial the telephone number of the station

(or the station dials the computer) and then the computer transmits the polling characters for that station.

Teleprocessing and BTAM Concepts 15

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In this discussion of contention versus polling, systems, the distinction between the two was based on establishing contact for the purpose of receiving inpu~ data from a remote station. The distinction is less clear in the case of output data. In either a contention or a polling system, the central computer must send a station identifier to select the specific station that is to receive an outgoing message.

The station identifier in this case is called addressing characters (or selection characters), and the process is called addressing (or selection). As with polling characters, the first addressing character may identify the station and the second, a particular component.

Sll'li tched Lines

It should be understood that, in the case of a switched line, the polling and addressing functions are independent of whether the central computer or a remote station initiated the telephone connection.

Typically, the operator at a remote station dials the computer only when the remote station has data to send to the computer, and the computer would therefore poll the station after the line connection is established. Similarly, the central computer might dial a remote station only when the computer has data to send, and would therefore address (or select) the remote station. These conventions do not always pre- vail, however. For example, some applications require that certain stations be polled after working hours when the stations are unattended. with the proper common carrier equipment at the station, the computer can dial the station, then poll the input devices that the operator pre- viously loaded with, for example, a deck of cards or a paper tape.

In establishing contact over a switched line, two situations should be avoided.

First, dialing a wrong number can result in establishing contact with a station other than the one intended. Second, an unau- thorized station, if provided with the telephone number of the central computer, could establish contact (assuming that the polling or addressing characters corre- sponded to the characters for authorized stations) •

To prevent message transmission under either of these circumstances, identification verification may be used. (This is an optional facility available for certain kinds of stations.) In order to use this facility, each remote station that is per- mitted to call the computer over a specific swi tched line termination (i. e,., by call ing 16 OS BTAM SRL

a specific telephone number), must have an identification sequence that it automatically sends after the line connection has been established. The program compares the received sequence against a programmer- defined sequence. If they match, message transmission can proceed; if they differ, BTAM signals the fact by setting a flag bit, and inhibits message transmission.

The user's program must check the flag bit and take appropriate action, which

ordinarily will be to break the line connection.

If the remote station is a computer, the identification sequence is provided by the programmer, and each computer, central and remote, can check the identity of the other. If the remote station is a terminal, the sequence is mechanically or elec- trically established when the terminal is installed, and only the central computer can perform the checking function.

Because the central computer has no way of uniquely identifying a station that calls it, all polling and addressing characters and identification sequences must be the same for any station that is to be per- mitted to call in over a given switched line termination.

The function of identification verification is not applicable to nonswitched lines, since the user determines, when the TP system is installed, which stations are to be connected to a specific nonswitched line.

Terminal Lists

When establishing contact with a remote station, the BTAM program gets the telephone numbers, polling or addressing characters, and identification sequences needej from a control table called a terminal list, which the programmer generates at assembly time using a BTAM macro instruction provided for this purpose. The structure and contents of the terminal list vary according to the kind of line configuration and rem6te station for which the list is being generated. (Terminal lists are not used for contention systems.)

Positive and Negative Responses

The discussion on how contact is established between stations has up to this pOint considered only the action taken by the originating station (i.e.,the station that initiates the contact). Before mes-