THE DESIGN OF SYSTEM FOR TELECOMMUNICATIONS BETWEEN SMALL AND LARGE COMPUTERS Robin B. John Thesis presented for the Degree of Doctor of Philosophy Faculty of Science, University of Edinburgh November, 1973. 0 m 8 x3 ABSTRACT This thesis describes the development of a data communication system for small computers to enable them to link to large computers. The particular advantages and additional facilities made available to computer users through the use of such a link are described. A detailed description is given of the hardware and software components needed to achieve this link, together with the reasons for choosing the particular techniques employed. The discussions given highlight the problems involved in this type of operation. Some of these problems, such as lack of standardization, are short-term and will be overcome with the natural evolution of computer systems, while others are of a more fundamental nature related to the use of data transmission over long distances. The system was designed to be applicable to a number of different small computers. This has resulted in a system which is easily transferable between machines, through the careful choice of interfaces to other componentso This is seen as a step towards a more flexible and more modular method of system construction whereby complete software systems for arbitrary configurations can be put together using 'off-the--shelf' components already well-developed and tested. This contrasts with the present situation in which whole new systems are developed for a new computer, frequently duplicating systems already developed on other hardware. A detailed description of the factors involved in producing machine-indepezxd.'n, easily-transferable system components is given as a guide to other developments in this direction. It is felt that there is need for a better-engineered approach to the construction of software systems and it is hoped that the work described makes some contribution towards this end. 1 CONTENTS Chapter 1 . • 0 . • . 1 TELECOMMUNICATIONS BETWEEN SMALL AND LARGE COMPUTERS 1.1 Introduction . 1.2 Small computer capabilities . 1 1.3 Central computers ..................3 1.4 Coznmunications requirement .............3 1.5 Summary of developments . 4 1.6 The General System approach . 6 Chapter 2 . 8 USER APPLICATIONS FOR SMALL COMPUTER COMMUNICATION LINKS 2.1 ERCC PDP-8/L . ... 8 2.2 PhysIcs PDP-8 . , , . • • . • . • . 10 2.3 Social Medicine PDP-8........,...... 13 2.4 Computer Science CAD PrOject PDP-7 . • . 16 2.5 IBM 1130 in ABRO and Dept. of Statistics . 19 2.6 ConclusIons • • • • • • • , • • • • • • • • • • . 21 Chapter 3 . S • • S • S • • S • • • 690609409606 so 24 GENERAL COMMUNICATIONS HARDWARE CONSIDERATIONS 3.1 Main features of data communication hardware . 24 3.2 Asynchronous communications adapter . • . • . 26 3.3 Synchronous communations adapter . • . • . • 27 3.4 Telegraphic signalling . • . • . • • . • . 29 3.% 5 Modems . • • . • . • • • . • . 30 3.6 Electrical circuit facilites . • • • . • • , . 33 3.7 Conclusions . • . 0 0 . 0 . • 0 0 0 . • . 9 . 35 Chapter 14 . • . • • . , . 37 SYNCHRONOUS V. ASYNCHRONOUS FOR COMPUTER-COMPUTER COMMUNICATIONS .L Introduction ....................37 14.2 Handling of transmission errors • . • . 38 14 3 Error detection ..................39 14.14 Block-oriented error detection . • . • .. 141 4. 5 Comparisons • • e • • • . • . • . 142 14 .6 Conclusions . • . 143 Chapter 5 . S S • S • • S. • • S S S S • S • S • S • S • S • 1414 LARGE COMPUTER COMMUNICATIONS HARDWARE 5 .1 Introduction • . • . • . 1414 5.2 Communications controllers • • . • . 1414 5.3 IBM synchronous controllers . • . 146 5 5 14 ICL synchronous controller . • . 0 . 147 5.5 Problem of compatibility. 0 . • • • . • . 148 5 .6 Conclusions . • • • . • • . • 149 Chapter 6 • . • • • . • • • . • . 50 INTERFACING TO THE SMALL COMPUTER 6 .1 Introduction • • . • . • • • • . • • 50 6.2 The 'uniform requirement . • . • 50 6.3 Small conmuter input/output interfaces . 52 6.14 PDP-8 i53 6.5 Modular One . .. • . • • . • . • . 56 6.6 ICL bioo • .. • . • . • . • • . 58 6.7 IBM 113O. • . • . • • . • . • . • . 61 iii Chapter 6 . ' . S • • • • • • . • • . 50 INTERFACING TO THE SMALL COMPUTER (cont.) 6.8 Common facilities'of the input/outpxt'interface.. 614 6.9 Generalized interfaâe 'description .'..'. .. 66 6.10 General applicabi].ity............... 67 Chapter SMALL COMPUTER COMMUNICATIONS HARDWARE 7.1 Introduction . , . ,. , . ... . ... 69 7.2 Flexibility requirement . 69 7.3 Basic functional requirements . TO 7,14 Extra functions ' . 9 , * 0,0 . 72 7.5 Preliminary functiona]. specification . 75 7.6 Non—interrupt ncIe of operation'. 77 7,7 Program interface . .. . .'. . 77 7.8 Communications interface specification..'. 82 7.9 Experience of first implementtion. 85 7.10 Improved functional specification . • . 89 7.11 Interface test facility . 91 7.12 Conclusions on small computer. communications 92 hardware Chapter 8.'. • • • •', , 914 SPECIFICATION FOR SYNCHRONOUS COMMUNICATIONS INTERFACE FOR SMALL COMPUTERS 8.1 Basic requirement ................. 914 8.2 Modem interface consierations. 914 8 .3 Program control . 0 . 0 0 0 0 0 0 0 0 0 0 0 0 95 iv 914 Chapter 8 . S S S • • • S S S • I S • I I S I • I S SPECIFICATION FOR SYNCHRONOUS COMMUNICATIONS INTERFACE FOR SMALL COMPUTERS (cont.) 8 . 14 General • • • • • • • • • • • • • • • • • • • • • • 95 6.5 Receive (RX)channel ............... 95 8.6 Modem interface considerations forRX channel . 98 8.7 Transmit (TX) channel . '98 8.8 Modem interface considerations for TX channel • • • 100 8 ,9 Timeout control . , . • . • . '. • . 101 8.10 Interrupt control . .. , • • • • • ISS .101 Chapter 9,. • • • • • 0S ••••• . .102 COMMUNICATIONS SOFTWARE FOR THE SMALL COMPUTER 9.1 Overview . • . .' . • , • • . 102 9.2 ' General requirements. $ . 103 9.3 Package implementation—genera], details. 104 9.4 The user interface • .,............ io6 9.5 The communications hardware interface . • . 112 9.6 The executive interface . e . 119 9.7 Remote host computer interface . • . 120 9.8 Conclusions , , • • , • • • • • • • , • . 123 V Chapter 10 .. a S •S S • • a a s • a a U • a U • • • S S a a • 1214 IMPLEMENTATION DETAILS IN ACTUAL SY87kMS 1091 Summary . .. 1214 10.2 PDP-8 with ERCC communications controller . .. 126 1003 PDP-8 with Data Dynamics 6310 controller . .. 128 io.14 ICL 14100 with ERCC communications controller . • 132 10.5 Modular One with. 1.61 communications knultiplexor. 133 10.6 PDP-11 with DP11 communications controller . 1140 10.7 Conclusions . • . • . .. .1.44 Chapter 11 . • a • • • • • . 1146 IMPLEMENTING THE COMMUNICATIONS SYSTEM ON A NEW SMALL COMPUTER 11.1 Introduction . '. • • • • . .. .. .. .. 1146 11.2 General • . • , . • . .. • •• . .. .• .. • . 1147 11.3 Difficulty of 'lie' testing . .. .. .• .. • 1147 11 .4 User program. • . • . a a • a • a a a • a 1148 115 Communications hardware . • .. .. .. .. .. • . 150 11.6 Communications software . .. .. .. .. .. .. • . 153 11.7 Hardware interfacing routines .. .. .. .. .. .. 173 11,8 User interface routines and executive interface • 177 11.9 Conclusions about transferability . • . 179 Chapter 12 . • . a a • • • • • . • . a a 180 COMMUNICATIONS PROTOCOLS FOR INTER-COMPUTER WORKING 12.1 Introduction . a • • • • . • • ... 180 12.2 General characteristics of protocols . • • 181 12.3 Examination of existing protocols • • . • a • • . 182 vi Chapter 12 . • • • • . .• . 180 COMMUNICATIONS PROTOCOLS FOR INTER-qoPqTER WORKING 12. 14 Applicability of protocols . o • a a a .' . 183 12.5 Efficiency of protocols e j ó • i i a • a j • . 183 12.6 Implementation complexity . ... ..... • . 187 12.7 Half-duplex protocol with one-way data traffic . 187 12.8 Half-duplex with two-way transmission interleaved 196 12.9 Full-duplex with two-way simultaneous traffic..•.. 211 12.10 Conclusions on coxnmijnications protocols ... ... 217 Chapter 13 • . • • . • . .• . • . 220 SYSTEM PROGRAMMING ON SMALL. COMPUTERS WITH HIGH-LEVEL LANGUAGES 13.1 Introduction , 220 13.2 Compilation on a different computer .... .' 222 133 Language facilities — general consideration . 22 13,14 Data elements 13.5 Data structures . • • . 229 13.6 Machine address manipulation . S • S i . 230 13.7 Program structure and statements • . • . • . 232 13.8 Expressions . • • . • . ....... 233 13.9 Conditional statements • • • . • • . • . • 2314 13.10 Programmed 1oops. • . a . 236 13.11 Programmed transfers . • •..•.... .236 13.12 Machine-code . .• . • . • . • • •• • . •. 238 13.13 Routines and functions . • •• •• •• • •. •. 239 13.114 Block stru'otjwe . • • . .• . • .• . • . 2141 13.15 Run-time environment . • . • . • . • • •. 2143 13.16 Conclusions • • • • • • • • • , • • . 244 vii Chapter lii . , . • • , • • • , , , 2147 CONCLUSIONS 114.1 Introduction . • . • . 2147 114.2 Uses of computer telecommunications . • '. '. 2147 114.3 Technical aspects of data communications • .. • . 250 114.14 System construction techniqee . • . 253 114.5 Transferable system components . • .• . 2514 14'.6 Interfaces • . • .• • . • . 256 114.7 Real-tine components . ,. ,• • • . 257 114.8 Use of h±gh-level languages . • . • . 258 114.9 Transferable' hardware . '. '. • . 258 114.10 System construction in the future . ..• . • . • 259 114 .11 Further developments • • . .. • • . .. • . 262 ACKNOWLEDGEMENTS . • . • . • . 2614 REYERENCES............;...... ........... 265 Appendix A Chronology of significant developments, indicating items of work involving other people. Appendix B Other available documents on particular implementations. -1- Chapter 1