Stratification of computer tasks 94 Example I 94 Example 2 96 Controllevels and computer input/output hardware 104 Leveli 105 Level 2 118 Level 3 118 Level4 118 Level5 119 Characteristics of process control computer systems 119 A survey of process control computer hardware 120 Communication codes and circuits 138 Channe1 capacity 138 Types of connection and communication hardware 140 Practical suggestions and recommendations 152 Rcferences 153 Part II: The Role of Software in Process Control Systems 155 Chapter 4: The relative roles of software and hardware 157 In troduction 157 Data processing 158 Hardware 159 Computingpower 163 Software for process control data processing 169 Process software 170 Intercomputer communication software 173 Message switching software 173 Software for engineering ca1culations 173 Extcnded real-time software 173 Software versus hardware 174 Program loop 175 References 183 Chapter 5: System software 185 Introduction 185 Basic concepts of real-time operating systems 186 Structure and functions of real-time operating systems 190 Data and symbols for the operating system 200 System software 204 Cost, safety and reliability of operating system software 208 References 209 Chapter 6: Application pro grams and databases 211 Introduction 211 Application program tasks 211 Structure and timing requirement of application programs 220 Direct communication 227 Multiprogramming constraints 228 Database and basic process software 233 Access to database 235 Basic faciJities of an on-line database 236 Database organization 240 Contention resolution 243 Distributed database 244 Extended real-time software 247 Referenees 257 Part III: The Man-Machine Interface 259