In this introductory chapter, we consider what is meant by the phrases ‘embedded system’ and ‘time-triggered system’ and we examine how these important areas overlap.
Introduction
Current software applications are often given one of a bewildering range of labels:
● Information system
● Desktop application
● Real-time system
● Embedded system
● Event-triggered system
● Time-triggered system
There is considerable overlap between the various areas. We will therefore briefly con- sider all six types of application in this chapter, to put our discussions of time-triggered embedded systems in the remainder of this book in context.
Information systems
Information systems (ISs), and particularly ‘business information systems’, represent a huge number of applications. Although many of the challenges of information system development are rather different from those we will be concerned with in this book, a
basic understanding of such systems is useful, not least because most of the existing techniques for real-time and embedded development have been adapted from those originally developed to support the IS field.
As an example of a basic information system, consider the payroll application illustrated schematically in Figure 1.1.
This application will, we assume, be used to print the pay slips for a company, using employee data provided by the user and stored in the system. The printing of the cheques might take several hours: if a particularly complex set of calculations are required at the end of a tax year, and the printing is consequently delayed by a few minutes, then this is likely to be, at most, inconvenient. We will contrast this ‘incon- venience’ with the potentially devastating impact of delays in a real-time application in later examples.
ISs are widely associated with storage and manipulation of large amounts of data stored in disk files. Implementations in file-friendly languages, such as COBOL, were common in the 1960s and 1970s and such systems remain in widespread use, although most such systems are now in a ‘maintenance’ phase and new implementations in such languages are rare.
Modern IS implementations make far greater use of relational databases, accessed and manipulated using the SQL language. Relational database technology is well proven, safe and built on a formal mathematical foundation. While the design and implementation of large, reliable, relational database systems is by no means a trivial activity, the range of skills required to develop applications for use in a home or small business is limited. As a consequence, the implementation of such small relational
Refer to Appendix A for details of this notation
database systems has ceased to be a specialized process and relational database design tools are now available to, and used by, many desktop computer users as part of stan- dard ‘office’ packages.
However, new demands are being placed on the designers of information systems. Many hospitals, for example, wish to store waveforms (for example, ECGs or auditory evoked responses) or images (for example, X-rays or magnetic resonance images) and other complex data from medical tests, alongside conventional text records. An exam- ple of an ECG trace is shown in Figure 1.2.
For the storage of waveforms, images or speech relational databases systems, opti- mized for handling a limited range of data types (such as strings, characters, integers and real numbers), are not ideal. This has increased interest in object-oriented database systems (’object databases’), which are generally considered to be more flexible.
