9 Digital computation and intelligent devices

 

This chapter is concerned with introducing the principles of digital computation and its application in measurement systems. Digital computers have been used in conjunction with measurement systems for many years in the typical control system scenario where a computer uses data on process variables supplied by a measurement system to compute a control signal that is then applied to an actuator in order to modify some aspect of the controlled process. In this case, the computer is not actually part of the measurement system but merely works with it by taking data from the system. However, the rapid fall in the cost of computers has led to their widespread inclusion actually within measurement systems, performing various signal processing operations digitally that were previously carried out by analogue electronic circuits.

In early applications of digital signal processing, the computer remained as a distinctly separate component within the measurement system. However, the past few years have seen the development of measurement systems in the form of intelligent devices in which the computational element (usually called a microcomputer or microprocessor) is much more closely integrated into the measurement system. These devices are known by various names such as intelligent instruments, smart sensors and smart transmitters. However, before discussing these in detail, the basic principles of digital computation need to be covered first.

 

9.1 Principles of digital computation

 

9.1.1 Elements of a computer

The primary function of a digital computer is the manipulation of data. The three elements that are essential to the fulfillment of this task are the central processing unit, the memory and the input–output interface, as shown in Figure 9.1. These elements are collectively known as the computer hardware, and each element exists physically as one or more integrated circuit chips mounted on a printed circuit board. Where the central processing unit (CPU) consists of a single microprocessor, it is usual to regard the system as a microcomputer. The distinction between the terms ‘microcomputer’, ‘minicomputer’ and ‘mainframe computer’ is a very arbitrary division made according to relative computer power. However, this classification has become

somewhat meaningless, with present day ‘microcomputers’ being more powerful than mainframe computers of only a few years ago.

The central processing unit (CPU) part of a computer can be regarded as the brain of the system. A relatively small CPU is commonly called a microprocessor. The CPU determines what computational operations are carried out and the sequence in which the operations are executed. During such operation, the CPU makes use of one or more special storage locations within itself known as registers. Another part of the CPU is the arithmetic and logic unit (ALU), which is where all arithmetic operations are evaluated. The CPU operates according to a sequential list of required operations defined by a computer program, known as the computer software. This program is held in the second of the three system components known as the computer memory.

The computer memory also serves several other functions besides this role of holding the computer program. One of these is to provide temporary storage locations that the CPU uses to store variables during execution of the computer program. A further common use of memory is to store data tables that are used for scaling and variable conversion purposes during program execution.

Memory can be visualized as a consecutive sequence of boxes in which various items are stored, as shown in Figure 9.2 for a typical memory size of 65 536 storage units. If this storage mechanism is to be useful, then it is essential that a means be provided for giving a unique label to each storage box. This is achieved by labelling the first box as 0, the next one as 1 and so on for the rest of the storage locations. These numbers are known as the memory addresses. Whilst these can be labelled by decimal numbers, it is more usual to use hexadecimal notation (see section 9.1.2).

Two main types of computer memory exist and there are important differences between these. The two kinds are random access memory (RAM) and read only memory (ROM). The CPU can both read from and write to the former, but it can only read from

the latter. The importance of ROM becomes apparent if the behaviour of each kind of memory when the power supply is turned off is considered. At power-off time, RAM loses its contents but ROM maintains them, and this is the value of ROM. Intelligent devices normally use ROM for storage of the program and data tables and just have a small amount of RAM that is used by the CPU for temporary variable storage during program execution.

The third essential element of a computer system is the input–output (I/O) interface, which allows the computer to communicate with the outside world by reading in data values and outputting results after the appropriate computation has been executed. In the case of a microcomputer performing a signal processing function within an intelligent device, this means reading in the values obtained from one or more sensors and outputting a processed value for presentation at the instrument output. All such external peripherals are identified by a unique number, as for memory addresses.

Communication between these three computer elements is provided by three electronic highways known as the data bus, the address bus and the control bus. At each data transfer operation executed by the CPU, two items of information must be conveyed along the electronic highway, the item of data being transferred and the address where it is being sent. Whilst both of these items of information could be conveyed along a single bus, it is more usual to use two buses that are called the data bus and the address bus. The timing of data transfer operations is important, particularly when transfers take place to peripherals such as disk drives and keyboards where the CPU often has to wait until the peripheral is free before it can initialize a data transfer. This timing information is carried by a third highway known as the control bus.

The latest trend made possible by advances in very large-scale integration (VLSI) technology is to incorporate all three functions of central processor unit, memory and I/O within a single chip (known as a computer on a chip or microcomputer). The term ‘microprocessor’ is often used to describe such an integrated unit, but this is strictly incorrect since the device contains more than just processing power.