Page 121 - Mechanical Engineers Reference Book

314 Microprocessors, instrumentation and control
3.2 Microprocessors Intel 8086 microprocessor, introduced in 1979, has 29 000
transistors packed on the 225 mm2 chip.
3.2.1 System architecture The main feature of the microprocessor is the arithmetic
and logic unit (ALU). The ALU allows the arithmetical
Since the mid-1970s there are currently a number of different manipulation of data, addition and subtraction with 8-bit
microprocessor designs available in several versions. The systems and multiplication with 16-bit systems. Logical opera-
‘popular’ designs are produced by a few manufacturers, and tions (AND, OR, etc.) can also be performed. In addition to
these include such companies as Intel, Motorola, Rockwell. the ALU. the CPU contains a number of temporary data
Texas Instruments and Zilog. storage registers to fetch, decode and execute instructions,
Microprocessor-based systems require additional family and to time and control circuits to manage the internal and
support chips and. in true digital form, all microelectronic external operation of the complete microprocessor system.
components which constitute a microcomputer are designated The processing power of the CPU is influenced by such
numerically rather than by name. Some of the more popular factors as word length, instruction set, addressing modes,
microprocessors available are as follows:
number of available registers and information transfer rates.
&bit: Binary data handled in a word, 8-bits wide, defining For word processors or the manipulation of large quantities of
the accuracy of the number handling representation. e.g. data as in CAD packages, 16- or 32-bit microprocessors are
Intel 8080 and 8085 essential. In the field of measurement and control, 8-bit
Motorola 6800 series systems are usually adequate.
Rockwell 6502 series The system clock, accurately controlled by a quartz crystal
Zilog Z80 maintaining a constant frequency, acts as the heartbeat for the
16-bit: Binary data handled in a word of 16-bits width, e.g. system and provides all the timing reference points.
Intel 8086 All the basic components (CPU, memory and I/O) and their
Motorola 68000 interconnections may reside in a complete microcomputer
Zilog 28000 system encompassing the traditional keyboard, monitor, etc.
Alternatively, they may reside on a single card, or even on a
single chip to give a single-chip microcomputer.
The 8086 is one of the most powerful and versatile 16-bit
microprocessors available, and it has been widely adopted by
industry. Further enhancements include the 32-bit ver- 3.2.2 Bus structure
sions - 80286, 80386 and 80486 - which provide increased As outlined in Section 3.2.1, the connection between the
processing power. system components is made by an arrangement of three buses:
Although it is unnecessary for the user of the technology to
understand in detail how each individual chip actually func- 1. The data bus transmits the words in binary form represent-
tions, it becomes essential to have at least a working ing the program instructions for the CPU to execute. It
knowledge of the logical organization of the system hardware can also carry the information transmitted between ?he
and how each component relates to each other. The composi- CPU and memory or I/O devices. Although the popular
tion of this hardware structure is known as the system architec- PCs use a 16-bit data bus, 8-bit data transfer operations
ture. remain the norm in data acquisition and control applica-
A digital computer system comprises three main constituent tions.
parts: the microprocessor, the memory and the inputloutput. 2. The address bus transmits the memory address related to
Digital signals which have a common function are transmitted the data bus information. In 8-bit systems this bus com-
between the main components by a group of wires or conduc- monly has 16 lines to give 64K of addresses. PCs usually
tion tracks, termed a bus. In a microcomputer there are three have an effective 20 lines to give 1M byte of available
buses, i.e. the data bus, the address bus and the control bus. addresses, although software limitations often restrict this
The interconnection between the basic hardware components to 640K.
in a microcomputer is illustrated in Figure 3.1. 3. The control bus transmits the collection of timing and
The microprocessor is a very large-scale integrated circuit control signals which supervise the overall control of the
(VLSI), which is the brain of the microcomputer system and system operation.
acts as the central processing unit (CPU). Integrated circuits The physical format of a bussing system is basically a circuit
are generally classified according to the number of compo- board with a number of connectors. Different types of
nents on the silicon chip, and VLSI has tens of thousands. The microprocessors require different hardware interfaces, and to
alleviate the problems, standard bus structures have been
Address bus developed in order to facilitate the connection of hardware
components. In industrial-type systems, cards for various
microcomputer functions such as processor, memory, digital
and analogue I/O, power switching, etc. slot into a standard
t t backplane or motherboard rack. This offers the advantage of
Memory Data - being able to plug any specific card, designed to the bus
c-
RAM 4 - Microprocessor Data + 110 standard, into a free slot in the rack to build up the system as
ROM bus bus required.
The physical form of the bus is represented by its mecha-
t t nical and electrical characteristics. Such information as card
dimensions, input and output pin-out connections, signal
levels, loading capability and type of output gates must be
known.
Control bus Standard buses are compatible with cards from different
manufacturers, and the most popular bus structures include
Figure 3.1 Basic components in a microcomputer Multibus, S-100 Bus, STD Bus and the STE Bus.