Appendix 1 UNDERSTANDING MICROCOMPUTER BUSES A1.1 WHAT IS A MICROCOMPUTER BUS? for example, is connected to the microcomputer bus via a parallel or serial interface, which takes the The main 'building blocks' of a microcomputer form of a board connected to the bus, see Fig. 50. comprise the CPU, internal memory and I/O, see This book features two types of bus structure: the Fig. 49. Apple II family and the IBM PC, but there are many These 'building blocks' pass data to one another others. Each bus would originally have been designed under the control of the CPU via a data highway by the computer manufacturer in question, but called the bus. In a typical microcomputer each of because certain computers have become very popu­ these building blocks will be realised as one or lar, their buses have become de facto industry more microcomputer boards. Each peripheral (I/O), standards. This is certainly true for the Apple II and Microcomputer 1---------------------------, I CPU I I I I Control block I Data from Data and control external devices I I to external devices (eg. keyboards, (eg. printer, visual disk drives) I display unit) I I I I I I I Memo~ I L ~ Fig.49 The main internal units of the microcomputer User's Interface Motor Solenoid C)CI) o~ valve -Q)co> ~~ Processors Memo~ Digital 110 Analog 110 Peripheral Industrial 1/0 8088 RAM Input ports DACs interface 8086 ROM Output ports ADCs 6502 280 etc. Fig. 50 The System Bus, with its power, data and microcomputer system can be assembled by adding address and control lines. A special purpose the right combination of boards 239 Expanding and Networking Microcomputers IBM PC. In other cases, bus structures have been infrequent, or important peripherals, to secure the adopted by the US Institute of Electronic and attention of the CPU. Electrical Engineers (IEEE),to be redefined and firmly Some microcomputer buses will allow more than established as an industry standard. Typical examples one device to gain control of the bus lines. A system are the 5-100 bus (lEEE-696), the GPIB bus (lEEE­ may have more than one processor or variety of 488) and the Multibus (lEEE-795). intelligent cards, such as disk controllers, on the A bus is a way of connecting boards together. It bus. Any device that is capable of controlling bus has a physical (hardware) and procedural (software) operation is usually referred to as a bus master and part and we have seen that for the Apple II and IBM passive devices on the bus, such as memory, I/O PC there are over 300 boards available for each ports etc., are called slaves. A system will always machine which, either singularly or in combination, have at least one master and one slave, and it may can give these microcomputers very specific capabili­ have several of each. ties. These boards will conform rigidly to the physical and electrical specifications laid down by Apple and A1.3 SYSTEM EXPANSION IBM respectively for their bus structure. Within the microcomputer several types of buses With the Apple II and the IBM PC expansion occurs are used to transmit power, data, address and via the motherboard. This is a large printed-circuit control signals. Collectively they are referred to as board that distributes bus signals to connectors the system bus, see Fig. 50. which mate with system boards. The appeal of both these microcomputers is that it is quite easy to A1.2 BUS STRUCTURES enhance their performance by slotting in a new board. It is now fashionable to refer to this as open system architecture, although when the Apple II first A1.2.1 Address Lines came out in 1977, several add-in boards were A bus carries electrical signals to and from the required before the machine could begin to do microprocessor and these consist of address, data anything useful. However, both as a result of this and control information. The address lines are used 'openness' and the ease of access to the Apple by the processor to indicate to memory and other motherboard, many small engineering firms were peripherals the location with which it wishes to encouraged to developed special purpose boards. communicate. 8 bit processors such as the 6502 This increased the appeal of the Apple, a lesson that used in the Apple, or the Z-80, have 16 address IBM had learnt when the PC was launched some lines, generally labelled AO-A15, that are divided into five years later. two 8 bit bytes. AO-A7 comprise the low-address Apart from 5-100 based systems, most other byte and lines A8-A15 form the high address byte. microcomputers do not have the expansion capability Thus the maximum addressing capability of an 8 bit of the Apple II and IBM PC. Indeed, with the latest processor is 216 bits, or 65536 bits expressed as 64 generation of Apple products - the Lisa and Kb. Not all the address lines may be used by any Macintosh, there has been a deliberate move away one board. The Intel 8086 and the Motorola 68000 from this approach. Both the Apple and IBM PC are 16 bit processors that may contain up to 24 have well designed power supplies which produce address lines. the necessary voltages and provide the extra current required when a new board is added. Power A1.2.2 Data Lines requirement is a critical factor in a system that is The data lines, usually designated 00-07 on 8 bit being expanded. buses and 00-015 on 16 bit buses, carry instructions and data between the processor and all the A1.4 DEVELOPMENT OF MICROCOMPUTER BUSES peripherals, including memory. All processors have bidirectional data lines which carry information both into and out of the processor. The direction of A1.4.1 The S-100/IEEE-696 Bus information flow on these lines is usually under In 1974 a series of articles appeared in the processor control. Most buses, with the notable U5 magazine Radio Electronics which described exception of 5-100 and the Digital Group buses, construction plans for a computer called the Mark maintain bidirectional data lines. The 5-100 splits 8. This was based on the now obsolete Intel the data lines into eight data input and eight data microprocessor, the 8008, and it represented the output lines. The Digital Group bus splits them into first time a computer had become available to anyone eight memory-data in, eight memory-data out, eight other than an employee of a large corporation. I/O in, eight I/O out. Earlier in the year that the Mark 8 article had appeared, Intel announced the 8080, an enhanced A1.2.3 Control Lines version of the 8008 processor. Les Soloman, editor The control lines coordinate the operation of the of the rival magazine Popular Electronics also decided system components connected by the bus. Most to publish a series of articles on microcomputer buses include a master clock line that indicates to construction, but based on the 8080. It was arranged peripherals that the address placed on the bus is that the kit would be sold by a small electronics settled and valid. company, Micro Instrumentation and Telemetry The direction of the flow of data on the data lines Systems (MITS), and the product was to be called is determined by one or more processor signals. the Altair 8800. The first Altair article appeared in Most buses include reset lines whilst interrupt inputs the January 1975 issue of Popular Electronics and are common to all systems. These inputs allow the kit was an instant success. 240 Appendix 1 108 The Altair was built around a motherboard with I I I I I I I I add-in boards interfacing to the system bus. MITS I I I I called its bus the Altair bus and the designers chose Back- I I I I """""---- plane ---- I I I I a connector for the motherboard that had 100 pins, buses because they were readily available at a discounted Local Area Networks price. The design was largely ad hoc, mainly because ,...... there was no prior experience, but at least it worked. II I I I I I I Success created competition and another company, II IEEE-488 I I I I I II I IMSAI, launched an improved version of the Altair, I but fortunately it used the same bus design. ----+ I I ~----t---~---- I I I I I I In a relatively short period a number of firms I I I I I I were producing Altair look-alikes and other names I I I I I I such as Cromemco, Polymorphic and Processor RS23~C ~---~---~---- e----t---t I I I I I Technology were not uncommon at the time. I I I II I A degree of rationalisation was required and one I I I I I I of the directors of Cromemco, Roger Millan, decided ----+----t---~---~---~~--~---- I I I II I a generic name was needed for the bus. He proposed I I I I I I to call it the Standard 100 Bus in view of the pin I I I I I I I I I I I I I count, or S-100 for short, and the name caught on. 4 For the most part the manufacturers of S­ 10' 10 100 compatible products adhered to the bus/pin Range/metres arrangements quite closely but the relative timings of the various bus signals varied considerably from Fig. 51 Performance characteristics of the different one manufacturer to the next.