Acorn Computers Limited BBC A, Руководство по оборудованию, Страница: 19 / 47

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CHAPTER 3 :- COMPLETE CIRCUIT DESCRIPTION.
3.1 The Power Supply Unit.
The power supply unit fitted to the BBC Microcomputer is a switch mode type. This unit pr5 volts
at 3.75 amps and -5 volts at 0.l amps, which are supplied to the main printed circuit board (main PCB) via
seven flying leads. These leads are connected to the main PCB using 0.ll" blade type push-on
receptacles. The three red leads carry +5 volts to various points on the board, which are all connected
together electrically by copper tracks on the underside. The reason for this is to overcome the need for
large, high current carrying, copper tracks on the printed circuit board. The mauve coloured lead carries
the -5 volt supply and the three black leads connect the zero volt line to the board. Note on the early,
linear type, supply the -5V lead is coloured brown.
A six pin connector, which is mounted on the case of the power supply unit on the underside of the
microcomputer (see photograph chapter 5), provides auxiliary power for accessories such as a floppy
disc drive unit. This socket provides a +12 volt supply in addition to the supplies carried to the main PCB.
Details of the connections to this socket are given in chapter 5 (fig 5.2).
Early BBC Microcomputers, mainly issues 1 and 2, were fitted with a linear type power supply unit which
employed a conventional mains transformer and bridge rectifier, the regulation of the voltage being
performed by several 7800 series voltage regulators. These power units tend to get rather hot, even in an
unexpanded model A machine and do not provide any auxiliary power. The linear supply can be
distinguished from the later switch mode type by its black painted case and the absence of an auxiliary
power socket.
Both types of supply contain no user-serviceable components and should therefore be exchanged if they
are found to be faulty. The early linear supply should be exchanged for a switch mode type if it overheats
or is faulty.
3.2 The Microprocessor (MPU)
The Microprocessor Unit (MPU) used in the BBC Microcomputer is the 6502A, which is a 2 MHz version
of the popular 8 bit 6502, introduced by MOS Technology in 1975. Although a 2 MHz device is used, and
indeed many devices such as the Random Access Memory (RAM) are accessed at this speed, the MPU
does in fact run at 1MHz when addressing slow devices within the computer. Examples of these 1MHz
devices are the Versatile Interface Adapters (VIA's) and the Analogue-to-Digital (A to D) converter.
3.3 The Clock Circuitry.
The microcomputer uses a master clock frequency of 16MHz, which is generated by a crystal controlled
oscillator. The oscillator circuitry is designed around a high speed TTL hex inverter type 74S04 (IC43).
Two of the inverters within IC43 form an RC oscillator which is forced to run at 16 MHz by a 16MHz
crystal (X1). The output of this oscillator is buffered by one of the remaining inverters within IC43 before
being conditioned by a high-speed NAND gate, part of IC40. This provides the 16 MHz clock signal for
both the video ULA, the floppy disc circuitry and the serial ULA.
One section of the video ULA is used to subdivide the 16MHz clock into four in-phase signals of 1,2,4 and
8 MHz, thus providing the required clock signals for the remaining circuitry within the microcomputer.
A 6MHz clock is required by the SAA5050 Teletext character generator (IC5) and also the "Econet"
circuitry on early versions of the machine. This is produced in a rather unique way using the 4MHz and
8MHz clock signals. A set-reset (SR) bistable is formed using two cross coupled NAND gates (part of
IC40) and two inverters (part of ICs 37 and 43) The 8MHz signal is inverted by part of IC37 (74LS04) and