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It must be stressed that any monitor connected to either the R.G.B. or video B.N.C. sockets must be 

electrically isolated from the mains supply. The easiest way to check this is to consult the manufactures 

of the monitor in question. 

 

Colour television receivers usually require a UHF modulated PAL encoded signal. Such a signal is 

provided by the BBC Microcomputer from the socket marked "UHF out". An Astec UM1233 modulator, 

which has a video bandwidth of 8MHz, is used to convert the composite video signal to the required UHF 

signal. 

 

The PAL encoding is performed using discrete circuitry. A 17.73 MHz signal is generated by a crystal 

controlled oscillator consisting of a transistor (Q10) and a 17.73 MHz crystal (X2). The signal produced is 

then inverted using a NAND gate (part of IC50) and divided by four using a 74574 bistable (IC46) to 

provide the 4.4325 MHz colour subcarrier frequency. The quadrature colour-difference signals "U" and 

"V" are produced on pin 9 of IC46 and pin 11 of IC48 respectively. The horizontal synchronisation pulses 

produced by the CRTC (IC2) use an exclusive OR gate (part of IC48) to gate the "V" colour difference 

signal. This produces the required alternating phase signal for each horizontal line scan. Exclusive OR 

gates (IC47 and IC48) are then used to generate the yellow, cyan, green, magenta, red and blue colours 

according to the phase relationships of the "U" and "V" signals. These signals are then "mixed" using six 

NAND gates (IC49 and part of IC50). The colour burst gate signal is generated using an inverter (part of 

IC37) and a NOR gate (part of IC4). This arrangement is timed by a CR network (C45 and R189) thus 

producing a burst gate pulse in the back porch period of the line scan. The burst gate signal is also mixed 

using the NAND gate mixer 

 

 

 

3.8 Cycle stretching. 

 

Certain devices within the BBC microcomputer need to be serviced at 1MHz instead of the normal 2MHz. 

These devices are: 

 

i) 

Analogue-to-digital converter (IC73) 

ii) 

System and user VIAs (IC3 and IC69). 

iii) 

Sideways ROMs (optional) (ICs 52,88,100 and 101). 

iv) 

MC6850 ACIA (IC4) 

v) 

6845 CRT controller (IC2). 

vi) 

1MHz extension bus (PL11) 

 

 

The circuitry used to perform this function is designed around IC's 23,29,30,3l,33 and 34. Whenever a 

1MHz device requires servicing, the relevant input of an 8 input NAND gate IC23 (74LS30 please note 

this gate is drawn as a NOR gate with

 

 

Содержание BBC A

Страница 1: ...r The BBC Microcomputer Written By A D Derrick B Sc D S Harding B Sc S D Middleton M P Smith Edited and Published by Wise Owl Publications Printed by Queenprint Ltd 64 74 Holderness Road Hull HU9 lEQ...

Страница 2: ...ions or errors No liability is assumed for any damages or injury whatsoever resulting from the use of information contained within this publication All references to the BBC Microcomputer in this publ...

Страница 3: ...eveal some of the many modifications upgrades and circuit changes that can be performed Some modifications only require extra components to be inserted whilst others require careful work with a solder...

Страница 4: ...nglish Street Kingston Upon Hull HU3 2DU Tel 0482 29894 Code F Farnell Electronic Components Ltd Canal Road Leeds L512 2TU Tel 0532 636311 Code RR The Radio Resistor Co Ltd St Martins Way Industrial E...

Страница 5: ...g Circuitry 3 7 The Video Driving Circuitry 3 8 Cycle Stretching 3 9 The Econet Circuitry 3 l0 The Keyboard Circuitry 3 11 The Cassette and R5423 Serial Interfaces 3 12 The Floppy Disc Interface 3 13...

Страница 6: ...1 The 1MHz Bus 5 22 Further Expansions 5 23 The Floppy Disc Expansion 5 24 The Speech Expansion 5 25 The Econet Expansion 5 26 The Second Processor Add Ons I CHAPTER 6 HINTS TIPS AND MODIFICATIONS 6 1...

Страница 7: ...oth of these diagnostic and test devices are produced by Acorn Computers Ltd and are presently only available to official Acorn dealers Until late 1983 the machine was available in two options model A...

Страница 8: ...al system If you intend to use the machine for any serious work then you will probably find the cassette system rather inadequate A floppy disc or Econet system will provide a much more suitable solut...

Страница 9: ...formation storage and retrieval device This system enables a large number of users to share expensive peripheral devices and also provides each one with the facility to communicate with any other user...

Страница 10: ...hout the book to simplify wherever possible the descriptions and explanations of the internal circuitry of the machine In some instances it has been extremely difficult to describe in simple terms wha...

Страница 11: ...11 p11 intentionally left blank for diagram...

Страница 12: ...12 p12 intentionally left blank for diagram...

Страница 13: ...able height such that you can sit down whilst working It is advisable to use a conductive cover placed over the surface of the workbench the reasons for this are detailed in the section below on anti...

Страница 14: ...circuit boards it is essential that you conduct a detailed visual inspection of both the work which you have carried out and the remaining areas of the PCB It is quite likely that small pieces of wir...

Страница 15: ...bject that is known to be at Earth potential The chrome portion of a central heating radiator valve or water tap are ideal iv If possible you should carry out the work on a conductive surface This wor...

Страница 16: ...a task that should be mastered fairly easily by the majority of readers This is especially the case if the general guidelines that follow are carefully observed A good quality soldering iron is of cou...

Страница 17: ...lly opposite pins so that localised overheating of the component and PCB is avoided 2 5 Desoldering Occasionally it will be necessary to desolder components or clear PCB holes which are blocked with s...

Страница 18: ...h should be applied using a stiff brush It is recommended that the manufacturers instructions supplied with the defluxing fluid are carefully read and understood since the solvents used can be harmful...

Страница 19: ...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 overhe...

Страница 20: ...s This CR network delays the reset action of the bistable thus producing a modified 4MHz waveform see figure 3 1 The phase modified 4MHz waveform produced by the bistable is exclusively ORed using par...

Страница 21: ...21 Figure 3 1 Generation of 6MHz Clock signal p21 left blank for diagram...

Страница 22: ...system supplied with machines fitted with a disc interface were contained within two 8 kilobyte EPROMs on a small carrier board When replacing this with a Series 1 Operating System ROM it should be n...

Страница 23: ...ut of a D type bistable half of IC44 The 4 and 8MHz clock signals provide the input to this bistable The 6845 cathode ray tube controller CRTC IC2 or the microprocessor may have control of the RAM add...

Страница 24: ...n stored in the relevant locations in the screen memory These bit patterns are interpreted by the video ULA in a similar way to the character font information Within the video ULA the serialised bit p...

Страница 25: ...r refresh address lines of the CRTC This offset is specified by the system VIA IC3 under the control of the operating system and a two bit code produced via an addressable latch IC32 74LS259 This two...

Страница 26: ...elected to be positive or negative going by a circuit board link S31 is also fed to this socket along with ground and 5V connections The horizontal and vertical components of the composite synchronisa...

Страница 27: ...exclusive OR gate part of IC48 to gate the V colour difference signal This produces the required alternating phase signal for each horizontal line scan Exclusive OR gates IC47 and IC48 are then used...

Страница 28: ...5 of this socket whilst pins 1 and 4 carry the data Pin 2 is connected to 0 volts The data line is bi directional thus both transmitted and received data is carried along the same pair of wires in hal...

Страница 29: ...f links S11 in conjunction with IC96 This is detailed in the chapter 4 which deals with link functions The Econet network must be terminated at each end with its characteristic impedance to prevent si...

Страница 30: ...column in turn directly into IC1 using outputs PA0 to PA3 of IC3 the system VIA thus interrogating each column in turn At the same time outputs PA4 5 and 6 of the system VIA are used to load data into...

Страница 31: ...or via a DC blocking capacitor C34 The CASMO cassette motor output of the serial ULA drives a transistor Q3 which in turn operates the cassette motor relay and also the keyboard cassette lamp LED The...

Страница 32: ...of an 8 inch drive the index pulses enter on pin 8 of this connector The option of whether the index pulse input to the P8271 FDC is taken from pin 4 5 1 4 inch drive or pin 8 8 inch drive of the con...

Страница 33: ...er supply rail The signal at the junction of the CR network provides a reset signal RSTA which is low at switch on and rises to 5V when the capacitor is fully charged Whenever the break key is pressed...

Страница 34: ...technique used is pitch excited Linear Predictive Coding LPC The data from the VSM is decoded within the VSP to reconstruct the digital speech information An 8 bit digital to analogue converter insid...

Страница 35: ...ich is the low level DIN audio output connector The same signal is also fed to a pre set potentiometer VR1 which acts as a volume control for the internal power amplifier of the computer This power am...

Страница 36: ...36...

Страница 37: ...ot PaGe FC and NPGFD Not PaGe FD which are often referred to as FRED and JIM respectively 3 19 FRED This signal is decoded from the machines address bus such that when the address bus holds a valid ad...

Страница 38: ...us is being accessed Circuitry within the BBC Microcomputer is used to effectively stretch the 2 MHz normal 6502A clock cycle into the 1 MHz clock 1 MHz E cycle required by 1 MHz peripheral devices at...

Страница 39: ...wn in figure 3 5 below can be used to eliminate this problem FIGURE 3 5A CLEANING UP PAGE SELECT LINES The circuit shown in figure 3 5 a is basically a gated S R flip flop which will only allow the cl...

Страница 40: ...an interrupt going unrecognised 3 22 NPGFD JIM The NPGFD signal is very similar to the NPGFC line the only difference being that this line is active when the address bus holds a valid address in the r...

Страница 41: ...41 p41 fig 3 5B and 3 5C...

Страница 42: ...42...

Страница 43: ...h which it deals Figure 4 1 at the end of this chapter is a shadow diagram of the main PCB which shows the positions of the various links In addition to the links on the main PCB there are 8 more link...

Страница 44: ...m the photographs in chapter 5 The next step in removing the keyboard assembly is to carefully unplug the interconnecting ribbon cable from the main PCB PL13 and also the loudspeaker connector from PL...

Страница 45: ...however a number of points to note i Do not forget to replace the two wires on the BNC video out connector correctly i e the ground wire to the tab and the signal wire to the centre pin ii When recon...

Страница 46: ...check again 4 4 Link Functions Link No Position Function 1 Printer strobe select Only present on issue 4 boards onwards Selects the type of printer strobe output to pin 1 of PL9 printer port SOUTH Pr...

Страница 47: ...device to the 1MHz bus is via a 600 mm length of 34 way ribbon cable terminated at each end with a 34 way IDC header socket i e RS part no 467 302 The peripheral device should allow all of the signal...

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