Acorn Prestel Adapter Service Manual
Contents
1. ACORN COMPUTER Prestel Adapter Service PRESTEL ADAPTER SERVICE MANUAL Part No 415 001 Issue 2 August 1984 Within this publication the term BBC British Broadcasting Corporation is used as an abbreviation for Copyright ACORN Computers Limited 1984 Neither the whole or any part of the information contained in or the product described in this manual may be adapted or reproduced in any material form except with the prior written approval of ACORN Computers Limited ACORN Computers The product described in this manual and products for use with it are subject to continuous development and improvement All information of a technical nature and particulars of the product and its use including the information and particulars in this manual are given by ACORN Computers in good faith However it is acknowledged that there may be errors or omissions in this manual A list of details of any amendments or revisions to this manual can be obtained upon request from ACORN Computers Technical Enquiries ACORN Computers welcome comments and suggestions relating to the product and this manual All correspondence should be addressed to Technical ACORN Comp Newmarket Enquiries uters Limited Road Cambridge CBS 8PD All maintenance and service on the product must be carried out by ACORN Computers authorised dealers ACORN Computers can accept no liability whatsoever for any2. 2 1 R20 90 505 333 RESISTOR 33K 2 1 R19 53 1 zi m eri Acorn Computers Limited Fulbourn Road Cherry Hinton Cambridge 1 4JN England
3. In Fe a Di S S a 6 3 0 E 5 L Ics R _ 554 IC 819 LM 386 pacc Ov Ov A a SP 00407 TX 1200 VI RING 1 t 20201 Mp ULN 2003 45 5 9 ICh 0404 7 oS Va 1014 ICH UL N2003 04071 8 10 cem AST a 00 1C10 OP Ak 1 16 1 320125 DELAYED RTS 13 Di RU A 812 RLJ 1 6 C0c071 004071 19 DIAL 12 CD4081 108 204081 gt mm E o ULN 20034 TENABLE PRESTEL Adap PCB Circuit Diag 70 70 70 70 70 70 70 70 00000 o 8 eo D nn P all g20 89 21 AAA 21 E 9 7 CT 14 28551 5 90 21 30 PRESTEL Adapter PCB Component Layout 41 OOOO oe LU e zu tis 1022 425 37 25 n E STA 1 1 Ics gt e E tra ac Cab ICIO Iti 4 t SS LI z cal COMPUTER D en O O PRESTEL Adapter PCB Silk Screen 43 NOTE SPEAKER INSERTED UNDER MOUNTING LUGS 1165 BENT DOWN AND ADHESIVE APPLIED TO SECURE SPEAKER TO LUGS PRESTEL Adapter Final Assembly PRESTEL Adapter Case Lower Assembly PRESTEL A
4. Green and yellow Earth Blue Neutral Brown Live As the colours of the wires may not correspond with the coloured markings identifying the terminals in your plug proceed as follows The wire which is coloured green and yellow must be connected to the terminal in the plug which is marked by the letter E or by the safety earth symbol or coloured green or green and yellow The wire which is coloured blue must be connected to the terminal which is marked with the letter N or coloured black The wire which is coloured brown must be connected to the terminal which is marked with the letter L or coloured red If the socket outlet available is not suitable for the plug supplied the plug should be cut off and the appropriate plug fitted and wired as previously noted The moulded plug which was cut off must be disposed of as it would be a potential shock hazard if it were to be plugged in with the cut off end of the mains cord exposed The moulded plug must be used with the fuse and fuse carrier firmly in place The fuse carrier is of the same basic colour as the coloured insert in the base of the plug Different manufacturers plugs and fuse Carriers are not interchangeable In the event of loss of the fuse carrier the moulded plug MUST NOT be used Either replace the moulded plug with another conventional plug wired as previously described or obtain a replacement fuse Carrier from an authorised BBC Microcomputer dealer In th
5. If their states are incorrect check the clock input IC7 pin 10 This should be a continuous 4 43 MHz clock signal Note this is a fairly fast signal for CMOS logic and slow edges to the clock signal are to b xpected If this 13 is not present check 6 8 where the clock should also be seen and that IC6 pin 9 is at a logic 0 This signal goes to a logic 1 only when the RTS input is held continuously high thus disabling the clock input short pulses such as STEP and RESET pulses on the RTS input will not affect it If the clock is still not present then check for open circuits track shorts around the crystal XTAL1 and its connections To continue checking the decode circuitry set the scope to trigger the falling edge of the RTS input pulse and observe the signal on IC9 pin 6 This should be a very short negative pulse of not less than 250 nSec note this is a single short pulse that occurs every few milliseconds so high level on the scope intensity and a fast timebase is required to observe it It is this pulse that is used as a clock input to step through the counter IC10 whose outputs control the Adapter state see Fig i If the RESET LOOP is again applied to the adapter f3 a similar signal should be seen at IC9 pin 5 Ensure that this pulse is also present at IC10 pin 15 If it is not check that 2 pin 6 is at a logic 1 this input is a power on reset signal
6. and Prestel Adapter Enter PRESTEL Prestel x display 3 v Press for call and input Prestel phone number press 15 adapter message displayed 9 Dial tone for two seconds Relays clicking Call established ie Prestel welcome 28 Response to keyboard 9 v FINISH Master Flowchart 19 No Prestel display Is Prestel ROM Install Prestel version 1 0 or ROM version higher fitted 1 0 or higher 2 Does HELP display ROM fitted Check ROM installed correctly no bent pins Replace ROM Adapter error displayed Is Prestel Adapter connected and on Make all connections switch on Mains fuses in plug and on rear of unit OK Replace fuse s If blowing repeatedly check mains switch and wiring 5V across C21 5 across C25 Replace fuse s Check internal f If blowing internal fuses tedly 53 54 RS423 BBC Micro Check RS423 cable connections OK necessary BBC Micro fault repair BBC Micro Make and check phone line connections ponte Check BT line working Speaker connections OK Repair as necessary Check VR4 set approximately mid way Signal and go
7. and pin 12 will go low Step 8 takes it to the Recieve at 1200 baud mode and pin 12 returns high Subsequent STEP pulses will toggle both pins 12 and 13 between logic 0 and 1 The modem chip modes for combinations of these two inputs are TXT1 TXT2 Modem IC mode 0 0 Tx at 1200 1200 1200 0 Tx at 600 not used 1 0 TX at 75 1200 75 1 1 Rx at 1200 1200 1200 Pin 11 is the Transmit analog output It is the modulated output corresponding to the state of the TXD input pin 14 Power Supply Checks Before examining signals on the PCB it should be ensured that the power supply is providing the correct voltages Use a DVM to ensure that TP9 is at 5 volts 100mV and that TP7 is at 5volts 200mV Also check that this voltage is present across fuses FS5 and FS6 respectively a drop of up to 0 1 volt across each fuse may be expected If a fuse has blown the Adapter should be switched off and the fuse replaced If it continues to blow check for short circuits on the pcb If the voltages are not within specification then the PSU circuitry may be checked as follows Firstly disconnect the Adapter from the mains supply Remove the 6 screws securing the PCB and disconnect the speaker Remove the 4 M3 bolts washers and nuts securing the plastic enlosure covering the PSU components If the voltage on one of the Test Points is only slightly out of spec then the voltage r
8. loss or damage caused by service or maintenance by unauthorised personnel This manual is intended only to assist the reader in the use of this product and therefore ACORN Computers shall not be liable for any loss or damage whatsoever arising from the use of any information or particulars in or any error or omission in this manual or any incorrect use of the product This manual is for the sole use of ACORN Computers authorised dealers and must only be used by them in connection with the product described within ACORN is a trade mark of ACORN Computers Limited First published 1984 Published by ACORN Computers Limited CONTENTS Page 1 INTRODUCTION 2 PACKAGING AND INSTALLATION 3 SPECIFICATION 4 DISASSEMBLY AND ASSEMBLY 5 CIRCUIT DESCRIPTION 6 DETAILED CIRCUIT OPERATION 7 FAULT FINDING ON THE PRESTEL ADAPTOR APPENDIX A De bugging Program APPENDIX B Diagnostic Flowcharts RS423 Cable Assembly Detail Telephone Cable Assembly Detail Speaker Assembly Detail PRESTEL Adapter Block Diagram Circuit Diagram PCB Component Layout PCB Silk Screen Final Assembly Case Lower Assembly Wiring Diagram Parts Lists Final Assembly Case Lower Assembly PCB Assembly 11 17 19 35 35 30 37 39 41 43 45 47 49 51 52 52 WARNING THE PRESTEL ADAPTER MUST BE EARTHED Important The wires in the mains lead for the Prestel Adapter are coloured in accordance with the following code
9. 0K AA Al A AT 22 23 23 22 23 22 A 22 22 22 2 A E 1 43 WW WW WW POTENTIOME TER 10K P CAPACITO CAPACITO CAPACITO CAPACITO CAPACITO CAPACITO CAPACITO CAPACITO CAPACITO CAPACITO CAPACITO DIODI DIODI DIODI gt A Af A 23 22 22 23 22 23 23 A Y RS423 d ET 10nF PLATE RAMIC 47nF 33nE 100nF DISC 220nF DISC 47pF MIN 212 250V D TO gt H 8 U OI um Ed 99 oe oe oe oe oe oe oe oe oe oe oe oe oe O UPLER RAMIC RAMIC D 0147 25V E ECT AXIAL 0 1 40V E ECT AXIAL F 10 16V EL AXIAL 2201 63V E ECT AXIAL 220pF 63V MIN BZY96C5V1 BZX70C12 BZX79C3V3 PLATE OTY R E p p pas GR IB E IB EB ES IO S EMARKS SLO BLO D3 4 5 6 D7 8 9 10 01 2 ITEM PART No DESCRIPTION REMARKS PRESTEL PCB Assembly SEE PAGES 41 amp 43 cont d 40 799 101 BRIDGE RECTIFIER 1KAB10 1 BR1 42 T
10. 6301 2 INTEGRATED CIRCUIT TCM3101 1 43 770 062 INTEGRATED CIRCUIT TLO62 1 IC2 45 733 091 INTEGRATED CIRCUIT DS3691 1 IC13 46 779 001 INTEGRATED CIRCUIT 1 320125 1 IC16 47 178 006 INTEGRATED CIRCUIT LM341P5 1 c15 48 770 386 INTEGRATED CIRCUIT LM386 il LGS 49 754 049 INTEGRATED CIRCUIT 4049B 1 IC4 50 754 071 INTEGRATED CIRCUIT 4071B 2 1 6 14 51 754 020 INTEGRATED CIRCUIT 4020B 1 IC7 52 754 081 INTEGRATED CIRCUIT 4081B 2 8 12 53 754 556 INTEGRATED CIRCUIT 4556 1 IC9 54 754 520 INTEGRATED CIRCUIT 4520B 1 0 55 772 003 INTEGRATED CIRCUIT ULN2003A 1 1 57 820 043 CRYSTAL 4 433619 MHz 1 XTAL 1 59 865 002 TRANSFORMER BARRIER 1 1 60 865 001 TRANSFORMER MAINS 1 T2 61 810 003 RELAY 1 RL1 62 810 005 RELAY 1 RL2 63 810 004 RELAY 1 RL3 66 815 016 FUSE 20mm x 5mm 63mA TYPE F 5 FS1 2 3 4 6 67 815 003 FUSE 20mm x 5mm 250mA TYPE F 1 FS5 68 815 910 HOLDER FUSE PCB MTG 12 70 800 200 FASTON TAB 0 110 9 71 800 203 FASTON TAB 0 250 2 72 800 250 PIN 2 WAY 2 LK1 73 800 051 PIN 3 WAY 1 LK3 74 800 070 SHUNT 2 LK1 LK2 76 885 103 PLASTIC BOX 1 79 800 108 SOCKET I C 8 WAY DIL 1 80 800 116 SOCKET I C 16 WAY DIL 1 FOR 83 882 972 WASHER M3 SHAKEPROOF 4 84 882 962 WASHER M3 PLAIN 8 85 882 902 NUT M3 FULL 4 86 882 122 SCREW M3 x 6mm PAN HD 4 87 505 683 RESISTOR 68K 2 1 R21 88 505 393 RESISTOR 39K 2 1 R18 89 505 563 RESISTOR 56K
11. 882 902 NUT M3 HEX FULL 6 14 882 972 WASHER M3 INT TEETH 17 890 000 FOOT STICK ON 4 51 IT PRESTEL Case Lower Assembly SEE PAG PRESTEL PCB Assembly 52 EM PART No 201 110 882 652 805 003 815 900 815 209 815 901 201 184 201 185 880 026 880 025 201 186 201 187 880 101 201 188 201 189 201 190 201 191 502 154 502 100 502 101 502 471 502 561 502 102 502 222 502 103 502 123 502 472 502 473 502 823 502 104 502 124 502 474 581 103 629 010 680 002 628 101 651 224 632 047 670 222 620 047 636 010 622 100 639 221 628 221 795 005 795 012 795 003 DESCRIPTION CASE PCB A SCRE SWITC ULDING EMBLY No6 MAINS LOWER HOLDE USE HROU Omm FUSE x 5mm FUSE ABE D w REAR MAINS HOLDER 400mA TYPI ES 47 amp 49 x 6mm SELFTAP El H C W PLUG TELEPHONE CABLE ET ET ASSE MAINS RS423 CABL Y TELEPHONE ASS EMB CABLE NE UTRAL o EV Ka AGQGGHAADDAANMNH D O 5 5 PER FUS D ES 4 eA aa A EE A 0 ESISTO ESISTO ESISTO ESISTO ESISTO ESISTO ESISTO ESISTO ESISTO ESISTO ESISTO ESISTO ESISTO ESISTO ESISTO 150K 10R 100R 470R 100K 120K 47
12. BREAK PULSE WIDTH 66mS 6mS Sg MAKE PULSE WIDTH 33mS 3mS 4 DIAL ON RELAY HOLD TIME gt 5mS 5 INTER DIGIT DELAY TIME gt 850mS Fig iii Dial Characteristics Timing Diagram 10 7 FAULT FINDING ON THE PRESTEL ADAPTER 7 1 General Any description or accompanying note giving details of the fault and how it came about should be read carefully Always ensure that there is a fault with the hardware and that it is not due to the user s software or the user s understanding of the PRESTEL Adapter which is the problem Always check the power supply voltages on the PCB as a matter of course 5 Power Supply Checks page 15 Also check all the fuses on the PCB itself there are six PCB mounted fuses which may become loose if the PRESTEL Adapter has been subjected to excessively rough handling If a fault has to be traced through on the PCB a useful debugging program for the BBC Microcomputer is given in Appendix A This routine configures the function keys as follows 0 STEP PULSE f1 RESET PULSE f2 CONTINUOUS STEP PULSES f3 CONTINUOUS RESET PULSES f4 RL1 ON 5 RL2 ON f6 RL3 ON Keys f2 and f3 may be used to trace the progress of a STEP or RESET pulse through the decoding circuitry Keys f3 f4 and f5 will activate relays RL1 RL2 and RL3 respectively so that their operation can be individually checked 7 1 Fault C
13. DAPTER PARTS LISTS NOTE Items indentified by are normally available as spare parts please contact your supplier for details of availability denotes SAFETY CRITICAL COMPONENT see below IMPORTANT In order to comply with BT approval regulations components used within this unit must only be replaced with exact equivalent parts All replacement parts identified in the list below are deemed to be Safety Critical and must be obtained from the Customer Services department of Acorn Computers Limited On no account may any other part be substituted Failure to comply with this may result in a breach of approval regulations and may cause the product to became unsafe SAFETY CRITICAL COMPONENTS ACORN PART NO DESCRIPTION 810 005 RELAY 810 004 RELAY 810 003 RELAY 865 002 BARRIER TRANSFORMER ITEM PART No DESCRIPTION QTY REMARKS PRESTEL Adapter Final Assembly SEE PAGE 45 3 SEE BELOW CASE ASSEMBLY LOWER 4 201 109 CASE MOULDING UPPER 4 5 201 182 LABEL CASE UPPER 1 7 882 946 NUT SPIRE No 6 2 8 882 665 SCREW No 6 x 13mm SELFTAP PAN HD 3 BLACK 10 201 183 PLATE SPEAKER FIXING 1 14 SPEAKER ASSEMBLY SEE PAGE 35 2 835 001 LOUDSPEAKER 16 OHM 0 2W 1 4 800 082 SHELL 2 WAY 1 5 800 080 CONNECTOR CRIMP 2 12 882 022 SCREW M3 x 8mm SL CH HD SCR 6 13
14. HTXDH amp 6A 120 RTSHTXDL amp 0A 130 DIMQ 100 140 FOR I 0 TO 2 STEP 2 150 0 160 OPT 15 170 180 XSTEP 190 LDA 1 200 JMP OSWORD 210 220 XRESET 230 LDA 0 240 250 OSWORD 260 STA PB 270 LDX PB MOD 256 280 LDY PB DIV 256 290 SEI 300 LDA amp 7B 310 JSR amp 320 CLI R 330 340 344 350 J 355 J RL1ON SR XRESET SR XSTEP MP XSTEP E 360 370 380 RL2ON 390 JSR RLION 400 LDA RTSHTXDH 410 STA UARTS 420 RTS 430 440 RL3ON 450 JSR RL20N 460 LDA RTSHTXDL 470 STA UARTS 480 RTS 490 500 PB EQUB 0 510 520 530 NEXT 540 PRINT N 550 PRINT FUNCTION KEY CONFIGURATION 560 PRINT f0 CALL STEP 570 PRINT f1 CALL RESET 580 PRINT f2 STEP LOOP 590 PRINT f3 RESET LOOP 591 PRINT f4 CALL RL1ON 592 PRINT f5 CALL RL2ON 593 PRINT f6 CALL RL30N 600 PRINT 610 K 0 CALL XSTEP M 620 K 1 CALL XRESET M 630 K 2 REPEAT CALL XSTEP UNTIL 0 M 640 K 3 REPEAT CALL XRESET UNTIL 0 M 641 K 4 CALL RLION M 642 K 5 CALL RL20ON M 643 K 6 CALL RL30N M 650 END 18 APPENDIX B DIAGNOSTIC FLOWCHARTS The letters in circles refer to the relevant flowcharts which follow START Connect Adapter to BBC Micro and phone line power up BBC
15. ar of the unit Before removing this fuse the PRESTEL Adapter must be disconnected from the mains supply Access to the fuse may be gained by undoing the round cover with the slot in it using a screwdriver The mains supply must not be reconnected until the fuse is relocated in its holder and the cover screwed hom 2 3 Do not use the PRESTEL Adapter in conditions of extreme heat cold humidity or dust or in places subject to vibration Do not block ventilation under or behind the computer Ensure that no foreign objects are inserted through any openings in the adapter Casing 3 SPECIFICATION The PRESTEL Adaptor Interfaces a BBC Microcomputer to any British Telecom attached 1200 75 baud dial up system e g PRESTEL MICRONET TELECOM GOLD Operates with any BBC Microcomputer model B with 1 2 MOS or later issue The unit is housed in a two part rigid injection moulded Case containing a PCB carrying the following Modem Auto Dialler RS232 423 compatible interface Telephone line interfac Power Supply Mains on off switch on rear panel Call set up and auto dialler fully computer controlled A 1200 75 baud modem interfaces to the RS423 serial port on the BBC Microcomputer and to th telephon lin via th telephon lin barrier All data processing is performed by software including the downloading of telesoftware files from MICRONET 80 Lo
16. d the RTS PRESTEL Adapter Control signals 4 Relay Drivers These are Darlington current drivers contained in a single IC package 1011 controlled by CMOS logic They are able to sink the high current levels required to drive the relay coils 5 Amplifiers The modulated data output and data input of the modem chip are buffered by amplifiers IC2 Also the telephone line may be audibly monitored through the loudspeaker which is driven by the amplifier 165 6 Control and Decode This section contains all the logic within the Adapter Driven from the main modem clock frequency 4 43 MHz the decode circuitry will detect one of two pulses of a predetermined width on the RTS input line The first type of pulse will cause all of the control circuitry to be reset to its initial on hook state The second type of pulse may then b repeatedly applied to the RTS line to step the control circuitry through a series of control states In this manner the following control functions are provided i Modem Mode select Transmit baud rates Originate Answer 11 Activation of Line Seize Dial on and Digit Relays 111 Loudspeaker on off state iv Control of CTS o p line such that it provides the function of Carrier Detect or Ring Indicator Also in conjunction with the Transmit Data line the Dial relays may be controlled to generate the digits required for Pulse dialling 7 PSU The power
17. e event of the fuse blowing it should be replaced after clearing any faults with a 3 amp fuse that is ASTA approved to BS1362 Exposure Like most electronic equipment the Prestel Adaptor should not be exposed to direct sunlight or moisture for long periods Not necessarily the same shade of that colour 1 INTRODUCTION This manual is intended to provide the information required to diagnose and repair faults on the PRESTEL Adapter a part of the BBC Microcomputer system which was designed by ACORN Computers Ltd of Cambridge England The information contained in this manual is aimed at ACORN dealers and servic ngineers who will be servicing the PRESTEL Adapter on behalf of ACORN Computers Ltd 2 PACKAGING AND INSTALLATION 2 1 The BBC Microcomputer PRESTEL Adapter is supplied in a two part moulded polystyrene packing which is further packaged within a cardboard sleeve 2 2 The following items are supplied with the PRESTEL Adapter PRESTEL ROM PRESTEL Subscriber s Application Form PRESTEL User Guide PRESTEL Prompt Card for BBC Microcomputer Function Keys Guarantee Registration Card Voucher redeemable against replacement of a BBC Microcomputer MOS ROM below version 1 0 2 3 A mains power switch is located at the rear of the PRESTEL Adapter 2 4 A 400mA slow blow type fuse is located at the re
18. econd of the two 4 bit counters The output 03 always enables the line seize relay and if LK3 is set as shown in the PCB assembly diagram will enable the second of the two 4 bit counters If the second of the two counters is enabled then the 00 output of the second counter provides additional mode control to the modem The link LK3 is provided to enable a Transmit Data at 1200 baud mode on the modem It should always be set to enable this mode as shown in the PCB Layout on page 41 items 73 74 LK3 fitted in position shown by solid black area Normally the link connects 03 on the first counter to the Enable input on the second counter and all states shown in Fig i are available This is the default setting for LK3 for normal PRESTEL operation If 1K3 is set such that 02 is connected to the enable input of the first counter then Q3 is never clocked high thus keeping the second counter in the reset state The PRESTEL Adapter can therefore only step through control states 1 to 8 see Fig 1 1 The operation of the relays in the telephone line interface is in the following manner The Line Seize relay RL1 is normally open and is activated closed to seize the line to enable data transmission and dialling To enable the data link only RL1 need be on closed To generate dialling signals the relays RL2 and RL3 operate as shown in Fig iii To dial a digit
19. ections at the rear of the board 11 Disconnect the two mains connection wires blue and brown from the PCB 111 Disconnect the twisted speaker cable from the PCB iv Undo the six fixing screws the PCB may then be lifted clear 5 The speaker plate assembly may be removed after the 6 retaining nuts and bolts are unscrewed and the speaker cable is disconnected from the PCB 5 CIRCUIT DESCRIPTION 1 This section should be read in conjunction with the block diagram and if necessary the circuit diagram in the Appendix 2 Modem The modem circuitry is based around the Texas TCM3101 single chip modem It is a medium speed frequency shift keying FSK modem which contains modulator demodulator and signal conditioning circuitry and is configured to operate on CCITT V23 standard frequencies The modem will always accept input RX data at 1200 baud The Transmit data speed is selectable to either 75 baud or 1200 baud depending upon the state of TXT1 and TXT2 pins In normal PRESTEL mode TXT1 will be at logic 1 and TXT2 will be at logic 0 Considering only the 1200 75 mode of operation i e transmit 75 baud receive 1200 baud the operation of the modem is as follows To transmit data the TXD input controls an output signal on the TXA pin which is a sine wave of one of two distinct frequencies A logic 0 on the TXD pin generates an output frequency of 450Hz and a logic 1 wi
20. ee Fig 11 54 t Q13 IC7 pin 2 1 b N 25 EN 014 IC7 pin 3 4 1 lt tpw reset 1 41mS 1 RTS i I tpw pulse 2 35mS RTS Fig ii RTS Pulse Timings Note Although the counter is reset as soon as the RTS pulse goes low i e all Q outputs of IC7 are set to zero internal gate delays allow the end count conditions on 013 014 to be decoded by IC9 before they are set to zero This produces a very short negative pulse on the Q1 or 02 outputs of the decoder 169 These internal Step and Reset pulses from IC9 are used to generate the control signals required to configure the states shown in Fig i These control signals are the outputs of two 4 bit binary counters 0 The Reset pulse simply resets the main counter on 1C10 pin 15 such that all outputs are low a power on reset is also provided by R16 and C31 The Step pulse is applied to the clock inputs of IC10 to sequentially enable disable the control outputs Q0 to Q3 in a binary count These outputs are gated through IC6 and IC8 to activate the relays as required in the correct sequenc 8 The output 00 is a speaker on off switch and can activate the line seize relay The output 01 always activates the line seize relay The output 02 always activates the line seize relay provides modem mod control and if LK3 is set will disable the s
21. egulator associated with that voltage is suspect and should be replaced g IC15 for the 5 volt rail and IC16 for the 5 volt rail 1 5 Reposition the PCB in the lower case ensuring that no washers bolts etc have been left in the base and switch on the Adapter Using a scope observe the 15V pin protruding from the PSU transformer to check for a 15 volt AC waveform If it is not present then disconnect from the mains and check the continuity of the mains wiring from cable to PCB If this is OK then replace the transformer If the 15 V AC is present check the ve terminal of C34 for a DC voltage of 20 volts with up to a 15 volt peak to peak ripple If the voltage is incorrect then replace the bridge BR1 and the regulator IC15 Repeat this for the negative rail by checking that the voltage at the ve end of C37 is at 22 volts with up to 5 volt peak to peak ripple If not then replace BR1 and IC16 If a fault still persists then check for short and open circuits around the PSU circuitry 16 APPENDIX A PRESTEL Adapter Debugging Program for the BBC Microcomputer model B 10 REM This listing configures the 20 REM function keys f0 and fl to 30 REM generate STEP and RESET 40 REM pulses to assist debugging of 50 REM PRESTEL Adapters 60 70 REM Note this routine expects a 80 REM PRESTEL ROM to be installed 90 100 UARTS amp FE08 110 RTS
22. elay opening and closing should be heard If the signal is switching but no clicking is heard check the resistance between the connections A and at the phone cable connections at the rear of the pcb It should be alternating between open circuit gt 1 Mega ohm when open and less than 100 ohms when closed If this is not so then replace 1 For RL2 press the 5 function key The resistance across the A and C terminals should be less than 25 ohms 14 For RL3 press the f6 function key The resistance across the A and C terminals should be gt 1 Mega ohm with RL3 active If any of these are incorrect then replace the appropriate relay To check the operation of the modem chip Pin 10 is the Carrier detect level adjustment and is set by R19 and R20 to be a constant 3 2 volts 0 2 volts Pin 7 is the Receive bias adjustment and is set by R18 and R20 to be a constant 2 7 volts 0 2 volts If either of these are incorrect then data transfer problems may be encountered Check and replace resistors if necessary Pin 3 is the Carrier Detect output and is low in the RESET state Providing 9 STEP pulses will switch the output to a logic 1 If this does not occur a fault exists within the modem chip or in the line amplifiers circuitry Pin 13 and 12 are the modem mode control inputs TXT1 and TXT2 They should both be a logic 1 on reset After 4 STEP pulses the modem will be configured to a transmit at 75 baud mode
23. ifiers Phone line interface To phone line TP2 XTAL 1 TPSO OO TP TP6 O 4 43361875 MHz R22 R2 LKI EJ o 16 15 Ri osc c2 R18 2 2208 CHA 10n Ri 5 AC 162 vR3 11662 100K Ov 5v R23 DM 102 TL 062 c30 3 R24 220n 10n 2 7 RXT LH RE d RS 0 12K 100K cs 120K Ov LM386 Ov Ov n 4 lt 5 1 4 04049 2 3 1 4 CD4049 GND GREEN 0 n F51 RI4 RTS ES E WHITE FS63mA 2X2 162 04 87 96 5 1 Y FS2 R15 ES EL BLUE FS63mA 2K2 7 DS B2Y96C5v1 06 8 96 1 V N FS3 RED FC 63mA 07 7 T 4081 08 NY CD40 81 9 a Se SEIZE 19 lt 3 5 YELLOW FS63mA 24 ene A N x002 ONE e e u e e e THESE COMPONENTS ezuroen Y 3 1 ISOLATION BOX De 51 6 8 1 4 9 FS5 OuT IN 26 cn C20 cn 5230m L GNO amp 7n amp In 1045 1 15 6v3 MII 4 C22 C24 C25 1 cas C 1 gt In 10u 6 3 FS6 10 al FS 63 OrrsO Lo llf TU 10401 RED L BLUE Oe m CENE Nip 4 nur GREEN 1 01 02 i R4 Rut 19401 gt lt o A WHITE 100R 82x79 4 10201 N C As Ov Re C5 lt 1 029 2 2 5708 1
24. is weak and the ear may need to be close to the Adapter to hear it It is produced by the mechanical action of the relays opening and closing and is not heard through the speaker This may be the result of Loose or faulty internal fuse 51 FS2 Circuit fault Internal power supply faulty Faulty relay RL2 or RL3 an Tp Failure to establish a call to a PRESTEL Computer after correct dialling This will be apparent by failure of the PRESTEL Welcome frame to appear failure of the online message to appear at the bottom of the screen and no answer tone heard through the speaker once a telephon connection is made This may be the result of Loose or faulty internal fuse FS3 Modem 1C1 faulty Circuit fault Internal power supply faulty Incorrect number being dialled Oo o 12 No response to keyboard This may be the result of Power Supply faulty Modem faulty Circuit fault BBC Micro fault DO mm Diagnostic Flowcharts for the above conditions are given Appendix These should be read in conjunction with the following Circuit Checks Circuit Checks It is assumed that the PRESTEL Adapter is connected to a BBC micro installed with a PRESTEL ROM The Adapter MUST NOT be connected to a phone line The following equipment is also required I 20MHz oscilloscope dual trace LE DVM to measure 5 volts to within 0 01 volts The fu
25. ll output a frequency of 390Hz This signal is buffered through IC2 and fed to the telephone line via Tl the line matching transformer Such signals are well suited to transmission through phone lines as they are well within the bandwidth of the telephone system and contain no high frequency harmonics Data is received from the telephone line amplified through IC2 and fed into the RxA input of the Modem It is again in the form of a sine wave of one of two distinct frequencies However as the RxData rate is 1200 baud the two frequencies used as a standard are different A logic 0 appears as 2100Hz frequency and a logic 1 as 1300Hz frequency Circuitry internal to the modem conditions this signal and outputs a logic 0 or 1 on the RxD pin of depending on whether 2100Hz or 1300Hz frequency is present at the input The CDT output of ICl is a signal which becomes active if a valid signal is present on the RxA input to the modem i e a sinusoidal signal Its primary function is to indicate when communication between the modem and an external device has been established 3 RS423 Line Interface This interface provides th interconnection between the PRESTEL Adapter and the BBC Microcomputer It translates the input signal levels from 5 volts to 5v CMOS logic levels and the converse for output signals It carries the Receiv Data Transmit Data CTS Carrier Detect and Ring Indicator an
26. n on on On on 1 2 3 4 5 6 7 8 0 1 12 13 Fig 1 PRESTEL Adapter State Diagram A pulse of approx 1 41ms will be internally decoded a RESET pulse and will reset the PRESTEL Adapter to its offline or initialised state Using these two pulse signals it is possible to configure the Adapter to any one of the modes shown in Fig 1 Internal decoding of these pulses is accomplished by IC7 and IC9 these are a 14 stage binary ripple counter and a 2 to 4 line decoder Th counter IC7 is clocked by the stable 4 43 MHz crystal controlled oscillator internal to the TCM 3101 Modem chip The final two stages of the counter Q13 and 014 drive the decoder The sequence of operation is as follows When a pulse is applied on the RTS input the counter IC7 is enabled RST pin 11 goes low and starts counting at 4 43 MHz and continues to do so as long as the pulse on RTS is active When the pulse goes low 013 and 014 will in a certain state dependent upon the width of the RTS pulse and the decoder IC9 will be enabled and will decode 013 014 to produce an output on 00 to 03 of IC9 Only 01 02 of 9 are used and they will produce a short negative pulse under the following states 1 Step pulse on 02 of IC9 if 013 low and 014 high 11 Reset pulse on 01 of IC9 if 013 high and 014 low Any other states 013 and 014 are not recognised S
27. nction of the Adapter circuitry is to decode input pulses from the RTS input and configure the phone line interfac relays and modem chip mode control as shown in the State Diagram Fig 1 A fault in this circuitry may manifest itself in a number of ways and could give rise to any of the faults stated above The best method of checking through the circuitry is by exercising the Adapter using the debugging program given in Appendix A see above This program will configure the function keys to produce RESET and STEP pulses and to produce a continuous stream of either The decode circuitry may be checked by observing the progress of pulses sent through the RTS line A continuous series of RESET pulses may be sent to the Adapter by pressing the RESET LOOP function key f3 he path of the signal from the RTS input to pin 11 of IC7 should be raced using a scope Triggering the scope on the rising edge of the TS input signal use FS1 a second scope probe should be attached to C7 pins 2 and 3 in turn Their states at the falling edge of the RTS ulse should be as shown in the RTS Pulse Timing Diagram Fig ii for reset pulse ress ESCAPE then press the STEP LOOP function key f2 Observe the signals on IC7 pins 2 and 3 again Their states at the falling edge of the RTS signal pulse should be as shown in Fig 11 for a STEP pulse
28. od contact to speaker Check IC5 and at pin Q associated components reset replace as necessary I Speaker Replace O C speaker 85423 connections to BBC Micro OK x Repair as necessary 5V across C21 5V across C25 Check RL1 1C11 pin 13 for switching current 1C6 pin 4 Relay RL1 switching 9 Check IC6 logic repair replace necessary BBC Micro fault check and repair as necessary x Refer BT line checks to BT personnel See fault finding guide text Relays not heard clicking while call being dialled Replace fuse s FS1 x If fuse s blows and FS2 OK repeatedly check RS 423 connections D3 D6 and IC12 RS423 connections OK x Repair as necessary 5V across C21 5V across C25 lt Check IC11 and replace if necessary See fault finding guide text Repair as necessary Replace RL2 RL3 Call not established Phone line Remake engaged call x RS423 x connections Repair OK as necessary Repair replace FS3 fuse s FS4 blown If fuse s blows or poor repeatedly check connection RS423 and circuitry gee text 5V across C21 5V across C25 Replace modem faults found and repaired 9 See fault finding text BBC Micro fault repair No res
29. onditions A PRESTEL Adapter failure can usually be related to one of the following fault conditions No PRESTEL display after entering PRESTEL This will occur if a A PRESTEL ROM version 1 0 or higher has not been installed in the BBC Microcomputer to which the PRESTEL Adapter is connected b The PRESTEL ROM is not installed in the BBC Micro The PRESTEL ROM has failed 11 Failure to recognise that the adapter is plugged in and switched on This will be shown by the Adapter error message being displayed by the BBC micro when dialling is attempted This may be the result of a The RS423 connector inserted into the BBC microcomputer socket incorrectly the cut out should be facing UP b The Mains fuse in the rear of the Adapter blown c Loose or faulty internal fuse FS4 d Internal power supply circuitry faulty Failure to seize the telephone line This will be apparent by failure to hear the dial tone through the Adapter s speaker when dialling starts This may be the result of a Poor or incorrect connection to the telephone line socket b Faulty speaker connections or speaker c Internal power supply circuitry faulty d Circuit fault e Faulty relay RL1 Failure to dial correctly This will be apparant by failure to hear the dail relays within the adapter clicking as dialling proceeds Note this sound
30. ponse to keyboard 5V across C21 5V across C25 Replace modem 1C1 Any faults found and repaired a Bee fault finding text BBC Micro fault repair PSU check Replace with identical type and value If blowing repeatedly check for S C components or PCB track shorts Check fuse holder making good contact Check fuse type correct and that fuse resistance lt 30 Replace as necessary Is voltage drop across FS5 greater than 0 1V Is mains switch and wiring OK from cable to PCB Repair replace as necessary Remove PSU cover Replace 15V AC on 15V pin of transformer transformer T2 secondary Rectified ua oltage OK and regulator text 1 15 16 see text Check for short open circuits on PCB FINISH O CTS N RxD M TxD L RTS GND GND GREEN YELLOW BLUE VIEW OF WIRING SIDE OF CONNECTOR RS423 Cable Assembly Detail RED BLUE WHITE GREEN gt PIN2 PIN 3 PIN WHITE GREEN 4 BLUE PIN 5 RED Telephone Cable Assembly Detail Speaker Assembly Detail 35 To BBC Micro RS423 rn GND RTS CTS TD RxD RS423 line interface Carrier detect Modem Mode select Decode and control Speaker control Relay Speaker drivers Ampl
31. supply unit produces 5 volts at around 200mA 5volts at around 50mA It requires a 240v 50Hz AC mains input and is a linear power supply circuit using three terminal regulators and a mains transformer The circuitry is protected by fuses on both positive and negative output rails Note The Line Interface signal is RS232 423 compatible The PRESTEL Adapter 5 pin DIN domino plug connects to the RS423 serial port on the BBC Micro for clarity the interface signal is therefore referred to as RS423 throughout this manual 6 DETAILED CIRCUIT OPERATION The operation of the PRESTEL Adapter is controlled by the RTS input signal from the RS423 interface A pulse of approx 2 35mSec width on the RTS line will cause the internal circuitry to step through the various control states shown in Fig i This is called a STEP pulse babyy by ly by by Ly by by by by Ly Reset pulse Step pulse Qo IC10 pin 3 Oe IC10 pin 11 9 IC10 pin 12 9 IC10 pin 13 Qs IC10 pin 14 1 1 1 o o j aL dba 2 3 5 29 Y 5 E Modem 4 213 3 86 22 3 88 85 tate 58 2 5 3 3 5 ee 5 E bus 81 4 1 8 2 21 2 5 Speaker on or on om on on or om or ox BER or on on on on On O
32. the Dial on relay is closed and the Dial Relay RL3 is opened and closed the same number of times as the digit to be dialled with timings as shown in Fig 111 IC8 allows the TxD line from the RS423 interface to control the Dial relay RL3 when the Dial on relay RL2 is closed Thus to dial a digit the Dial on relay needs to be closed and a series of pulses sent along the TxD line with timings as shown in Fig 111 The Ring Detector IC3 provides a logic 0 signal if a ringing signal is detected on the telephone line whilst the Line Seize relay is open This signal is gated through to the CTS output on the RS423 interface The gates IC14 and IC8 control the CTS output such that it provides both a Carrier Detect and a Ringing Detect output depending upon the state of the Line Seize and Dial on relays If the line is seized and the Adapter is not dialling then the Carrier Detect output from the Modem is gated onto CTS If the line is not seized then the Ring Detecter signal is gated onto CTS 1 Not fitted to current production RTS Reset IC9 pin 5 Step IC9 6 Dial 1 10 pin 12 TD 24 RL3 Dial relay RL2 Dial on relay i Make lt 502 Phone line impedance Hold lt 3002 Break gt 5 Digit 3 3 breaks Digit 2 2 breaks Example shows the digits 3 and 2 being dialled 1 DIAL ON RELAY SET UP TIME gt 5mS 2
33. udspeaker monitor during call set up Format Esla MAS Speed 1200 baud receive operating frequency 75 baud transmit Operating temperature 10 35 C Power In 240 50Hz 10 13amp moulded on plug Signal Connections Cable to 5 pin DIN domino plug RS423 interface Cable to BT series 400 line jack plug telephone connection Complies with BS415 Class 1 British Telecom approval no S 1462 3 D 021864 Dimensions 7 70mm High 210mm Wide 350mm Deep Weight Unit only 1 5kg Packed 1 7kg see page 6 4 DISASSEMBLY AND ASSEMBLY 1 In order to service the PRESTEL Adapter first switch off and disconnect it from the mains supply also disconnect both the RS 423 cable and telephone connector from the BBC Microcomputer and telephone jack socket respectively 2 The lid of the PRESTEL Adapter may be removed after undoing three fixing screws two on the rear panel and one underneath When reassembling press the lid down at the rear whilst tightening the two rear fixing screws Take care not to lose the two spire nuts pushed on to the case lid into which the rear fixing screws locate 3 Inside the PRESTEL Adapter are two sub assemblies the printed circuit board and the speaker plate assembly 4 The printed circuit board may be removed by the following procedure 1 Disconnect the RS 423 and telephone cables from the PCB by removing the 9 faston tab conn
34. which should be high at all times To check the action of the counter From the reset state repeatedly press the CALL STEP function key f0 and observe the counter outputs from IC10 pins 11 12 13 and 14 These outputs should perform a binary count from 0 through to 8 binary 1000 as each STEP pulse is applied from the f0 function key Once it reaches a count of 8 then further STEP pulses should have no further effect on the counter However further STEP pulses after a count of 8 should then start to cause the signal on IC10 pin 3 to toggle between 0 and 1 If the link LK3 is in the wrong position then the counter outputs from IC10 will only count up to binary 4 and 1C10 pin 3 will be permanantly low To check the action of the relays The debugging program given in Appendix A configures the function keys f4 5 f6 to activate RL1 RL2 and RL3 respectively Thus alternately pressing the respective function key and f1 RESET will cause that relay to open and close This sound should be audible and is a way of checking their operation However the contact resistance of each one should also be checked with a meter RL1 may be checked for operation by alternately pressing the f4 key and the fl key Using the scope observe 11 pin 13 which should be switching between 0 and 1 note its logic 0 output may be up to 1 volt Also listen carefully and the mechanical action of the r
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