BBC Microcomputer Model B+ Service Manual Contents 1 - Introduction 1 2 - Packaging and installation 2 3 - Specification 3 3.1 The microcomputer 3 3.2 Power supply 5 3.3 Video outputs 5 3.4 RS423 5 3.5 Cassette interface 6 3.6 Analogue to digital convertor 6 3.7 ECONET 6 3.8 CENTRONICS compatible printer interface 6 3.9 Audio output 6 3.10 Environment 7 3.11 Dimensions 7 4 - Disassembly and assembly 8 5 - Circuit description 10 5.1 General 10 5.2 CPU timing 11 5.3 Reset circuitry 13 5.4 Address decoding and memory 13 5.4.1 ROM operation 15 5.4.2 Paged RAM operation 16 5.4.3 RAM.
access 16 5.4.4 RAM circuitry 18 5.5 Disc interface 19 5.5.1 8271 FDC 19 5.5.2 1770 FDC 21 5.6 Video circuitry 21 5.6.1 RGB 21 5.6.2 Composite video 21 5.6.3 UHF 22 5.7 CENTRONICS compatible printer interface 23 5.8 User port 23 5.9 1MHz extension bus 23 5.10 TUBE interface 23 5.11 ECONET 24 5.12 Cassette and RS423 ports 24 5.13 Analogue to digital convertor 25 5.14 Audio circuitry 25 5.15 Keyboard 26 6 - Upgrading the PCB 27 6.1 1770 disc option 27 6.2 8271 disc option 28 6.3 ECONET 30 6.4 Speech 31 7 - Selection links 33