5 Circuit description This circuit description has been kept as simple as possible as the detailed fault finding section (section 9) should prove to be of more use for servicing. A detailed description is given of those features of the BBC Microcomputer B + which are new. 5.1 General The microcomputer uses the 6512 CPU (IC42) which allows more accurate timing of the logic circuitry than did the 6502, see 5.2. The 6512 requires two clock signals at 'MOS' voltage levels, in all other respects it functions in the same way as the 6502. The computer clocks are derived from a 16MHz crystal controlled oscillator circuit (X1 and half of IC26), and divider circuitry in the video processor ULA (IC53).
The 6512 accesses 31 1/4Kbytes of ROM, 3/4Kbyte of memory mapped input/output, and up to 44Kbytes of RAM. 64Kbytes of RAM are installed on the PCB, the extra 20Kbytes being used for the screen memory in shadow mode, see 3.1. The memory mapped I/O is located in pages &FC, &FD, and &FE of the CPU address space. There are five sideways ROM sockets installed on the PCB, each capable of taking an 8, 16, or 32Kbyte ROM or EPROM (ICs 35 44 57 62 68). When used with a 32K ROM, each sideways ROM socket is decoded as two 16K sideways ROM slots. A sixth ROM socket IC71 holds a 32Kbyte ROM which contains the operating system and BASIC.
'The number of the ROM currently in use is held in the ROM select latch (IC45). 64Kbytes of RAM are installed on the board in eight 64K by 1 bit DRAM chips, (ICs 55 56 60 61 64 65 66 67). Of this RAM, 32Kbytes are always accessible to the CPU, 12Kbytes can be paged into the sideways (paged) ROM space, and the remaining 20Kbytes are used as screen memory in shadow mode. Both CPU (IC42) and 6845 cathode ray tube controller ( IC78) have access to the RAM. Each can access the RAM at full 2MHz clock speed by. interleaving the accesses on alternate phases of the 2MHz clock. The RAM is thus being accessed at 4MHz. The 6845 accesses the RAM sufficiently to perform the refresh function.
Screen display is provided through the 6845 (IC78), video processor ( IC53), and various encoding circuits. Three display outputs are available: RGB consists of CSYNC and RED GREEN BLUE at TTL voltage levels. Each colour is either on, off, or flashing, giving sixteen displayable colour effects, ie eight static colours and eight flashing colours. VIDEO output is a summation of RGB to give a grey scale (luminance only). If link S26 is made the chrominance component (colour information) is added to the VIDEO output. UHF output is obtained by mixing luminance, chrominance and SYNC signals, and then feeding the result to a UHF modulator. 10