Serial input/output is provided by the cassette port and RS423 port. Both are controlled by the 6850 asynchronous communications interface adapter (IC82) and a ULA called the serial processor (IC85). Analogue input is fed to the four-channel 10 bit ADC chip (IC84). A local area network facility is provided by the ECONET circuitry, centred on the 68B54 advanced data-link controller (IC81). Two build options are available for the floppy disc circuitry. One is based on the 8271 floppy disc controller (IC15) as used on all BBC Microcomputers issues 1 to 7. The second option is based on the 1770 floppy disc controller (IC16). Some components are common to both options.
The 1770 operates in either single density (FM) or double density (MFM) mode, and includes a data separator and disc speed decision logic. The 1770 controller interface is therefore simpler than the 8271 controller interface. ICs 1, 2, 3, 4, and 9 are not required with a 1770. The CENTRONICS compatible printer interface is based on half (the A port) of a 6522 versatile interface adapter (IC10). IC5 buffers data sent to the printer. The User Port is connected directly to the B port of the same 6522 ( IC10). The 1MHz extension bus is a fully buffered interface to the CPU, operating with lus transfer cycles.
The bus appears as a 512 byte address block in the processor I/O space at pages &FC and &FD. The TUBE interface provides buffered address and data lines for connection to a second processor. The TUBE itself is a fast parallel bidirectional FIFO and is resident in the 2nd processor unit. The keyboard is read through half (the A port) of a 6522 versatile interface adapter (IC20). Sound is produced by the 76489 (IC38), a four-channel sound generator chip. Speech may be generated using an optional 5220 speech processor (IC29) and 6100 word PHROM (IC37).
5.2 CPU timing A 16MHz crystal controlled oscillator (X1 and half of IC26) generates clock pulses which are divided by circuitry within the video processor ULA (IC53). Pins 4, 5, 6, and 7 of the video processor provide 1MHz, 2MHz, 4MHz, and 8MHz outputs respectively. 8MHz, 4MHz and 2MHz are used to generate RAS and CAS for the DRAMs, and 6MHz for the TELETEXT chip IC59. 2MHz is used to generate the main system clock, 2E. 1MHz is used directly by the TELETEXT chip, and also in conjunction with 2MHz to generate the phase shifted 1MHz system clock, lE from IC25. 11