BBC Microcomputer Service Manual Oct 1985 Section 1 BBC Micro ...

In the other screen modes, the information is stored in RAM as actual
bit patterns for every character that is written to the screen. This is
expensive in terms of memory usage, (between 8K and 20K in the
different modes) but it makes it extremely versatile, especially when
mixing graphics with text. The addressing of the RAM for the different
modes is performed by the 6845 CRT controller, whilst the data is taken
from the RAM and serialised by a custom designed circuit, known as the
video processor. This data is not used directly to produce RGB
information, but can be thought of as a set of logical colour numbers
which are passed to an area of high speed RAM within the video
processor referred to as the colour palette. This determines, for each
logical colour number, which combination of red, green and blue is
produced, and whether or not the colour is flashing. The video
processor is also responsible for selecting either the RGB signals
coming from the Teletext chip or the signals coming from the palette
and sending them out to the RGB buffers and the PAL encoder.
This RGB information is presented, after buffering, on the RGB
connector. To provide a UHF output, the RGB signals are combined with
the sync signals and fed into a UHF modulator. A video output is also
provided which consists of a summing of the RGB signals in such a way
as to give an appropriate grey scale. On issue 4 boards onwards, the
option is given of adding colour to the video signal in order to
provide a PAL encoded video output. [See section 3.4]
Moving on round in an anti-clockwise direction we come to the two
serial interfaces, the cassette interface and the RS423. These
facilities are both provided by a standard ACIA (Asynchronous
Communications Interface Adaptor) - the 6850, and a custom designed
circuit referred to as, the serial processor. This processor contains
the programmable baud rate generators for transmit and receive which
provide the clocks for the ACIA. The ACIA itself is responsible for
serialising the data, providing the control lines for the RS423 and
generating interrupts, whilst the serial processor switches these data
and control lines between the cassette and RS423 interfaces. The serial
processor also provides data separator and sinewave synthesis circuits
for the cassette recorder as well as a means of detecting the presence
of the incoming tone from the recorder. [See section 3.5]
The next section is the analogue input port which is a four channel 12
bit converter which is discussed in more detail in section 3.9 and the
interfacing survey (see chapter 7).
In the NW corner is the Econet section which centres around a 68B54
Advanced Data-Link Controller (ADLC). This is a sophisticated serial
communications device allowing the sending and receiving of data at a
variety of speeds between as many as 254 computers. The data transfer
is synchronised by a clock signal fed to all the computers as a
differential signal on one pair of cables, whilst the data itself uses
another pair of cables. Data is both transmitted and received on the
same pair of cables, but obviously only one computer at a time is able
to "broadcast" onto the data highway. [See section 3.12]
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