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

7 Interfacing Survey
7.1 Purpose of each interface
Since there are so many different interface connections on the BBC
microcomputer, it may be a help to look at each in turn and talk about
possible applications for each. Working from left to right on the back
of the computer, we start with the UHF output, which provides a PAL
colour TV signal for use with a normal colour television. Next is a
video output on a BNC connector which is intended to be used with a
black and white video monitor. However it is possible to introduce the
colour burst information onto this signal in order to produce a PAL
composite video signal. On circuit boards issue 4 onwards, it is
possible to introduce this signal by adding a simple link, S39. On
previous issue boards it is necessary to introduce a 470pF capacitor
from the emitter of Q9 to the base of Q7. This capacitor would have to
be soldered directly on to the circuit board.
The third connector which is provided for video output is a 6-way, 240
degree DIN plug. This provides the red, green, blue and sync signals
needed for an RGB monitor. The sync signal is a 5 volts, negative going
pulse of 4.7uS duration, but it can be changed to positive going by
changing link S31. Also provided on this connector are a 0 volt and a
+5 volt supply, but these should not be used for providing more than a
few milliamps to external circuits.
The next connector is a serial port of the RS423 standard. This is a
standard which has superior drive capabilities to the RS232 interface
and is run in this case between +5 volt and -5 volt levels. The speed
is software selectable at 75, 150, 300, 1200, 2400, 4800 or 9600 baud.
There is a higher speed of 19200 baud, but this is not guaranteed to be
error-free. It is also possible to get the interface to work at 110
baud, but this requires a modification which would also change the
speed of the cassette interface. (See section 7.3 on hardware hints and
tips for more information.) The control signals provided are the normal
CTS and RTS lines, the RTS output also working on +5 volt and -5 volt.
NB When making up a connector for the RS423, note that connections as
shown on the circuit diagram refer to the socket. For the plug
connections, refer to page 504 of the User Guide which gives the
connections as seen from outside the case.
Apart from using the RS423 interface to run a serial printer it is also
possible to use it to communicate with other computers. For example, it
is possible to communicate with a mainframe computer either directly
within a building on a wired link or, by using the telephone network,
to a computer in another building or even another country. This would
of course require the use of an acoustic coupler.
There are various levels at which this link could be used. Firstly the
computer could be used as a "dumb terminal" which would simply be
capable of sending characters typed on the keyboard to the mainframe
computer and receiving characters from the mainframe and printing them
on the screen. The following program will allow you to do so.
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