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

The other way in which the RS423 can be used is to link two BBC
computers together. One reason for doing this would be to enable
software to be downloaded from a disc system to another computer which
does not have a disc interface. A 15K program can be downloaded using
an RS423 link in approximately 20 seconds which is clearly faster than
using a cassette to cassette link. The only software involved in doing
this is to type in, on the receiving computer:
NEW
*FX2,1
(This sets the RS423 as input instead of the keyboard.)
and then on the sending computer you would type in:
*FX3,7
LIST
*FX3,0
(This enables the RS423 as the output and sends a listing of the
program so that, to the receiving computer,, it is as if it were being
typed in from the keyboard.)
If you have a number of transfers to do, then these commands could be
programmed onto a single key on each machine. When the program has been
sent down, you simply have to press BREAK on the receiving computer,
type OLD and then the program is ready for use. If you are
doing a BBC to BBC link over a short distance and want to use the full
speed of the interface then you will have to connect the hand-shake
lines as well as the data lines. The "data out" from one computer
should be connected to the "data in" of the other computer and viceversa,
and in a similar way for the control lines, the RTS on one
should be connected to the CTS on the other and vice-versa. If you are
working over a longer distance and want to use only three cables, data
in, data out, and ground and are prepared to work at a slower speed
without any handshaking, then you have to loop back the RTS to the CTS
on each of the computers so that each is permanently enabled for
sending. If you do not do so, the RS423 output buffer fills up and
printing stops after a number of characters have been sent to the
screen.
If you want to use the RS423 interface over a long distance at high
speed using the hand shake lines, it might be necessary to terminate
the receivers by making links S23 and S24. This terminates the line
with its characteristic impedance of 180 ohms.
The cassette interface is a standard CUTS cassette interface. It has
two speeds, 300 and 1200 baud, controllable by software. There is also
motor control provided on pins 6 and 7 of the 7-way DIN plug. The
rating of the relay contacts is 24V at 1A DC, and they should not,
under any circumstances, be used to switch mains voltages, no matter
how small the current. If you wish to get two BBC Microcomputers to
send programs to each other on the cassette system, then it is possible
to do so by a direct connection, provided a 1.5k resistor is connected
between the signal line and ground.
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