Acorn Computers Limited BBC A, Hardware Manual, Page: 31 / 47

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outputs respectively. The RXD, DCD and RXC outputs of the serial ULA are fed into the RXD, DCD and
RXC inputs of IC4, an Asynchronous Communication Interface Adapter (ACIA) type MC6850. The ACIA
converts the data to a parallel form, which is then presented to the data bus of the microprocessor, on
demand.
The cassette clock signals are derived from the 16MHz master clock. The 16MHz signal is firstly divided
by 13 using a 74LS163 synchronous binary counter (IC42) and then further subdivided within the serial
ULA.
The data to be saved on to the cassette recorder is taken from the data bus and converted into serial
form by the ACIA. This serial information is then fed into the TXD input of the ULA. Within the ULA, the
serial data is converted to a synthesised sine wave signal, suitable for feeding to a standard cassette
recorder. The synthesised sine wave signal is buffered using an operational amplifier (part of IC35) and
then fed to pins 1 and 4 of the DIN cassette connector via a DC blocking capacitor (C34)
The CASMO (cassette motor) output of the serial ULA drives a transistor (Q3), which in turn operates the
cassette motor relay and also the keyboard "cassette" lamp (LED). The normally open contacts of this
relay are used for automatic motor control switching of the cassette recorder. The connections from these
contacts are available on pins 1 and 7 of the cassette DIN socket (SK5)
The solid state switches within the serial ULA are used to route either the cassette signals or the R5423
signals to the ACIA. This switching is determined by the control register of the ULA which is controlled
from the data bus. The serial ULA also contains the programmable baud rate generator, which is required
by the R5423 interface.
The Data Out (D OUT) and Request to Send (RTS) signals from the ULA are converted from a TTL level
to R5423 compatible form by a DS3691N line driver (IC75). This device requires both a +5 and -5 volt
supply. The data-in (DIN) and clear-to-send (CTS) input signals from the R5423 interface are converted
from R5423 to TTL levels by a line receiver IC type DS88LSl20N (IC74).
Connection to the R5423 interface is by a 5-pin DIN "domino" socket (SK4)