Acorn Computers Limited BBC A, Руководство по оборудованию, Страница: 30 / 47

30
3.10 The Keyboard Circuitry.
The keyboard consists of a ten by eight matrix of normally open contact switches, which are mounted on
a metal plate. Connections to the contacts of these switches are made by a glass-fibre printed circuit
board. This board also carries ICs 1, 2, 3 and 4, the loudspeaker, three LED's, the keyboard connector
(to PL13) and the ROM cartridge socket (if fitted)
The keyboard circuitry is based on the "walking zero' 0 technique. IC1 is a synchronous binary counter
(74L5163), which is clocked by the 1MHz system clock. The outputs from this IC are decoded by IC3,
which is a BCD to decimal decoder (7445). The ten outputs of this decoder are connected to the column
lines of the keyboard matrix. In this way, each column in turn is pulsed low then high thus producing the
"walking zero' 5" pattern. Depression of any key results in the output of IC4, an 8 input NAND gate
(74LS30), pulsing high as the walking zero passes the column to which that key is connected. The output
from IC4 interrupts the microprocessor using the CA2 line of the system VIA (IC3). On recognition of this
interrupt, the computer executes the keyboard reading routine to discover which key was depressed. This
is achieved by latching the BCD address of each column in turn directly into IC1 using outputs PA0 to
PA3 of IC3, the system VIA, thus interrogating each column in turn. At the same time outputs PA4, 5 and
6 of the system VIA are used to load data into IC2, which is a data selector (74LS251). Each row is
selected in turn by the three bit code present on PA4, 5 and 6. The logic level on a particular row appears
at the output of the data selector when selected. In this way, the keyboard matrix is scanned for the
coincidence created when a key has been depressed.
3.11 The Cassette and RS423 Serial Interfaces.
The cassette interface has two software selectable baud rates, 0 and l200 baud. The majority of the
circuitry for this interface is contained within a custom designed Uncommitted Logic Array (serial ULA-
IC7)
The output signal from the cassette recorder enters the microcomputer on pin 3 of the 7-pin DIN socket
SKS. Three of the operational amplifiers contained within IC35 (LM324) are used to condition this signal
before it is fed into the cassette input (pin 12) of the serial ULA. Once inside the ULA this signal is
presented to both a data /clock separator and a run-in header detector.
Solid state switches within the ULA, under the control of the microprocessor, are used to route the logic
signal from the run-in detector to the data carrier detect output of the ULA (see fig 3.3). Similarly, the
clock and data logic outputs are routed to the receive clock (RXC) and receive data (RXD)