Radio Shack TRS-80, Service Manual, Page: 16 / 44

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KEYBOARD INTERFACE
TV SWITCH BOX
Figure 4. Antenna Switch Box Schematic
To read the keyboard, only one column is
enabled by writing a zero to one of the eight
CPU I/O lines that corresponds to that column,
and by writing ones to all the other lines. If a
key has been pressed in that column, one of
the input lines will be a zero and the key loca-
tion will correspond to the bit that is low. By
scanning each column in the keyboard, all of
the keys may be checked. The Keyboard interface for the MC-10 utilizes
the I/O lines of the 6803 CPU to reduce the logic
required to a single six input CMOS buffer and
seven pull-up resistors.
From the computer, the signal is connected to a
balun in the switch box which matches the
modulator’s 75-ohm output impedance to a
TV’s 300 ohm antenna input impedance. This
signal is then connected to the switch. The
switch is specially designed to provide the
60dB of isolation required between the com-
puter and the TV antenna. The antenna switch box consists of a switch
and a balun, with connectors provided for
attachment to the computer, the TV antenna,
and the home TV. The switch box is connected
to the customer’s TV through the 300 ohm twin
lead output. The TV antenna is attached directly
to the switch box. The computer output is con-
nected through a 75-ohm coaxial cable to the
phone plug input on the switch box. Figure 4
shows a schematic of the antenna switch box.
COMPUTER
INPUT
60dB
ATTENNA INPUT
BALUN
300 0hm
SWITCH
TWIN LEAD OUTPUT
TO THE TV