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The TR_DATA line is connected directly to the data line of the transceiver.
Data Lines D0 through D7 can be individually set up as either inputs or
outputs. In this example, D4 and D5 have buttons connected to them, so
they will be set up as inputs, and D0 and D6 are set up as outputs. D0 is
connected directly to a piezoelectric buzzer. Line D6 will activate a relay
through a transistor buffer when it goes high. A buffer like this may be
needed if the load requires more than 25mA of current or a higher voltage
source to activate. The transcoder will turn on the transistor, which can be
selected to provide the appropriate drive levels to activate the relay.