Texas Instruments 99/4A - Page 88

C ASSETTE INTERFACING LIRC! 1 T TPV
GENERAL DESERIFT I ON
The cassette interfacing circuit enables the F9/4A to store and
read data from a normal cassette recorder- The interfacing ci r —
cuitry consists of 3 distinct parts: the control circuitry, the
write circuitry and read circuitry.
CASSETTE CONTROL CIRCUITRY
This part consists of two solid state switches which are used for
remote controlling of up to two cassette recorders.
WRITE CIRCUITRY
The write circuitry supplies digital data which has been wave 
shaped cassette recorder. It uses a T M S FF 01 output gate and a
second order filter to supply a signal of the desired shape and
amplitude. If two cassette recorders are used, then both will use
use the same write signal, but only one recorder is selected at a
time by the control circuitry.
DATA FORMAT
The vS/4 uses
d a t a s t r e a m . A
a biphase
standard
encodi ng
sof twar e , in the
coding technique to generate the serial
time interval (bit cell) is used in the
fol lowing way :
A. A binary one is represented by 2 bit cells both bit cells
of opposite polarity.
B- A binary zero if represented by 2 bit cells, both bit
cells of the same polarity.
C. After a binary bit is written, the next bit will start
with an opposite polarity referenced to the last bit
cell.
AUDIO GATE
A special feature is incorporated which consists of a solid state
switching circuit. This circuit can switch the audio signal of the
cassette recorder to the sound processor.