Texas Instruments 99/4A - Page 89

C O N TR 0 L C IR C U ITR Y
Sines the two control circuits are identical, only a description
of one cassette control circuit is given (see figure 54, p. 134).
Rin 19 (F6) of the TNG 9901 programmable system interface is used
to enable or disable D402. When enabled, the LED in opto-couplor
U401 turns on. R415 is used to reduce the current through the
LED. Outputs 4 and 5 of the opto — coupler are used to put tran 
sistor 0401 in the conductive state, thus enabling the motor drive
of the cassette recorder.
WRITE CIRCUITRY
The write signal is generated on pin 28 of 407 the TMS 9901. The
differentiating filter consisting of R402, C408, C407, R400, R401
and C400 shapes the signal.
READ CIRCUITRY
First part of the read circuitry consists of an operational ampli 
fier which performs amp 1 ification and pulse shaping of the read
signal. R408 and R410 determine the gain of the amplifier. C402,
C409 and C411 are for pulse shaping purposes. R406 is the resis 
tive load for the cassette recorder.
GROUNDING
Since the definition for ground and signal on
the cassette recorder depends on the type used,
allow operation with both types of recorders.
the output jack of
R409 is applied to
HYSTERESIS CIRCUIT
The hysteresis circuit consists of a second amplifier with posi 
tive feedback, the feedback network consisting of R412 and R411.
C410 is used to make the circuit less susceptible to undesired
signals. CR402 is used to limit the negative output swing, thus
avoiding damage to the TMS 9901. R413 is used to limit the cur 
rent through diode CR402.
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