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DESCRIPTION AND OPERATION
MODULE CIRCUITRY
I-E96-302B
2 - 11
To transmit, the microprocessor enables the FSK transmit
gate. The microprocessor transmits a digital signal that passes
through encoding logic to an oscillator circuit. A digital value
on the input of the oscillator causes it to transmit a keyed fre-
quency. As the digital input to the oscillator changes its state,
the oscillator shifts its output frequency to represent that new
state. For example, the oscillator outputs a frequency of 19
kilohertz when its input is a logic one. It outputs a frequency of
30.5 kilohertz when its input is a logic zero.
BASEBAND COMMUNICATION
The microprocessor can directly encode and decode baseband
communication. The ASI communication circuitry shapes the
baseband signal before it reaches the AC coupler circuit. The
AC coupler combines the baseband and analog signals for
point-to-point transmission.
Analog-to-Digital Converter
The FBS module can digitize analog values within the range of
-10 to +10 volts. Figure
shows the analog-to-digital conver-
sion circuitry. The analog-to-digital control chip provides the
logic needed to control the dual slope integrator. The ana-
log-to-digital conversion takes place in three stages: input inte-
gration, de-integration with a +10 or -10 volts reference and
zeroing the integrator/de-integrator.
The microprocessor signals the ADC circuitry to begin convert-
ing inputs. The ADC circuitry selects the input, reference volt-
age and conversion operation through the select multiplexer.
The input signal from the select multiplexer passes through
buffers and a differential amplifier that converts the input
Figure 2-8. FSK Signal and Decoded Digital Equivalent
FSK INPUT SIGNAL AT 9600 BAUD
t = 0
t
t
DIGITAL OUTPUT FROM DECODING LOGIC
TP25184A
f
1
1
f
1
1
f
1
1
f
1
1
f
1
1
f
0
0
f
0
0
f
0
0