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With C105 ON, the output impedance is 600 or 150 ohm, while with C105
OFF the output impedance has a value higher than 10 Kohm.
The line transformers assure galvanic isolation (1500 Vrms) and adaptation
between balanced line circuits and unbalanced internal circuits.
Jumpers U2 and U8 select two different values of transmission impedance:
150 ohm (for speeds higher or equal to 2400 bit/ s) and 600 ohm (for speeds
lower than 2400 bit/ s).
Two Metal oxide varistors provide protection against high-energy transients
from the net.
1.5.
RECEIVER CIRCUIT
1.5.1.
Analog section
The telephonic balanced receive line is connected to the transformer through
the jumpers U12 and U15, that select the input impedance value.
The unbalanced signal coming out of the transformer is protected against
overcurrents by a limitation circuit.
It is possible to increase input impedance to a value higher than 10 Kohm
setting the jumper U13 in 2-3 position.
The received signal is amplified, filtered and in case equalized.
Equalizer adaptation is automatic for speeds between 2400 bit/s and 19200 bit/s.
The filter output is squared in order to adapt the analog signal to the digital
input of the demodulator and the carrier detector.
1.5.2.
Digital section
The receiver digital section consists of the demodulator, the PLL circuit for
clock recovery and the carrier detector.
The recovered clock is driven to the C115 circuit.
The carrier detector consists of a high-band digital filter and a timing circuit.
The fast carrier detector circuit (DCD fast), when inserted (DSD5 ON),
detects absence of transitions on the modulated signal for a time of 1,5 bit time
(±0,5). In this case, the C104 circuit is forced to mark.
This function is useful in half-duplex two wire applications.
The fast carrier detector can eliminate problems caused by incoming spurious
characters.
The following figure illustrates an example of line micro-interruption, in
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