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Fig.: 1.2. MODULATION WAVEFORMS
1.4.
TRANSMISSION CIRCUIT
1.4.1. Digital section
An internal logic regulates the transit of data to/from the Digicom ASIC, then
data transit through the A/S converter.
The ON position of dipswitch DSC3 activates the synchronous mode.
The OFF position of dipswitch DSC3 activates the asynchronous mode.
In synchronous mode the C103 signal is sent directly to the modulator, in
asynchronous mode first it is converted to synchronous (A/S converter) and
then sent to the modulator.
The A/S converter adds the bits necessary to compensate the difference in
speed between the asynchronous data coming from the DTE and the modulator
TX time base.
The modulator clock is derived from the internal C114 circuit or from the DTE
C113 circuit.
In some cases, the modulator clock is derived from the receiver time base.
The transmission speed is chosen predisposing the dipswitches DSC 5-6-7-8.
The DSD 4 dipswitch allows to choose either CCITT V.1 or CCITT V.1
inverted standard (see figure 1.2).
If DSD 7 is in OFF position the carrier is always present, if it is in ON position
the modem is in controlled carrier (it follows the C105 signal).
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