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Theory of Operation
4.4 Digital Interfaces
The modem interface module is a daughter card that plugs onto the demodulator board. It
provides the interface for terrestrial data and overhead signals, and provides the fault
reporting output of the modem.
RS-422/449, V.35, RS-232-C, and ASYNC interfaces are available for input and output
of terrestrial data.
Both baseband and interface loopbacks are provided.
Terrestrial data rates from 19.2 to 128 kbit/s are supported.
4.4.1 RS-422 Interface
The RS-422 digital interface provides level translation, buffering, and termination
between the internal modem signals and the interface connector on the rear panel.
Electrical characteristics of the RS-422 interface signals are defined in EIA STD RS-422,
and details of the mechanical interface are found in EIA STD RS-449. For the electrical
and mechanical specifications, refer to Sections 4.4.1.1 and 4.4.1.2.
Refer to Figure 4-6 for a functional diagram of the interface.
The RS-422 interface provides a Send Timing (ST) clock signal at the modem data rate.
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In either case, the phase relationship between the clock and data is not important as long
as it meets the jitter specifications of RS-422/449. This is because a clock phase
correction circuit is provided, which shifts the clock away from the data transition times.
The clock phasing is jumper selectable at JP1. Refer to Table 4-1 for jumper settings.
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4–14
In the INTERNAL clock mode, the data to be transmitted, Send Data (SD), must
be synchronized to ST.
In the EXTERNAL clock mode, the clock is accepted on the Terminal Timing
(TT) input to clock-in the data to be transmitted.
The AUTO setting is used when there is no jitter on the clock source.
The NORMAL setting is used when standard specifications on clock and data
relationships exist.
The INVERT mode is used when the incoming clock is inverted from the
standard clock and data relationship.
SDM-100A Satellite Modem
Rev. 0
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