Comtech EF Data SDM-100A - Manual (Page 130)

Theory of Operation SDM-100A Satellite Modem
4–14 Rev. 0
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.
• 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.
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.
• 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.