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SDM-309B Satellite Modem
2.0 Digital Interfaces
2.1 RS422 & MIL-STD-188-114 Interface
2.1.1 Functional Description
The RS422 and MIL-STD-188-114 digital interface (Figure C-9) provides the level
translation, buffering and termination between the internal modem signals and the RS422
or MIL-STD-188-114 interface on the rear panel. Electrical characteristics of the RS422
interface signals are defined in EIA STD RS422. Electrical characteristics of the MIL-
STD-188-114 interface signals are defined in MIL-STD-188-114 in conjunction with
MIL-STD-188-100. MIL-STD-188-114 defines signal levels, offsets, termination
resistors, etc., while MIL-STD-188-100 specifies such characteristics as signal quality,
and clock/data phase relationships and EIA STD RS449 provides details of the
mechanical interface. Both the electrical and mechanical specifications are summarized
in Sections 2.1.2 and 2.1.3. A functional block diagram of the interface is shown in
Figure C-10.
The RS422 and MIL-STD-188-114 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, 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 RS422 or MIL-STD-188-100 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. When there is no jitter on the clock
source, the AUTO setting is used. 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.
Data received by the modem is output on the Received Data (RD) lines while the
recovered clock is output on the Receive Timing (RT) lines. For applications that require
the rising edge of the clock to occur in the middle of the data bit time, Receive Clock
NORMAL mode should be selected. INVERT mode puts the falling edge of RT in the
middle of the data bit. This selection can be made from the front panel in the
configuration menu.
The Request to Send (RS) lines are hardwired to the Clear to Send (CS) lines, since the
modem does not support polled operation. Data Mode (DM) indicates that the modem is
powered up. Receiver Ready (RR) indicates that an RF carrier is being received and
demodulated with a sufficiently low error rate for the decoder to remain locked.
MN/U-SDM309B Rev. #
External Channel Unit
C-C–11
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