Comtech EF Data SDM-650B - Manual (Page 51)

SDM-650B Satellite Modem Configuration
Rev. 5 3–9
The RS-422 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, 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 relationship meets the
jitter specifications of RS-422 or MIL-STD-188-100.
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 Receive Data (RD) lines.
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 (RTS) lines are hard-wired to the Clear to Send (CTS) lines, since
the modem does not support polled operation.
Data Mode (DM) indicates that the modem is powered-up.
Receive Ready (RR) indicates that an RF carrier is being received and demodulated with
a sufficiently low error rate for the decoder to remain locked.
The RS-422 and MIL-STD-188-114 interface also provides bi-directional relay loopback
of both the clock and data at the DCE interface. In Loopback:
• From the DTE side, SD is connected to RD, and either ST or TT (in Internal or
External mode) is looped back to RT.
• From the modem side, the received data and recovered clock are routed back to
the modulator input for re-transmission.