Comtech EF Data SDM-9000 - Manual (Page 127)

Operation SDM-9000 Satellite Modem
4–54 Rev. 4
4.3.2 ECL/MIL-STD-188 Interface Clocking
Refer to Figure 4-22 for the clocking diagram.
4.3.2.1 TX Timing
The TX timing for the ECL/MIL-STD-188 interface is always provided with the send
data supplied by the terrestrial network. The TX clock provided by the terrestrial
network is used as the reference to send information over the satellite network.
The modem provides a send timing reference clock to the terrestrial equipment. This
clock is internally generated by the modem and can be used to time the send data to the
modem. Optionally, the send timing reference can be switched to loop timing mode. The
loop timing mode provides the RX (satellite) clock as the send timing reference.
4.3.2.2 RX Timing
The RX (satellite) clock is recovered from information demodulated and decoded from
the satellite network.
4.3.2.2.1 RX Timing (with Buffer)
If a plesiochronous/Doppler buffer is used, data received from the satellite network is
clocked into the buffer using the RX (satellite) clock. Data is clocked out of the buffer
and to the terrestrial network referenced to one of the four selectable clock sources listed
in the table below:
INTERNAL CLOCK Generated internal to the modem.
TX TERRESTRIAL CLOCK Provided by the terrestrial network.
EXT. REF. CLOCK External reference supplied by the terrestrial network.
RX CLOCK (See note) Recovered from the RX satellite network.
Note: When the RX clock is selected as the buffer clock source, the input and output
of the buffer are clocked by the same source. In this mode, the buffer is virtually
bypassed with a delay defined by the selected buffer size.
4.3.2.2.2 RX Timing (without Buffer)
If a plesiochronous/Doppler buffer is not used, data received from the satellite network is
clocked to the terrestrial network using the RX (satellite) clock.