Comtech EF Data SDM-100A - Manual (Page 186)

Options SDM-100A Satellite Modem
B–4 Rev. 0
4. Once modem A has re-acquired the carrier, it will start sending commands to
modem B to achieve the desired E
b
/N
0
. During this time, modem B will be
sending commands to modem A to either increase or decrease power to maintain
modem B’s target E
b
/N
0
.
Notes:
1. Local carrier loss will always take priority over remote carrier loss.
2. The RX AUPC link is dead when carrier is lost.
B.1.1 Theory of Operation
B.1.1.1 Terrestrial Data Interfaces
Two I/O interfaces are provided for the terrestrial data source: RS-422 and V.35. The
ASYNC board is mounted upside-down inside the modem. Therefore, the user must
select the terrestrial interface type from the front panel under the UTILITY INTERFACE
menu. Once selected, I/O data is routed to and from the appropriate drivers and receivers.
B.1.1.2 ASYNC Data Interfaces
The RS-485 and RS-232-C I/O interfaces are provided for the ASYNC data source. The
ASYNC board is mounted upside-down inside the modem. Therefore, the user must
select the ASYNC data interface type from the front panel under the UTILITY
INTERFACE menu. Once selected, the I/O data is routed to and from the appropriate
drivers and receivers.
B.1.1.3 Multiplexer Operation
The multiplexer receives terrestrial and ASYNC data from the selected receivers. The
terrestrial data flows into a small First In/First Out (FIFO) buffer. The FIFO buffer aids in
the rate exchange between the terrestrial data rate and the overhead rate. The data can be
clocked into the MUX by the terrestrial clock or an internal clock.
ASYNC data is received by the RX section of a Universal Asynchronous Receiver/
Transmitter (UART) programmed by the M&C for the correct data protocol. The
incoming ASYNC data is sampled with a 16x clock in the middle of the bit time.
AUPC data is received from a serial M&C interface. The overhead clock is generated
from the terrestrial data clock by a phase-locked loop. Inside the multiplexer, overhead
bits (1/16) are added to create a sub-frame, frame, and multi-frame structure. The AUPC
data from the M&C interface and the ASYNC data are inserted into the framing structure.
The framed data is output to the modulator card on the modem at the overhead rate.