Comtech EF Data SDM-650B - Manual (Page 84)

Configuration SDM-650B Satellite Modem
3–42 Rev. 5
The formula for calculating the buffer depth is:
Buffer Depth (bits) = Bit Rate (bit/s) x Max. Delay Variation
Contact the satellite company to determine the delay variation of a particular domestic
satellite.
The buffer depth capacity can be set either through the M&C or manually with a DIP
switch on the buffer. Jumpers and optional modem software determine if the control is
local or through the M&C.
When an overflow or underflow occurs, the buffer will automatically reset itself to the
center, and set the respective indicator.
The buffer is not plesiochronous.
Data framing is not required for proper operation.
The buffer is centered at power-up, and may be re-centered manually or through the
M&C, depending on jumper settings on the assembly.
Refer to Figure 3-14 for the Doppler buffer daughter card and jumper selections.
Refer to Table 3-8 to determine settings.
CAUTION
A problem can result in the switch system when using the buffer with the
source clock set to TX CLK. When the TX section switches over to the
back-up modem, the source clock is no longer present at the prime mode.
The internal oscillator in the prime modem will free-run at the frequency
± 50 PPM from the center. The prime modem running free will eventually
cause the Doppler buffer in the prime modem to overflow or underflow. Two
lines are provided on the connector called the Master Clock (MC-A and MC-
B) that prevent the modem from overflowing or underflowing.
The pinouts on the 15-pin connector are:
• Pin 7 for MC-A
• Pin 8 for MC-B