Comtech EF Data SDM-300A - Manual (Page 53)

SDM-300A Satellite Modem Revision 6
Functional Description MN/SDM300A.IOM
2–13
2.5.1 Available Interfaces
The following data interfaces are available for the SDM-300A:
• EIA-232 (25-Pin D Sub)
• EIA-422 (37-Pin D Sub)
• EIA-530 (25-pin D Sub)
• G.703 (BNC and DB15)
• V.35 (34-Pin Winchester)
2.5.2 Plesiochronous/Doppler/Buffer
Data from the DEMUX section is fed into a buffer. This buffer size is user-selectable in bit
increments that correspond to the length of an IESS-308 satellite superframe. The increments
range from 1 to 32 ms. Refer to the interface specifications for a list of valid entries for each of
the selected formats. The buffer automatically centers on resumption of service, either from the
front panel or remotely. The startup buffer will overfill when centering to match the satellite
frame to the terrestrial frame with a maximum slide of 0.5 ms.
The fill status is available as a monitor and is accurate to 1%. Overflows or underflows are
stored in the stored fault section of the M&C status registers, along with the date and time of the
incident (which are provided by the modem internal clock). These are stored in battery-backed
RAM.
A normal selection is to have the data clocked out of the buffer by an external clock. This
procedure removes the Doppler from the receive satellite data.
The operator may select from four other clock sources as a backup:
• Insert clock (D&I mode only)
• RX satellite
• Internal clock source
• TX terrestrial
Problems occurring on any of the selected clocks will cause the modem to substitute the satellite
clock and a fault will be signaled.