Plesiochronous Buffer - Comtech EF Data SDM-650B Installation And Operation Manual

SDM-650B Satellite Modem
F.1.4 Plesiochronous Buffer
Where the option is installed, data from the DEMUX section is fed into a plesiochronous
buffer. The buffer size is selectable in increments of 1, 2, 4, 6, 8, 12, 24, and 32 ms. The
buffer is automatically centered on resumption of service after an outage. The buffer can
also be commanded to center from the Config Interface menu of the front panel, or
remotely.
The start-up buffer will overfill upon centering, to match the satellite frame to the
terrestrial frame, with a maximum slide of 0.5 ms. For obvious reasons, manual centering
will generally not be plesiochronous.
The fill status is available as a monitor function, and is accurate to 1%. Overflow or
underflow incidents will be momentarily indicated by red LEDs on the module, and are
stored in the "StFaults" section of the M&C status registers, along with the date and time
of the incident. These are stored in battery-backed RAM.
To synchronize the receive data output with the satellite data, the user will normally
select Receive Clock. The data will then be clocked out of the buffer by the clock
recovered from the receive data input. The user may select from two other clock sources
as a backup:
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•
Problems on either the recovered receive data input clock or the external clock (if
selected) will cause satellite clock to be substituted, and a fault will be signaled.
Rev. 5
User-supplied external reference clock
Internal clock source
Asynchronous Overhead Channel Unit
F–7