The science data interface consists of a single serial channel at a nominal rate of 100 kHz (with capability of operating at 50 kHz or 200 kHz). Each data dump downloads a commanded number of bits, regardless of the amount of data actually contained in the science memory. The data are written out onto the serial line byte by byte (incrementing byte addresses sequentially through the memory), with each byte written LSB first. The data dump consists of a header, followed by the actual contents of the CUBIC science memory and ECC memory, followed by blank data values (0xFF) to complete the data dump. One byte of ECC error correction codes is transmitted for each 16 bit word of science data. (These ECC bytes permit correction of SEUs and/or transmission errors during data processing on the ground.)
The command sequence from the Command Processor that begins the
CUBIC Science Data Dump consists of an ``RF Contact'' command
(0x800y, see Fig 5.3.3.3 of the SAC-B ICD), a ``Data Dump Size''
command (0x4001 - 0x401F) specifying the dump size in Mbits,
a delay of at least 30 seconds,
and a ``Dump Data'' command (0x7FE0).
When CUBIC receives the ``RF Contact'' command
from the command processor, the ICP will complete
any pending CCD readout, disable the CCD frame interrupts,
perform a housekeeping check, and wait for a CUBIC
``Dump Data'' serial command (0x7FE0) from the command processor.
The ``Data Dump Size'' command specifies a dump size of up to 31 Mbits
(where 1 Mbit is defined to be 
bits, or
131,072 bytes, or 128 Kbytes).
When the instrument receives the ``Dump Data'' serial command,
it begins writing the Science Data Dump to the Science Data
FIFO.
During data transmission, the ICP keeps the FIFO at least half
filled until the requested number of bits have been
written out or
the science memory dump has been completed.
If the science memory dump is complete
before 
Mbits
have been written out to the Telemetry Processor,
the FIFO will empty and tri-state its outputs. Pull-up resistors on
the FIFO outputs will then present 0xFF to the Telemetry Processor for
the remaining bits of the dump.
On completion of the memory dump the ICP enables the CCD frame interrupts, flushes the CCDs, and checks the command buffer for additional ground commands. If additional commands are present in the command buffer they will be executed. Otherwise, the ICP returns to its normal housekeeping checks.
The sequence of commands to perform a CUBIC data dump is shown in Table 27. Note that each serial command sequence should include 2 CCD Readout/Calibration mode commands to set the CCD readout modes for the following orbit. (NOTE: the Frame Compression mode command is reset in on-board software after execution of one full CCD frame to ensure that only one full frame is collected). Other CUBIC serial commands may be added to the list given in Table 27 as required.
Table 27: CUBIC Data Dump Command Sequence