In order to properly mark the last word in data blocks on the link
when performing transfers from the processing
DSPs (1-4) to the Formatting DSP (5) and from
Formatting DSP to the Output FPGA
it is necessary to add additional information to the DSP serial
output.
We have chosen to do this by appending an extra 8-bit control byte to
each 32-bit data word transmitted out of the DSPs on their serial
ports.
In the DSP operating system, this is accomplished by doing a DMA
transfer of five bytes for each 32-bit data word to be transferred
out.
The DSP FPGA uses the Control Byte to generate a
C-bit on the link,
marking the last word in a data block,
for the data word associated with it.
This scheme has the drawback that it slows down the DSP output,
which is already a bottleneck in the system,
by a factor of 4/5.
However, no other prospects for data output that preserve the block
structure appear to be possible.