Curtiss-Wright FibreXtreme SL240, Hardware Reference Manual

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SL240 PROTOCOL
C.4 Data Transmission and Flow Control
As SL240 is seen as a point-to-point link from the transmitter, there is no need to log into
the receiver node to begin sending data. SL240 boards can begin transmission as soon as
they are started and data is available in the Transmit FIFO. Using the frames described
above, the transmitter sets up a constant stream of frames, into which it inserts data as it
becomes available. Data is only inserted if the flow control signal from the remote end is
GO—if it is STOP, then the data waits in the Transmit FIFO until the signal changes.
Curtiss-Wright Controls’ SL240 boards use the same protocol when transmitting from
either end to allow the link to operate bi-directionally. Since these data streams are
independent, the maximum throughput on the link would be 494 MB/s for SL240.
The receiver should transmit the STOP signal when it has space for the data contained in
20 km of fiber or less left. Assuming 5 µs/km for the speed of light, this gives us 100 µs
of data. For SL240, each 32-bit word (40 bits on the link) takes 16 ns, so there are 6250
words stored in 20 km of cable. The first 10 km is reserved for sending the STOP signal
to the transmitter, and the second 10 km is for the data already contained in the receive
fiber.
Copyright 2006 C-5 FibreXtreme Hardware Reference Manual