Vaisala RVP900

USER’S MANUAL__________________________________________________________________
258 _________________________________________________________________ M211322EN-D
options are then chosen using LRMSK, and SOPRM, and receiver noise
samples are taken with SNOISE. The noise levels are not automatically
sampled on power-up, so SNOISE must be issued at least once by the user.
LFILT is executed if clutter filters are needed. If data rays are to be
synchronized with antenna motion, then LSYNC is used to specify a table
of antenna angles. After all setups are complete, PROC commands are
issued to actually collect, process, and output the data. Errors detected
during the execution of commands are noted by the RVP900 and can be
monitored using GPARM.
The RVP900 contains a 4096-word sfirst-in-first-out (FIFO) buffer
through which all output data flow. This buffer is included to simplify the
requirements of the user's interface hardware. The FIFO holds each
sequential word generated by the RVP900 until such time as the user is
ready to accept it. Thus, when reading from the processor, it is permissible
to fall behind by as many as 4096 words before any slowdown in
performance occurs. The RVP900 writes to the FIFO at full speed as long
as it is not full, and the internal processing is not affected by the exact
speed at which user I/O actually occurs. This continues as long as the
average I/O rate on, perhaps 10ms intervals, matches the average rate at
which data are being produced.
The sequence of events described above is altered when the FIFO becomes
completely full. Then, when the processor generates the next output word,
it waits in an idle loop until the user makes room in the FIFO by reading
out one or more words. Until this space becomes available, the RVP900
simply waits and does not proceed any further with its internal processing.
This, of course, leads to a slowdown in performance, but it is not a
disastrous one. The user always obtains correct data no matter how long it
takes to read it. One could take advantage of this fact to synchronize the
acquisition of data by the RVP900 with the post-processing and display of
that data by the user. In this case, RVP900 would be instructed to output
data at the maximum rate, the user would read these words at the user's
maximum rate, and the overall system would automatically run at the
slower of those two speeds.
When the output FIFO is full and the RVP900 has the next word ready for
output, there is another way that the idle wait loop can be exited, that is, if
the processor detects that the user is performing a write I/O cycle. Since the
user should have been reading data by now, the presence of a write cycle
is taken to mean that some more important condition has arisen. As such,
the wait loop is terminated and the RVP900 accepts the write data soon
afterward. If the new data are commands, they are executed right away, but
any output they try to produce may be lost in a similar manner. The net
effect is that the processor continues to execute all commands correctly,
but that their output is discarded.