Vaisala RVP900

Chapter 3 _______________________________________________________ Hardware Installation
VAISALA_______________________________________________________________________ 95
the user has chosen, usually through the Pb and Ps plotting setup
commands.
Following the burst window is a fixed-length sequence of 25 serial data
bits, which convey information from the IFDR. The first four data bits
form a characteristic (0,1,1,0) marker pattern. The first zero in this pattern
effectively marks the end of the variable length burst window, and the
other three bits should be checked for added confidence that a valid bit
sequence is being received. Table 8 on page 93 provides the interpretation
of the serial data bits.
The period τ
s
of the serial data is (128/f
aq
) , where f
aq
is the acquisition
clock frequency given in the Mc section of the RVP900 setup menu. For
the default clock frequency of 71.9502 MHz, the period of the serial data
is 1.779 μsec. The logic that is receiving the serial data should first locate
the center of the first data bit at (0.5 × τ
s
) past the falling edge at the end of
the burst window. Subsequent data bits are then sampled at uniform τ
s
intervals.
The actual data sampling rate can be in error by as much as one part in 75
while still maintaining accurate reception. This is because the data
sequence is only 25-bits long, and therefore, the last data bit would still be
sampled within ±1/3 bit time of its center. Having this flexibility makes it
easier to design the receiving logic. For example, if a 5 MHz or 10 MHz
clock were available, then sampling at 1.8 µsec intervals (1:85 error)
would be fine. Similarly, one could sample at 1.75 µsec based on a 4 MHz
or 8 MHz clock (1:61 error), but only if the first sample were moved
slightly ahead of center, so that the sampling errors were equalized over the
25-bit span.
Table 8 Bit Assignments for the IFDR Coax Uplink
Bit(s) Meaning
1 to 4 Marker Sequence (0,1,1,0). This fixed, 4-bit sequence
identifies the start of a valid data sequence following the
variable-length burst window.
5 to 20 16-bit, multi-purpose data word, MSB is transmitted first (see
below)
21 Reset Request. This bit sets in just one transmitted sequence
whenever an RVP900 reset occurs.
22 If set, then interpret the 16-bit data word as 4-bits of command
and 12-bits of data, rather than as a single 16-bit quantity (see
below)
23 to 24 Diagnostic select bits. These are used by the RVP900 power-
up diagnostic routines; they are both zero during normal
operation.
25 Green LED Request; 0=Off, 1=On. The state of this bit
normally follows the "Downlink Detect" LED on the IFDR.