Stanford Research Systems SR865A, Operation Manual

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172 Data Streaming and Capture Appendix D
SR865A DSP Lock-in Amplifier
The header word consists of the following fields:
Header bits
31
–
24 (MSB) 23
–
16 15
–
12 11
–
8 7
–
0 (LSB)
Status
Bit 24: Overload
Bit 25: Error
Bit 28: Little-end
Bit 29:Checksum
Sample rate
0: 1.25 MHz
1: 625 kHz
2: 312.5 kHz
…
N: 1.25MHz/2
N
,
…
31: 0.582 mHz
Packet length
0: 1024 bytes
1: 512 bytes
2: 256 bytes
3: 128 bytes
Packet content
32 bit floats:
0: X
1: XY
2: Rθ
3: XYRθ
16 bit integers:
4: X
5: XY
6: Rθ
7: XYRθ
Packet counter
0 – 255
Packet counter: allows the receiving computer to detect lost or dropped packets. Each
UDP packet has a packet count that is one greater than the previous packet, modulo 256.
Packet content: indicates the data contained in the packet.
Packet length
: the number of bytes of data that follow.
Sample rate: is the sample rate of the data in the packet. A value of N corresponds to a
sampling rate of 1.25 MHz/(2
N
), where N is a value between 0 and 31 inclusive. This
sample rate already takes into account the subsampling requested in the command
STREAMRATE.
Status: bit 24 is high when an input overload, sync filter overload (if sync filter is on), or
output overload (if integer data is contained in the packet) condition existed at the
beginning of the packet. Bit 25 is high when a PLL unlock or sync filter out-of-range
condition existed at the beginning of the packet.
Bit 28 is high if data is stored in little-endian format, and low if it is in big-endian format.
Bit 29 indicates that integrity-checking (UDP packet checksumming) is enabled.
After reading the header information, it is possible to correctly interpret the data in the
rest of the packet. If you need to do endian-conversion, floating-point data must be
converted 32 bits (4 bytes) at a time, whereas integer data must be converted 16 bits (2
bytes) at a time.
XYR θ data is recorded in that order, followed by the next sample of XYR θ . As an
example, let's say the packet contains floating-point XYR θ data:
Data bytes
Byte 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 …
Data X(0) Y(0) R(0)
θ
(0) X(1)
…