C.3 generation of signature – Campbell Scientific CR23X Micrologger User Manual

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APPENDIX C. BINARY TELECOMMUNICATIONS

C-5

DATA TYPE AND SECOND BYTE FORMAT

A,B,C, = 1 - Start of Output Array, G & H are the most
significant bits of the Output Array ID. All 8 bits of the
2nd byte are also included in the ID.

C = 0 - First byte of a 4 byte value.

A,B = 0; C = 1 - Third byte of a 4 byte value.

A = 0; remaining bits = 1 - First byte of a 2 byte
"dummy" word. The CR23X always transmits a 0 for
the 2nd byte, but the word can be decoded on the basis
of the 1st byte only.

HIGH RESOLUTION FORMAT

Continuing to use the A-H bit representation, the four byte number is shown below as two two byte pairs.

AB0111GH

XXXXXXXX

001111GH

XXXXXXXX

BITS, 1ST BYTE,
1ST PAIR

DESCRIPTION

CDEF = 0111

Code designating 1st byte pair of four byte number.

Polarity , 0 = +, 1 = -.

G,H,A,

Decimal locator as defined below.

2nd byte

16th - 9th bit (left to right) of 17 bit binary value.

ABCDEF = 001111

Code designating 2nd byte pair of four byte number.

Unused bit.

17th and MSB of 17 bit binary value.

2nd byte

8th - 1st bit (left to right) of 17 bit binary value.

CSI defines the largest allowable range of a
high resolution number to be 99999.

Interpretation of the decimal locator for a 4 byte
data value is given below. The decimal equivalent
of bits GH is the negative exponent to the base 10.

BITS

DECIMAL FORMAT

G H A

5 digits

0 0 0

XXXXX.

0 0 1

XXXX.X

0 1 0

XXX.XX

0 1 1

XX.XXX

1 0 0

X.XXXX

1 0 1

.XXXXX

C.3 GENERATION OF SIGNATURE

At the end of a binary transmission, a signature
is sent. The signature is a 2 byte integer value
which is a function of the data and the
sequence of data in the Output Array. It is
derived with an algorithm that assures a
99.998% probability of detecting a change in the
data or its sequence. The CR23X calculates
the signature using each transmitted byte
beginning with the Final Storage format data
(for K command, echo and carriage return line
feed are not included) until the 2 byte signature
itself. By calculating the signature of the
received data and comparing it to the
transmitted signature, it can be determined
whether the data was received correctly.