Campbell Scientific TX320 Transmitter - Manual (Page 62)

Appendix E. High Resolution 18-Bit Binary Format
E-2
Where the 18-bit data point is made by combining the three bytes
after bit 7 and bit 6 of each byte have been dropped.
Where 0 represents bit 0 - the least significant bit
Where 17 represents bit 17 - the most significant bit and is used to
determine the sign.
Converting the 18-bit data point to an integer can be done manually. Don’t
forget the 18-bits are numbered 0 through 17. Bit 17 is the sign bit, when bit 17
is set, the number is negative. If bit 17 is set, subtract 1 from the number then
take the complement of the number. If bit 17 is not set, simply convert the
number to its decimal equivalent.
Example positive data point conversion:
Byte Label byte 3 byte 2 byte 1
Actual data point 01000101 11110010 11010010
Drop first 2 bits of
each byte
000101 110010 010010
Combine the 3
bytes into one word
000101 110010 010010
Convert from Binary to Decimal 23698
Example of a negative data point conversion:
Byte Label byte 3 byte 2 byte 1
Actual data point 01111010 11001101 11101101
Drop first 2 bits of
each byte
111010 001101 101101
Notice bit 17 is set,
Combine the 3
bytes into one word
111010 001101 101101
Subtract 1 from the number 111010 001101 101100
Take the complement of
each bit
000101 110010 010011
Convert the binary value
into a decimal value, don’t
forget the negative sign
–23699