Rockwell Automation 1771-IR , D17716.5.76 RTD INPUT MODULE - Manual (Page 54)

Data Formats
Appendix C
CĆ3
Two’s complement binary is used with PLC–3 processors when performing
mathematical calculations internal to the processor. To complement a number
means to change it to a negative number. For example, the following binary
number is equal to decimal 22.
10110
2
= 22
10
First, the two’s complement method places an extra bit (sign bit) in the
left–most position, and lets this bit determine whether the number is positive or
negative. The number is positive if the sign bit is 0 and negative if the sign bit is
1. Using the complement method:
0 10110 = 22
To get the negative using the two’s complement method, you must invert each
bit from right to left after the first ”1” is detected.
In the above example:
0 10110 = +22
Its two’s complement would be:
1 01010 = –22
Note that in the above representation for +22, starting from the right, the first
digit is a 0 so it is not inverted; the second digit is a 1 so it is not inverted. All
digits after this one are inverted.
If a negative number is given in two’s complement, its complement (a positive
number) is found in the same way:
1 10010 = –14
0 01110 = +14
All bits from right to left are inverted after the first ”1” is detected.
The two’s complement of 0 is not found, since no first ”1” is ever encountered
in the number. The two’s complement of 0 then is still 0.
Two's Complement Binary