Rockwell Automation 1771-IL/B , D17716.5.91 ISOLATED ANALOG INPUT Module User Manual

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Data Formats

Appendix C

C-3

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.