Rockwell-automation 1771-OFE/B Analog Output Module User Manual Manuel d'utilisateur télécharger pdf (Page 77)

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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

= 22

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

1 2 ... 72 73 74 75 76 77 78 79 80 81 82 83 84

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Rockwell-automation 1771-OFE/B Analog Output Module User Manual Manuel d'utilisateur Page 77