Rockwell Automation 1771-N SERIES High Resolution Analog Module User Manual - Manual (Page 45)

4–5 Configuring the Module
Publication 1771ĆUM127B-EN-P - December 2002
Two's Complement Binary
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.
Real Time Sampling
Real time sampling is set using word 4 of the block transfer write
data file. The real time sampling (RTS) mode of operation provides
data from the module at a fixed time period for use by the processor.
RTS is invaluable for time based functions (such as PID and
totalization) in the processor. It allows accurate time based
calculations in local or remote I/O racks.