FX Series Programmable Controllers Devices in Detail 4
4-45
4.14.4 Two’s Compliment
Pro
rammable controllers, computers etc, use a format called 2’s compliment. This is a
mathematical procedure which is more suited to the micro processors operational hardware
requirements. It is used to represent ne
ative numbers and to perform subtraction operations.
The procedure is ver
simple, in the followin
example “15 - 7” is
oin
to be solved:
Step1: Find the binar
values (this example uses 8 bits)
15 = 00001111
7 = 00000111
Step2: Find the inversion of the value to be subtracted.
Procedure
: invert all 1ís to 0ís and all 0ís to 1’s.
7 = 00000111
Inverted 7 = 11111000
Step3: Add 1 to the inverted number.
Procedure:
add 1 to the ri
ht hand most bit. Remember this is binar
addition hence, when a
value of 2 is obtained 1 is moved in to the next left hand position and the remainder is set to 0
(zero);
Inverted7
11111000
Additional1
00000001
Answer
11111001
This result is actuall
the same as the ne
ative value for 7 i.e. -7.
Step4:
Add the answer to the number the subtraction is bein
made from (i.e. 15).
Procedure:
Remember 1+1 = 0 carr
1 in base 2 (binar
).
Ori
inal value15
00001111
Answer found in step3
11111001
Solution
(1)00001000
The “(1)” is a carried “1” and is i
nored as this example is onl
dealin
with 8 bits.
Step 5:
Convert the answer back.
00001000 = 8
The answer is positive because the MSB (the most left hand bit) is a 0 (zero). If a quick mental
check is made of the problem it is indeed found that “15-7 = 8”.
In fact no subtraction has taken place. Each of the steps has either converted some data or
performed an addition. Yet the answer is correct 15 - 7 is 8. This example calculation was
based on 8 bit numbers but it will work equall
well on an
other quantit
of bits.
