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An actual data location within any data area but the TC area is designated by its
address. The address designates the bit or word within the area where the de-
sired data is located. The TC area consists of TC numbers, each of which is used
for a specific timer or counter defined in the program. Refer to 3-8 TC Area for
more details on TC numbers and to 5-14 Timer and Counter Instructions for in-
formation on their application.
The rest of the data areas (i.e., the IR, SR, HR, DM, AR, and LR areas) consist of
words, each of which consists of 16 bits numbered 00 through 15 from right to
left. IR words 000 and 001 are shown below with bit numbers. Here, the content
of each word is shown as all zeros. Bit 00 is called the rightmost bit; bit 15, the
leftmost bit.
The term least significant bit is often used for rightmost bit; the term most signifi-
cant bit, for leftmost bit. These terms are not used in this manual because a
single data word is often split into two or more parts, with each part used for dif-
ferent parameters or operands. When this is done, the rightmost bits of a word
may actually become the most significant bits, i.e., the leftmost bits in another
word,when combined with other bits to form a new word.
Bit number
IR word 000 0000000000000000
IR word 001 0000000000000000
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
The DM area is accessible by word only; you cannot designate an individual bit
within a DM word. Data in the IR, SR, HR, AR, and LR areas is accessible either
by word or by bit, depending on the instruction in which the data is being used.
To designate one of these areas by word, all that is necessary is the acronym (if
required) and the two-, three-, or four-digit word address. To designate an area
by bit, the word address is combined with the bit number as a single four- or five-
digit address. The following table show examples of this. The two rightmost dig-
its of a bit designation must indicate a bit between 00 and 15, i.e., the rightmost
digit must be 5 or less the next digit to the left, either 0 or 1.
The same TC number can be used to designate either the present value (PV) of
the timer or counter, or a bit that functions as the Completion Flag for the timer or
counter. This is explained in more detail in 3-8 TC Area.
Area Word designation Bit designation
IR 000 00015 (leftmost bit in word 000)
SR 252 25200 (rightmost bit in word 252)
DM DM 1250 Not possible
TC TC 215 (designates PV) TC 215 (designates completion flag)
LR LR 12 LR 1200
Data Structure Word data input as decimal values is stored in binary-coded decimal (BCD);
word data entered as hexadecimal is stored in binary form. Each four bits of a
word represents one digit, either a hexadecimal or decimal digit, numerically
equivalent to the value of the binary bits. One word of data thus contains four
digits, which are numbered from right to left. These digit numbers and the corre-
sponding bit numbers for one word are shown below.
Bit number
Contents 0000000000000000
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
Digit number 3210
When referring to the entire word, the digit numbered 0 is called the rightmost
digit; the one numbered 3, the leftmost digit.
Data Area Structure Section 3-2