Automationdirect.com DL05, User Manual

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CPU Specifications
and Operation
4–21
CPU Specifications and Operation
Variable memory (called “V-memory”) stores data for the ladder program and for
configuration settings. V-memory locations and V-memory addresses are the same
thing, and are numbered in octal. For example, V2073 is a valid location, while
V1983 is not valid (“9” and “8” are not valid octal digits).
Each V-memory location is one data word wide, meaning 16 bits. For configuration
registers, our manuals will show each bit of a V-memory word. The least significant
bit (LSB) will be on the right, and the most significant bit (MSB) on the left. We use the
word “significant”, referring to the relative binary weighting of the bits.
0 1 0 0 1 1 1 0 0 0 1 0 1 0 0 1
MSB LSB
V-memory data
(binary)
V-memory address
(octal)
V2017
V-memory data is 16-bit binary, but we rarely program the data registers one bit at a
time. We use instructions or viewing tools that let us work with decimal, octal, and
hexadecimal numbers. All these are converted and stored as binary for us.
A frequently-asked question is “How do I tell if a number is octal, BCD, or hex”? The
answer is that we usually cannot tell just by looking at the data... but it does not really
matter. What matters is: the source or mechanism which writes data into a
V-memory location and the thing which later reads it must both use the same data
type (i.e., octal, hex, binary, or whatever). The V-memory location is just a storage
box... that’s all. It does not convert or move the data on its own.
Since humans naturally count in decimal (10 fingers, 10 toes), we prefer to enter and
view PLC data in decimal as well. However, computers are more efficient in using
pure binary numbers. A compromise solution between the two is Binary-Coded
Decimal (BCD) representation. A BCD digit ranges from 0 to 9, and is stored as four
binary bits (a nibble). This permits each V-memory location to store four BCD digits,
with a range of decimal numbers from 0000 to 9999.
0 1 0 0 1 0 0 1 0 0 1 1 0 1 1 0
4 9 3 6
V-memory storage
BCD number
In a pure binary sense, a 16-bit word can represent numbers from 0 to 65535. In
storing BCD numbers, the range is reduced to only 0 to 9999. Many math
instructions use Binary-Coded Decimal (BCD) data, and
Direct SOFT and the
handheld programmer allow us to enter and view data in BCD.
Hexadecimal numbers are similar to BCD numbers, except they utilize all possible
binary values in each 4-bit digit. They are base-16 numbers so we need 16 different
digits. To extend our decimal digits 0 through 9, we use A through F as shown.
8 9 10 11 12 13 14 15 0 1 2 3 4 5 6 7
8 9 A B C D E F 0 1 2 3 4 5 6 7
Decimal
Hexadecimal
A 4-digit hexadecimal number can represent all 65536 values in a V-memory word.
The range is from 0000 to FFFF (hex). PLCs often need this full range for sensor
data, etc. Hexadecimal is just a convenient way for humans to view full binary data.
1 0 1 0 0 1 1 1 1 1 1 1 0 1 0 0
A 7 F 4
V-memory storage
Hexadecimal number
V–Memory
Binary-Coded
Decimal Numbers
Hexadecimal
Numbers