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Glossary
address
The location in memory where data is stored. For data areas, an address
consists of a two-letter data area designation and a number that designates
the word and/or bit location. For the UM area, an address designates the
instruction location (UM area).
allocation
The process by which the PC assigns certain bits or words in memory for
various functions. This includes pairing I/O bits to I/O points.
AND
A logic operation whereby the result is true if and only if both premises are
true. In ladder-diagram programming the premises are usually ON/OFF
states of bits or the logical combination of such states called execution condi-
tions.
BCD
Short for binary-coded decimal.
BCD calculation
An arithmetic calculation that uses numbers expressed in binary-coded deci-
mal.
binary
A number system where all numbers are expressed to the base 2, i.e., any
number can be written using only 1’s or 2’s. Each group of four binary bits is
equivalent to one hexadecimal digit.
binary calculation
An arithmetic calculation that uses numbers expressed in binary.
binary-coded decimal
A system used to represent numbers so that each group of four binary bits is
numerically equivalent to one decimal digit.
bit
A binary digit; hence a unit of data in binary notation. The smallest unit of
information that can be electronically stored in a PC. The status of a bit is
either ON or OFF. Different bits at particular addresses are allocated to spe-
cial purposes, such as holding the status input from external devices, while
other bits are available for general use in programming.
bit address
The location in memory where a bit of data is stored. A bit address must
specify (sometimes by default) the data area and word that is being ad-
dressed, as well as the number of the bit.
bit number
A number that indicates the location of a bit within a word. Bit 00 is the right-
most (least-significant) bit; bit 15 is the leftmost (most-significant) bit.
bus bar
The line leading down the left and sometimes right side of a ladder diagram.
Instruction execution proceeds down the bus bar, which is the starting point
for all instruction lines.
carry flag
A flag that is used with arithmetic operations to hold a carry from an addition
or multiplication operation, or to indicate that the result is negative in a sub-
traction operation. The carry flag is also used with certain types of shift op-
erations.
clock pulse
A pulse available at a certain bit in memory for use in timing operations. Vari-
ous clock pulses are available with different pulse widths.