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Glossary
*DM Indirectly addressed DM area. See indirect address and DM area.
1:1 link A link created between two PCs to create common data in their LR areas.
ACP See add count input.
add count input An input signal used to increment a counter when the signal changes from
OFF to ON.
address A number used to identify the location of data or programming instructions
in memory.
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 conditions.
area See data area and memory area.
area prefix A one or two letter prefix used to identify a memory area in the PC. All
memory areas except the IR and SR areas require prefixes to identify
addresses in them.
arithmetic shift A shift operation wherein the carry flag is included in the shift.
ASCII Short for American Standard Code for Information Interchange. ASCII is
used to code characters for output to printers and other external devices.
AR Area A PC data area allocated to flags and control bits.
AUTOEXEC.BAT An MS DOS file containing commands automatically executed at startup.
back-up A copy made of existing data to ensure that the data will not be lost even if
the original data is corrupted or erased.
basic instruction A fundamental instruction used in a ladder diagram.
baud rate The data transmission speed between two devices in a system measured
in bits per second.
BCD See binary-coded decimal.
BCD calculation An arithmetic calculation that uses numbers expressed in binary-coded
decimal.
binary A number system where all numbers are expressed in base 2, i.e., numbers
are written using only 0’s and 1’s. Each group of four binary bits is equivalent
to one hexadecimal digit. Binary data in memory is thus often expressed in
hexadecimal for convenience.
binary calculation An arithmetic calculation that uses numbers expressed in binary.
binary-coded decimal A system used to represent numbers so that every four binary bits is
numerically equivalent to one decimal digit.
bit The smallest piece of information that can be represented on a computer.
A bit has the value of either zero or one, corresponding to the electrical
signals ON and OFF. A bit represents one binary digit. Some bits at particular
addresses are allocated to special purposes, such as holding the status of
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 specifies
the data area and word that is being addressed as well as the number of the
bit within the word.
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