Glossary
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Ladder Support Software A software package installed on a IBM PC/AT or compatible computer to func-
tion as a Programming Device.
least-significant (bit/word) See rightmost (bit/word).
LED Acronym for light-emitting diode; a device used as for indicators or displays.
leftmost (bit/word) The highest numbered bits of a group of bits, generally of an entire word, or the
highest numbered words of a group of words. These bits/words are often called
most-significant bits/words.
link A hardware or software connection formed between two Units. “Link” can refer
either to a part of the physical connection between two Units or a software con-
nection created to data existing at another location (i.e., data links).
load The processes of copying data either from an external device or from a storage
area to an active portion of the system such as a display buffer. Also, an output
device connected to the PC is called a load.
logic block A group of instructions that is logically related in a ladder-diagram program and
that requires logic block instructions to relate it to other instructions or logic
blocks.
logic block instruction An instruction used to locally combine the execution condition resulting from a
logic block with a current execution condition. The current execution condition
could be the result of a single condition, or of another logic block. AND Load and
OR Load are the two logic block instructions.
logic instruction Instructions used to logically combine the content of two words and output the
logical results to a specified result word. The logic instructions combine all the
same-numbered bits in the two words and output the result to the bit of the same
number in the specified result word.
LR area A data area that is used in data links.
LSS See Ladder Support Software.
main program All of a program except for subroutine and interrupt programs.
mark trace A process in which changes in the contents of specific memory locations are re-
corded during program execution.
masked bit A bit whose status has been temporarily made ineffective.
masking `Covering’ an interrupt signal so that the interrupt is not effective until the mask
is removed.
megabyte A unit of storage equal to one million bytes.
memory area Any of the areas in the PC used to hold data or programs.
message number A number assigned to a message generated with the MESSAGE instruction.
mnemonic code A form of a ladder-diagram program that consists of a sequential list of the in-
structions without using a ladder diagram.
MONITOR mode A mode of PC operation in which normal program execution is possible, and
which allows modification of data held in memory. Used for monitoring or debug-
ging the PC.
most-significant (bit/word) See leftmost (bit/word).
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