Glossary
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decrement Decreasing a numeric value.
default A value automatically set by the PC when the user omits to set a specific val-
ue. Many devices will assume such default conditions upon the application of
power.
delay In tracing, a value that specifies where tracing is to begin in relationship to
the trigger. A delay can be either positive or negative, i.e., can designate an
offset on either side of the trigger.
destination The location where an instruction is to place the data on which it is operating,
as opposed to the location from which data is taken for use in the instruction.
The location from which data is taken is called the source.
differentiated instruction An instruction used to ensure that the operand bit is never turned ON for
more than one scan after the execution condition goes either from OFF to
ON for a Differentiate Up instruction or from ON to OFF for a Differentiate
Down instruction.
digit A unit of storage in memory that consists of four bits.
distributed control An automation concept in which control of each portion of an automated sys-
tem is located near the devices actually being controlled, i.e., control is de-
centralized and ‘distributed’ over the system. Distributed control is one of the
fundamental concepts of PC Systems.
download The process of transferring a program or data from a higher-level computer
to a lower-level computer or PC or between peripheral devices and the PC.
electrical noise Random variations of one or more electrical characteristics such as voltage,
current, and data, which might interfere with the normal operation of a de-
vice.
execution condition The ON or OFF status under which an instruction is executed. The execution
condition is determined by the logical combination of conditions on the same
instruction line and up to the instruction currently being executed.
execution time The time required for the CPU to execute either an individual instruction or
an entire program.
extended counter A counter created in a program by using two or more count instructions in
succession. Such a counter is capable of counting higher than any of the
standard counters provided by the individual instructions.
extended timer A timer created in a program by using two or more timers in succession.
Such a timer is capable of timing longer than any of the standard timers pro-
vided by the individual instructions.
fatal error An error that stops PC operation and requires correction before operation
can continue.
flag A dedicated bit in memory that is set by the system to indicate some type of
operating status. Some flags, such as the carry flag, can also be set by the
operator or via the program.
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