Glossary
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debug
A process by which a draft program is corrected until it operates as intended.
Debugging includes both the removal of syntax errors, as well as the
fine-tuning of timing and coordination of control operations.
decimal
A number system where all numbers are expressed to the base 10. In a PC
all data is ultimately stored in binary form, four binary bits are often used to
represent one decimal digit, via a system called binary-coded decimal.
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.
definer
A number used as an operand for an instruction but that serves to define the
instruction itself, rather that the data on which the instruction is to operate.
Definers include jump numbers, subroutine numbers, etc.
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 that is executed only once each time its execution condition
goes from OFF to ON. Nondifferentiated instructions are executed each cycle
as long as the execution condition stays ON.
differentiation instruction
An instruction used to ensure that the operand bit is never turned ON for
more than one cycle 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.
digit designator
An operand that is used to designate the digit or digits of a word to be used
by an instruction.
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.
DM area
A data area used to hold only word data. Words in the DM area cannot be
accessed bit by bit.
download
The process of transferring a program or data from a higher-level computer
to a lower-level computer or PC.
Duplex CPU
A CPU arrangement available for a C2000H PC in which there are actually
two CPUs. Each of the CPUs holds the same program and the same data.
One of the CPUs is active and controls current PC operation; the other CPU
serves as a backup and takes over PC operation if the active CPU fails.