Glossary
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definer A number used as an operand for an instruction but that serves to define the in-
struction itself, rather that the data on which the instruction is to operate. Defin-
ers include jump numbers, subroutine numbers, etc.
destination The location where an instruction places the data on which it is operating, as op-
posed to the location from which data is taken for use in the instruction. The loca-
tion 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. Non-differentiated instructions are executed for each scan 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 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 instruc-
tion.
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.
DIP switch Dual in-line package switch, an array of pins in a signal package that is mounted
to a circuit board and is used to set operating parameters.
distributed control A automation concept in which control of each portion of an automated system is
located near the devices actually being controlled, i.e., control is decentralized
and ‘distributed’ over the system. Distributed control is a concept basic to PC
Systems.
DM Area A data area used to hold only word data. Words in the DM area cannot be ac-
cessed bit by bit.
DM word A word in the DM Area.
downloading The process of transferring a program or data from a higher-level or host com-
puter to a lower-level or slave computer. If a Programming Device is involved,
the Programming Device is considered the host computer.
DR See
data register
.
Dummy I/O Unit An I/O Unit that has no functional capabilities but that can be mounted to a slot on
a Rack so that words can be allocated to that slot. Dummy I/O Units can be used
to avoid changing operand addresses in programs by reserving words for a slot
for future use or by filling a slot vacated by a Unit to which words have already
been allocated.
EEPROM Electrically erasable programmable read-only memory; a type of ROM in which
stored data can be erased and reprogrammed. This is accomplished using a
special control lead connected to the EEPROM chip and can be done without
having to remove the EEPROM chip from the device in which it is mounted.
electrical noise Random variations of one or more electrical characteristics such as voltage, cur-
rent, and data, which might interfere with the normal operation of a device.
EM Area Extended Data Memory Area; an area that can be optionally added to certain
PCs to enable greater data storage. Functionally, the EM Area operates like the
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