Glossary
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flicker bit
A bit that is programmed to turn ON and OFF at a specific frequency.
force reset
The process of forcibly turning OFF a bit via a programming device. Bits are
usually turned OFF as a result of program execution.
force set
The process of forcibly turning ON a bit via a programming device. Bits are
usually turned ON as a result of program execution.
function code
A two-digit number used to input an instruction into the PC.
hardware error
An error originating in the hardware structure (electronic components) of the
PC, as opposed to a software error, which originates in software (i.e., pro-
grams).
hexadecimal
A number system where all numbers are expressed to the base 16. In a PC
all data is ultimately stored in binary form, however, displays and inputs on
Programming Devices are often expressed in hexadecimal to simplify opera-
tion. Each group of four binary bits is numerically equivalent to one hexadeci-
mal digit.
increment
Increasing a numeric value.
indirect address
An address whose contents indicates another address. The contents of the
second address will be used as the operand. Indirect addressing is possible
in the DR area only .
initialize
Part of the startup process whereby some memory areas are cleared, system
setup is checked, and default values are set.
input
The signal coming from an external device into the PC. The term input is
often used abstractly or collectively to refer to incoming signals.
input bit
A bit that is allocated to hold the status of an input.
input device
An external device that sends signals into the PC System.
input point
The point at which an input enters the PC System. Input points correspond
physically to terminals or connector pins.
input signal
A change in the status of a connection entering the PC. Generally an input
signal is said to exist when, for example, a connection point goes from low to
high voltage or from a nonconductive to a conductive state.
instruction
A direction given in the program that tells the PC of an action to be carried
out, and which data is to be used in carrying out the action. Instructions can
be used to simply turn a bit ON or OFF, or they can perform much more com-
plex actions, such as converting and/or transferring large blocks of data.
instruction block
A group of instructions that is logically related in a ladder-diagram program.
Although any logically related group of instructions could be called an instruc-
tion block, the term is generally used to refer to blocks of instructions called
logic blocks that require logic block instructions to relate them to other
instructions or logic blocks.
instruction execution time
The time required to execute an instruction. The execution time for any one
instruction can vary with the execution conditions for the instruction and the
operands used within it.