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Glossary
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call A process by which instruction execution shifts from the main program to a
subroutine. The subroutine may be called by an instruction or by an interrupt.
carry flag A flag that is used with arithmetic operations to hold a carry from an addition
or multiplication operation or to indicate that the result is negative in a sub-
traction operation. The carry flag is also used with certain types of shift oper-
ation.
clock pulse A pulse available at a certain bit in memory for use in timing operations. Vari-
ous clock pulses are available with different pulse widths.
clock pulse bit A bit in memory that supplies a pulse that can be used to time operations.
Various clock pulse bit are available with different pulse widths.
condition An ‘instruction’ placed along an instruction line to determine how terminal
instruction on the right side are to be executed. Each condition is assigned to
a bit in memory that determines its status. The status of the bit assigned to
each condition determines, in turn, the execution condition for each instruc-
tion up to a terminal instruction on the right side of the ladder diagram.
constant An operand for which the actual numeric value is input directly and in place
of a data memory address would hold the value to be used.
control bit A bit in a memory area that is set either from the program or from a Program-
ming Device to achieve a specific purpose, e.g., a Restart bit is turned ON
and OFF to restart a Unit.
Control System All of the hardware and software components used to control other devices.
A Control System includes the PC System, the PC programs, and all I/O de-
vices that are used to control or obtain feedback from the controlled system.
controlled system The devices that are being controlled by a PC System.
control signal A signal sent from the PC to affect the operation of the controlled system.
counter Either a dedicated number of digits or words in memory used to count the
number of times a specific process has occurred or a location in memory ac-
cessed through a TC bit and used to count the number of times the status of
a bit or an execution condition has changed from OFF to ON.
CPU An acronym for central processing unit. In a PC System, the CPU executes
the program, processes I/O signals, communicates with external devices,
etc.
CPU Unit The CPU Unit contains the CPU and provides a certain number of I/O points.
cycle The process used to execute a ladder-diagram program. The program is ex-
amined sequentially from start to finish and each instruction is executed in
turn based on execution conditions.
cycle time The time required for a single cycle of the ladder-diagram program.
data area An area in the PC’s memory that is designed to hold a specific type of data,
e.g., the SR area is designed to hold flags and control bits. Memory areas
that hold programs are not considered data areas.