Glossary
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used or as an address to express the location in memory of the data to be
used.
operand bit A bit designated as an operand for an instruction.
operand word A word designated as an operand for an instruction.
operating error An error that occurs during actual PC operation as opposed to an initializa-
tion error, which occurs before actual operations can begin.
OR A logic operation whereby the result is true if either of two premises is true, or
if both are true. In ladder-diagram programming the premises are usually ON/
OFF states of bits or the logical combination of such states called execution
conditions.
output The signal sent from the PC to an external device. The term output is often
used abstractly or collectively to refer to outgoing signals.
output bit A bit in memory that is allocated to hold the status to be sent to an output
device.
output device An external device that receives signals from the PC System.
output point The point at which an output leaves the PC System. Output points corre-
spond physically to terminals or connector pins.
output signal A signal being sent to an external device. Generally an output 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.
overseeing Part of the processing performed by the CPU that includes general tasks re-
quired to operate the PC.
overwrite Changing the content of a memory location so that the previous content is
lost.
PC An acronym for Programmable Controller.
PC configuration The arrangement and interconnections of the Units that are put together to
form a functional PC.
peripheral device Devices connected to a PC System to aid in system operation. Peripheral
devices include printers, programming devices, external storage media, etc.
port A connector on a PC or computer that serves as a connection to an external
device.
present value The current value registered in a device at any instant during its operation.
Present value is abbreviated as PV.
printed circuit board A board onto which electrical circuits are printed for mounting into a comput-
er or electrical device.
program The list of instructions that tells the PC the sequence of control actions to be
carried out.
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