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Glossary
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output point The point at which an output leaves the PC System. Output points correspond
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.
parity Adjustment of the number of ON bits in a word or other unit of data so that the
total is always an even number or always an odd number. Parity is generally
used to check the accuracy of data after being transmitted by confirming that the
number of ON bits is still even or still odd.
PC An acronym for Programmable Controller.
PCB An acronym for printed circuit board.
PC configuration The arrangement and interconnections of the Units that are put together to form
a functional PC.
PCF Acronym for plastic-clad optical fiber cable.
PC Link System A system in which PCs are connected through PC Link Units to enable them to
share common data areas, i.e., each of the PCs writes to certain words in the LR
area and receives the data of the words written by all other PC Link Units con-
nected in series with it.
PC Link Unit The Unit used to connect PCs in a PC Link System.
PC System With building-block PCs, all of the Racks and independent Units connected di-
rectly to them up to, but not including the I/O devices. The boundaries of a PC
System are the PC and the program in its CPU at the upper end; and the I/O
Units, Special I/O Units, Optical I/O Units, Remote Terminals, etc., at the lower
end.
peripheral device Devices connected to a PC System to aid in system operation. Peripheral de-
vices include printers, programming devices, external storage media, etc.
port A connector on a PC or computer that serves as a connection to an external de-
vice.
present value The current value registered in a device at any instant during its operation. Pres-
ent value is abbreviated as PV.
printed circuit board A board onto which electrical circuits are printed for mounting into a computer or
electrical device.
Printer Interface Unit A Unit used to interface a printer so that ladder diagrams and other data can be
printed out.
program The list of instructions that tells the PC the sequence of control actions to be car-
ried out.
Programmable Controller A computerized device that can accept inputs from external devices and gener-
ate outputs to external devices according to a program held in memory. Pro-