Glossary
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overwrite Changing the content of a memory location so that the previous content is lost.
Parameter Area A part of System DM used to designate various PC operating parameters.
Parameter Backup Area A part of System DM used to back up the Parameter Area.
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.
parity check Checking parity to ensure that transmitted data has not been corrupted.
PC An acronym for Programmable Controller.
PC configuration The arrangement and interconnections of the Units that are put together to form
a functional PC.
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.
PCB An acronym for printed circuit board.
PC Setup A group of operating parameters set in the PC from a Programming Device to
control PC operation.
Peripheral Device Devices connected to a PC System to aid in system operation. Peripheral de-
vices include printers, programming devices, external storage media, etc.
peripheral servicing Processing signals to and from peripheral devices, including refreshing, com-
munications processing, interrupts, etc.
PID Unit A Unit designed for PID control.
pointer A variable or register which contains the address of some object in memory.
positive delay A delay set for a data trace in which recording data begins after the trace signal
by a specified amount.
power-off interrupt An interrupt executed when power to the PC is turned off.
power-on interrupt An interrupt executed when power to the PC is turned on.
present value The current value registered in a device at any instant during its operation. Pres-
ent value is abbreviated as PV. The use of this term is generally restricted to tim-
ers and counters.
printed circuit board A board onto which electrical circuits are printed for mounting into a computer or
electrical device.
PROGRAM mode A mode of operation that allows inputting and debugging of programs to be car-
ried out, but that does not permit normal execution of the program.
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-
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