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Glossary
109
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 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.
overflow The state where the capacity of a data storage location has been exceeded.
overseeing Part of the processing performed by the CPU Unit that includes general tasks
required 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.
parity check Checking parity to ensure that transmitted data has not been corrupted.
PC See 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 Units connected up to, but not including, the
I/O devices. The boundaries of a PC System are the PC and the program in its
CPU Unit at the upper end; and the I/O Units at the lower end.
PCB See 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.
port A connector on a PC or computer that serves as a connection to an external de-
vice.
positive delay A delay set for a data trace in which recording data begins after the trace signal
by a specified amount.
Power Supply Unit A Unit that connected to a PC that provides power at the voltage required by the
other Units.
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-
grammable Controllers are used to automate control of external devices. Al-
though single-unit Programmable Controllers are available, building-block Pro-
grammable Controllers are constructed from separate components. Such Pro-
grammable Controllers are formed only when enough of these separate compo-
nents are assembled to form a functional assembly.