Glossary
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I/O capacity
The number of inputs and outputs that a PC is able to handle. This number
ranges from around one hundred for smaller PCs to two thousand for the larg-
est ones.
I/O delay
The delay in time from when a signal is sent to an output to when the status of
the output is actually in effect, or the delay in time from when the status of an
input changes until the signal indicating the change in the status is received.
I/O device
A device connected to the I/O terminals on I/O Units. I/O devices may be either
part of the Control System, if they function to help control other devices, or they
may be part of the controlled system.
I/O interrupt
An interrupt generated by a signal from I/O.
I/O point
The place at which an input signal enters the PC System, or at which an output
signal leaves the PC System. In physical terms, I/O points correspond to ter-
minals or connector pins on a Unit; in terms of programming, an I/O points cor-
respond to I/O bits in the IR area.
I/O refreshing
The process of updating output status sent to external devices so that it agrees
with the status of output bits held in memory, and of updating input bits in mem-
ory so that they agree with the status of inputs from external devices.
I/O response time
The time required for an output signal to be sent from the PC in response to an
input signal received from an external device.
I/O Unit
The Units in a PC that are physically connected to I/O devices to input and out-
put signals. I/O Units include Input Units and Output Units, each of which is
available in a range of specifications.
I/O word
A word in the IR area that is allocated to a Unit in the PC System and is used
to hold I/O status for that Unit.
IBM PC/AT or compatible
A computer that has similar architecture to, that is logically compatible with,
and that can run software designed for an IBM PC/AT computer.
increment
Increasing a numeric value, usually by 1.
indirect address
An address whose contents indicates another address. The contents of the
second address will be used as the actual operand.
initialization error
An error that occurs either in hardware or software during the PC System star-
tup, i.e., during initialization.
initialize
Part of the startup process whereby some memory areas are cleared, system
setup is checked, and default values are set.
input
The signal coming from an external device into the PC. The term input is often
used abstractly or collectively to refer to incoming signals.
input bit
A bit in the IR area that is allocated to hold the status of an input.
input device
An external device that sends signals into the PC System.
input point
The point at which an input enters the PC System. Input points correspond
physically to terminals or connector pins.
input signal
A change in the status of a connection entering the PC. Generally an input sig-
nal is said to exist when, for example, a connection point goes from low to high
voltage or from a nonconductive to a conductive state.
instruction
A direction given in the program that tells the PC of the action to be carried out,
and the data to be used in carrying out the action. Instructions can be used to
simply turn a bit ON or OFF, or they can perform much more complex actions,
such as converting and/or transferring large blocks of data.
