Glossary
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holding area Words in memory designated to maintain status when the PC’s operating mode
is changed or power is turned off and then back on.
host interface An interface that allows communications with a host computer.
Host Link System A system with one or more host computers connected to one or more PCs via
Host Link Units or host interfaces so that the host computer can be used to trans-
fer data to and from the PC(s). Host Link Systems enable centralized manage-
ment and control of PC Systems.
Host Link Unit An interface used to connect a C-series PC to a host computer in a Host Link
System.
I/O allocation The process by which the PC assigns certain bits in memory for various func-
tions. This includes pairing I/O bits to I/O points on Units.
I/O bit A bit in memory used to hold I/O status. Input bits reflect the status of input termi-
nals; output bits hold the status for output terminals.
I/O Block Either an Input Block or an Output Block. I/O Blocks provide mounting positions
for replaceable relays.
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 largest
ones.
I/O Control Unit A Unit mounted to the CPU Rack to monitor and control I/O points on Expansion
CPU Racks or Expansion I/O Racks.
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, Special I/O Units, etc. 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 Interface Unit A Unit mounted to an Expansion CPU Rack or Expansion I/O Rack to interface
the Rack to the CPU Rack.
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 termi-
nals or connector pins on a Unit; in terms of programming, an I/O points corre-
spond 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 memory
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 table A table created within the memory of the PC that lists the I/O words allocated to
each Unit in the PC System. The I/O table can be created by, or modified from, a
Programming Device.
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