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Appendix B
Glossary
modules to the associated 16-bit word in the input image table without waiting
for the normal I/O scan.
Input Devices
Devices such as limit switches, pressure switches, push buttons, etc., that supply
data to a programmable controller. These discrete inputs are two types
those with common return and those with individual returns (referred to as
isolated inputs). Other inputs include analog devices and digital encoders.
Instruction
A command or order that will cause a PC to perform one certain prescribed
operation. The user enters a combination of instructions into PC memory to
form a unique application program.
Interfacing
Interconnecting a PC with its application devices, and data terminals through
various modules and cables. Interface modules convert PC logic levels into
external signal levels, and vice versa.
I/O Chassis
Same as chassis.
I/O Module
The printed circuit assembly that interfaces between the user devices and the
PC.
I/O Rack
Same as rack.
I/O Scan Time
The timer required for the PC processor to monitor all inputs and control all
outputs. The I/O Scan repeats continuously.
Keying
Keying bands installed in various backplane module sockets in order to ensure
that only certain modules can be inserted into designated sockets.
Ladder Diagram
An industry standard for representing control logic relay systems.
Ladder Diagram Programming
A method of writing a user’s PC program in a format similar to a relay ladder
diagram.
Language
A set of symbols and rules for representing and communicating information
(data) among people, or between people and machines.
Latch Instruction
A PC instruction which causes a bit to stay on, regardless of how briefly the
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