Glossary
Appendix D
D-14
Master
A device used to control secondary devices.
Master Control Relay (MCR)
A mandatory hardwired relay that can be de-energized by any
series-connected emergency stop switch. Whenever the master control
relay is de-energized, its contacts open to de-energize all application
I/O devices.
Master Control Reset (MCR)
See MCR Zones
MCR Zones
User program areas where all non-retentive outputs can be turned off
simultaneously. Each MCR zone must be delimited and controlled by
MCR fence codes (MCR instructions).
Memory
A group of circuit elements that can store or retrieve data.
Memory Map
A diagram showing a system’s memory addresses and what programs and
data are assigned to each section of memory.
Message
(1) A meaningful combination of alphanumeric characters which
establishes the content and format of a report. (2) An information unit
designated for transfer by a single command form the application to
the network.
MOS
Metal Oxide Semiconductor. A semiconductor device in which an electric
field controls the conductance of a channel under a metal electrode called
a gate.
Mnemonic
A term, usually an abbreviation, that is easy to remember.
Mode
A selected method of operation. Example: run, test, or program.
Modem
A contraction of Modulator/Demodulator. Equipment that connects data
terminal equipment to a communication line.
Module
An interchangeable, plug-in item containing electronic components
M
