2
The Hardware
2.1
General Introduction to PLCs
2.1.1
History & Development
Bedford Associates, founded by Richard Morley introduced the first Programmable Logic Con-
troller in 1968. This PLC was known as the Modular Digital Controller from which the MODICON
Company derived its name.
Programmable Logic Controllers were developed to provide a replacement for large relay based
control panels. These systems were inflexible requiring major rewiring or replacement when-
ever the control sequence was to be changed.
The development of the Microprocessor from the mid 1970’s have allowed Programmable Logic
Controllers to take on more complex tasks and larger functions as the speed of the processor
increased. It is now common for PLC’s to provide the heart of the control functions within a sys-
tem often integrated with SCADA (Supervisory Control And Data Acquisition), HMI (Human
Machine Interfaces), Expert Systems and Graphical User Interfaces (GUI). The requirements of
the PLC have expanded to providing control, data processing and management functionality.
2.1.2
The initial specification for the PLC
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Easily programmed and reprogrammed in plant to enable its sequence of operations, to be
altered.
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Easily maintained and repaired – preferably using ‘plug-in’ cards or modules.
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Able to withstand the rigorous Environmental, Mechanical and Electrical conditions, found
in plant environments.
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Smaller than its relay and “discrete solid state” equivalents.
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Cost effective in comparison with “discrete solid state” and relay based systems.
2.1.3
Comparison of PLC and Relay Systems
The Hardware
General Introduction to PLCs
Training Manual GX IEC Developer
2 - 1
Characteristic
PLC
Relay
Price per function
Low
Low - If equivalent relay program uses more than 10 relays
Physical size
Very compact
Bulky
Operating speed
Fast
Slow
Electrical noise
immunity
Good
Excellent
Construction
Easy to program
Wiring - time consuming
Advanced instructions
Yes
No
Changing the control
sequence
Very simple
Very difficult – requires changes to wiring
Maintenance
Excellent
PLC’s rarely fail
Poor - relays require constant maintenance
Содержание FX2N-32CAN
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Страница 18: ...Modular PLC Training Hardware Course Overview and Requirements 1 2 MITSUBISHI ELECTRIC ...
Страница 172: ...Monitoring Instances of Function Blocks Advanced Monitoring Functions 7 12 MITSUBISHI ELECTRIC ...
Страница 176: ...Device Edit 8 4 MITSUBISHI ELECTRIC ...
Страница 190: ...Assigning DUT Variables to Function Blocks Data Unit Types DUT 10 10 MITSUBISHI ELECTRIC ...
Страница 198: ...Array Example Single Dimension Array Arrays 11 8 MITSUBISHI ELECTRIC ...
Страница 219: ...Sequential Function Chart SFC SFC Elements Training Manual GX IEC Developer 14 3 ...
Страница 220: ...14 3 SFC configuration examples SFC configuration examples Sequential Function Chart SFC 14 4 MITSUBISHI ELECTRIC ...
Страница 230: ...Structured Text Program Example IEC Structured Text 16 4 MITSUBISHI ELECTRIC ...
Страница 248: ...Configuring GX IEC Developer to access the PLC on Ethernet Ethernet Communications 18 10 MITSUBISHI ELECTRIC ...
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