PlC_Lecture1 Flashcards
Plc Advantages Compared to relay
Eliminates much of the hardwiring associated with conventional relay control circuits
Increased Reliability - Since all the logic is contained in the PLC’s memory, there is no chance of making a logic wiring error
More Flexibility - It is easier to create and change a program in a PLC than to wire and rewire a circuit.
Lower Cost - Generally,
if an application has more than about a half-dozen control relays, it will probably be less expensive to
install a PLC
Faster Response Time - Machines that process thousands of items per second and objects that spend only a fraction of a second in front of a sensor require the PLC’s quick-response capability.
Easier to Troubleshoot - To find and fix problems, users can display the control program on a monitor and watch it in real time as it executes
Easier to Test Field Devices –
A PLC control panel has the ability to check field devices at a common point.
A PLC open architecture design allows
design allows the system to be connected easily to devices and programs made by other
manufacturers.
A PLC system with a closed architecture is
one whose design is proprietary, making it more difficult to connect to other systems
Fixed I/O
comes in one package with no separate removable units
Modular I/O
is divided by compartments
into which separate modules can be plugged.
The power supply in PLC provides
DC power to all modules that plug into the rack. For large PLC systems, this power supply does not normally supply power to the field devices
The processor (CPU) in PLC consists of
a microprocessor for implementing the logic
and controlling the communications among the modules
The processor in PLC requires memory for
storing the results of the logical operations performed by the microprocessor as well as the operating system and the PLC program
Memory is the part of a PLC controller that
stores data, instructions, and the control program.
Memory size is usually expressed in
K values: 1 K,6 K, 12 K, and so on.
The measurement kilo, abbreviated K, normally
refers to
1000 units
When dealing with computer or PLC memory,
however, 1 K means
1024, because this measurement is based on the binary number system (210 = 1024).
Depending on memory type, 1 K can mean
1024 bits, 1024 bytes, or 1024 words
The amount of memory required depends on
Number of I/O points Size of control program Data-collecting requirements • Supervisory functions required • Future expansion
Human Machine Interface (HMI)
software allows the user to monitor and control the process.
View a process - or a graphical representation of a
process on a monitor
Determine how the system is running, trend values,
and receive alarm conditions
