Modes Of Operation Of PLC - Electrical Machines
MODES OF OPERATION OF PLC
The various modes of operation of PLC's are
1. CLEAR program memory mode
2. PROGRAMMING mode
3. RUN mode
4. TESTING program mode
5. DIAGNOSTIC test mode
6. ENTER/CHANGE Access Code
When PLC is put in CLEAR mode, the user program stored in the PLC memory can be cleared. Now all the memory is free for entering a new program. A new program can now by bringing the PLC into PROGRAMMING mode. Program instructions are entered one by one. After entering the program, the program
execution can be checked by putti be checked by putting PLC in TEST program in test mode, PLC executes the user program but energise the outputs, only LED displays for outputs energised. Thus program working can be checked thout actually energising the outputs. This avoids damage machinery on account of incorrect program. The machine an be run continuously by bringing the PLC in RUN mode.
In RUN mode, the PLC first reads the status of input devices and store then in the memory. Then it executes the user program instructions one by one and if any outputs are to be energised, the memory location for that output device is updated by storing binary 1 at that memory location. When whole of the program is executed, the output devices are energised. After this the PLC will communicate with any connected devices like hand-held programmer, personal computer and will also perform any necessary memory management Memory management will include updating timers, counters, internal registers etc. This four step process is called processor scan. It is repeated over and over. The scan time of PLC is in milliseconds. The scan time can vary from fraction of milliscond to hundreds of milliseconds depending upon the size of the program.
PLC SCAN:
The processor in any PLC is to perform specific in a specific sequence and then continuously repeat equence is called as operating cycle. This operating also called as PLC scan. The scan cycle is performed sequentially and repeatedly when the processor is in un mode. The cycle time depends on the size of the program and the number of I/Os used.
A PLC scan consists of:
1. Input scan (or) processing
2. Program scan (or) processing
3. Output scan (or) processing
1. Input scan:
The input scan process. During this scan the PLC reads the status of inputs from input module and stores them in the input image table before execution of program. This information remains unaltered until the next scan of input module. That is, the change in the status of the input module will not be accepted and not stored in between scan sequence.
2. Program scan:
The program scan process, During the the PLC executes the user program by taking from the input image table. The instructions carried out in sequence manner and depending upon the outputs are produced. Then the status of the
touts are stored in the output Image table (memory) in bit pattern.
3.Outputs:
The output scan process. On
completing the execution of all instructions (program scan). The status of output image table is transferred to output module which will physically change the status of the output on the PLC module.
The various modes of operation of PLC's are
1. CLEAR program memory mode
2. PROGRAMMING mode
3. RUN mode
4. TESTING program mode
5. DIAGNOSTIC test mode
6. ENTER/CHANGE Access Code
When PLC is put in CLEAR mode, the user program stored in the PLC memory can be cleared. Now all the memory is free for entering a new program. A new program can now by bringing the PLC into PROGRAMMING mode. Program instructions are entered one by one. After entering the program, the program
execution can be checked by putti be checked by putting PLC in TEST program in test mode, PLC executes the user program but energise the outputs, only LED displays for outputs energised. Thus program working can be checked thout actually energising the outputs. This avoids damage machinery on account of incorrect program. The machine an be run continuously by bringing the PLC in RUN mode.
In RUN mode, the PLC first reads the status of input devices and store then in the memory. Then it executes the user program instructions one by one and if any outputs are to be energised, the memory location for that output device is updated by storing binary 1 at that memory location. When whole of the program is executed, the output devices are energised. After this the PLC will communicate with any connected devices like hand-held programmer, personal computer and will also perform any necessary memory management Memory management will include updating timers, counters, internal registers etc. This four step process is called processor scan. It is repeated over and over. The scan time of PLC is in milliseconds. The scan time can vary from fraction of milliscond to hundreds of milliseconds depending upon the size of the program.
PLC SCAN:
The processor in any PLC is to perform specific in a specific sequence and then continuously repeat equence is called as operating cycle. This operating also called as PLC scan. The scan cycle is performed sequentially and repeatedly when the processor is in un mode. The cycle time depends on the size of the program and the number of I/Os used.
A PLC scan consists of:
1. Input scan (or) processing
2. Program scan (or) processing
3. Output scan (or) processing
1. Input scan:
The input scan process. During this scan the PLC reads the status of inputs from input module and stores them in the input image table before execution of program. This information remains unaltered until the next scan of input module. That is, the change in the status of the input module will not be accepted and not stored in between scan sequence.
2. Program scan:
The program scan process, During the the PLC executes the user program by taking from the input image table. The instructions carried out in sequence manner and depending upon the outputs are produced. Then the status of the
touts are stored in the output Image table (memory) in bit pattern.
3.Outputs:
The output scan process. On
completing the execution of all instructions (program scan). The status of output image table is transferred to output module which will physically change the status of the output on the PLC module.
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