Wednesday, July 6, 2011

System Performance Scan Time of PLC





The time it gets for the CPU to resolve the program of ladder logic and to renew all the I/O under its control is defined as scan time. Scan time contains logic solve time, servicing time of I/O and the time it gets to do tasks of system overhead. The maximum time amount permitted for the PLC to scan a program of user logic one time is 250 ms. If the scan has not finished in that time amount, a watchdog timer in the CPU ends the appliance and transfers a message of timeout error to the panel of programming. This maximum limit of scan time avoids the PLC from entering endless loops in the program of logic.

Logic Solve Time
It gets the CPU to resolve the control logic in the program, autonomous of any administrative or service time, is defined as logic solve time. Logic solve time for the 110CPU411 and 110CPU311 Micro PLCs is 4.25 ms/K nodules of ladder logic solve time for the 110CPU612 and 110CPU512 Micro PLCs is 2.5 ms/K nodules of ladder logic.

Programming Note for 512XX and 612XX Controllers
In very small test situations of user logic such as, using a contact to change a coil as a fast oscillator, in Child or Single mode operation, the fast scan time may slow down right operation of the internal output device circuit and the internal hardware output LED circuit. Both circuits respond separately to user logic, so the LED might not imitate the actual output operation. The logic that is programmed, the longer of scan time will be; and both output circuits and LEDs will show the right programmed reaction.

Ask the hardware manual offered with the unit to decide the output device reaction or switching time. For instance, the relay of internal output has a maximum switching rate is 5 Hz. When the Micro is arrangement as a parent, this restriction of hardware should not be seen, while each Child Micro is added in the Parent configuration adds for 3 ms to the scan time.

Memory Allocation
The operating system of ladder logic establishes the method memory resources in a Micro PLC of Modicon are allocated. It splits accessible system memory into three classes:
• User program memory—the ladder logic program is built and edited
• User data memory—the variable data that modifies during program execution
• System configuration memory—for saving system data tables for instance the PLC setup values and I/O map.



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