<Table> <Tr> <Td> +------- +---- - ()-------------------- + A +---- Remote Unlock +------- + Remote Counter +------- +---- - ()-------------------- + B +---- interior unlock +------- + Interior counter +-------- +-------------------- + A + B +----------- into C +-------- + Adder </Td> </Tr> </Table> <Tr> <Td> +------- +---- - ()-------------------- + A +---- Remote Unlock +------- + Remote Counter +------- +---- - ()-------------------- + B +---- interior unlock +------- + Interior counter +-------- +-------------------- + A + B +----------- into C +-------- + Adder </Td> </Tr> <P> In this example, the system will count the number of times that the interior and remote unlock buttons are pressed . This information will be stored in memory locations A and B. Memory location C will hold the total number of times that the door has been unlocked electronically . </P> <P> PLCs have many types of special blocks . They include timers, arithmetic operators and comparisons, table lookups, text processing, PID control, and filtering functions . More powerful PLCs can operate on a group of internal memory locations and execute an operation on a range of addresses, for example, to simulate a physical sequential drum controller or a finite state machine . In some cases, users can define their own special blocks, which effectively are subroutines or macros . The large library of special blocks along with high speed execution has allowed use of PLCs to implement very complex automation systems . </P>

The different between plc physical ladder diagram and plc programming ladder diagram