PLC essence is one kind special at industrial pilot computer, its hardware structure basically as identical as the microcomputer, 1, central processing is unit (the control center that CPU) is PLC. The function that it gifts according to PLC system program is received and store from process designing implement the user program that key in and data; Check the status of power source, memory, I/O and cautionary timer, can diagnose the solecism in user program. When PLC investment moves, above all the status that it receives the spot to input device each with the means of scanning and data, stock I/O image area respectively, read one by one from inside user program memory next take user course, the regulation that dictates through commanding to be pressed after the explanation carries out logic or the result of count operation sends I/O image area or data register inside. Wait for all user programs to carry out after ending, the number inside the each output condition I/O image area or output register arrives according to conveying finally corresponding take-off, such loop moves, until stop to move. To raise PLC further but * sex, still use double CPU to make redundant system to large PLC in recent years, or use 3 CPU vote type system. Such, although a certain CPU appears breakdown, whole system still can move normally. 2, the memory that memory deposits systematic software calls systematic program memory. The memory that deposits application software calls user program memory. The memory type with commonly used PLC (1) RAM (RandomAssessMemory) this is read one kind / keep memory (random memory) , its access speed the fastest, by lithium battery support. (2) EPROM (ErasableProgrammableReadOnlyMemory) this is a kind but of erasure read-only memory. In cut off the power below the circumstance, all content inside memory keep changeless. (In ultraviolet ray successive illuminate falls but) of erasure memory content. (3) EEPROM(ElectricalErasableProgrammableReadOnlyMemory) this is a kind of report but of erasure read-only memory. Use process designing implement the content that stores easily to its undertakes modification. The allocation of PLC storage space although the largest space finding site of the CPU of all sorts of PLC each are not identical, but the working principle according to PLC, its store the space includes commonly the following 3 area: (1) systematic program memery block (2) systematic RAM memery block (include I/O image area and systematic software to wait) (3) memery block of program of system of user program memery block: The systematic program that is equivalent to computer operating system is being deposited in systematic program memery block. Include program of monitoring program, government, command to explain subprogram of program, function, system diagnoses a subprogram to wait. its by mill business solidify is in EPROM, the user cannot be accessed directly. It and hardware decided the function of this PLC together. Systematic RAM memery block: Systematic RAM memery block includes I/O image area and of all kinds software, be like: Logistic coil; Data register; Clock; Tally; Change location register; The memory such as accumulator. (1) I/O image area: After as a result of PLC investment moves, just inputting sampling phase ability is ordinal read in each to input condition and data, the condition that in output refresh phase just will output and data send to corresponding peripheral. Accordingly, it needs a certain quantity of location (the condition with depositing I/O mixes RAM) data, these unit call I/O image the area. I/O of quantity of a switch takes up one of location (Bit) , i/O of an imitate quantity takes up a word in location (16 Bit) . Because area of this whole I/O image can regard two parts to comprise: Switch measures I/O image area; Imitate measures I/O image area. (2) systematic software memery block: Besides I/O image trifling, systematic RAM memery block still includes PLC interior of all kinds software (register of logistic coil, clock, tally, data and accumulator) memery block. This memery block is divided again break phone to have retentive memory area and without break phone retentive memory area, former when PLC cuts off the power, by in-house lithium battery power supply, data won't lose; Latter cuts off the power when PLC when, data by to clear. 1) logistic coil and switch output are same, every logistic coil takes up one of systematic RAM memery block, but cannot direct drive peripheral, use in process designing for the user only, its action is similar to electric equipment to control the relay in circuit. Additional, different PLC still offers the special logic coil that the amount differs, have different function. 2) data register and imitate quantity I/O are same, every data register takes up a word in systematic RAM memery block (16bits) . Additional, PLC still offers the special data register that the amount differs, have different function. 3) clock 4) the user program that memery block of program of user of memery block of tally user program deposits an user to weave. The PLC of different type, its memory capacity each are not identical. 3, the power source of PLC of power source is having effect of very heavy good in whole system. If neither one is good, but * gets power source system cannot work normally, because the manufacturer of this PLC is right,make of power source also take seriously very. Wave motion of general alternating voltage is in + inside 10%(+15%) limits, need not take other step and receive PLC alternating current net to go up repeatedly directly CNC MillingCNC Machining