Internal flags and registers of Siemens PLC

① Internal flag bit M: The "internal flag bit" in Siemens PLC is marked with the English letter M. Its function is equivalent to the "internal relay" commonly mentioned in PLCs of other companies. It is used to store the intermediate calculation results of the PLC program. In the PLC program, the content of the internal flag bit can be continuously updated and changed during the execution of the program.

② Special internal flag SM: The special flag inside the PLC is marked with the English letters SM. The difference between it and the internal flag is that the special flag SM has a special meaning. It can reflect the working status and control information inside the PLC and can be used by the PLC user program.

The special flag SM is divided into two parts: "read-only" and "readable and writable". The "read-only" signal is automatically generated by the PLC, and only the result can be used in the program, and the "write" (or output) operation cannot be performed.

③Accumulator AC: S7.200 and S7-400 have four 32-bit accumulators ACCU0 to ACCU3: S7-300 has two 32-bit accumulators ACCU0 and ACCU1. The contents of the accumulator can also only be processed in the form of bytes, words or double words, and AC0 to AC3 are used as absolute addresses. When accessing the accumulator in bytes (or words), the lower 8 bits or lower 16 bits of the accumulator are used.

④ Sequential control relay S (S7-200 only): In S7-200 PLC, you can use the sequential control relay (SCR), which is marked with the English letter S and is used to organize PLC program operations or enter the corresponding program steps.

⑤Data block DB/DI (S7-300/400 only): In S7-300/400 series PLC, data block DB can be used to store program processing results or data. The data block can be specified in decimal form under the address of DB (or DI).

The content of the data block can also be used in the form of binary bits, bytes, words, and double words. For binary bits, the address is specified in the form of DBX+.+ (or DIX*.*); for bytes, words, and double words in the data block, the address is specified in the form of DBB**, DBW**, DBD** (DIB¨+, DIW**, DID***).