Internal flag bit M: The "internal flag bit" in Siemens PLC is marked with the English letter M, which is equivalent to the "internal relay" often said in other companies' PLCS, which is used to store the intermediate calculation results of PLC programs. In the PLC program, the content of the internal flag bit can be updated and changed continuously with the execution of the program.
② Special internal flag bit SM: PLC internal special flag bit is marked with the English letter SM, the difference between it and the internal flag bit is that the special flag bit 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 bit SM is divided into two parts: "read-only" and "read-write". The "read-only" signal is automatically generated by the PLC and can only be used in the program, and cannot be "written" (or output).
③ Accumulator 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 as bytes, words, or double-words, with AC0 to AC3 as absolute addresses. When accessing the accumulator in bytes (or words), the low 8 or low 16 bits of the accumulator are used.
Sequence control relay S (S7-200 only) : In S7-200 PLC, you can use the sequence control relay (SCR), which is marked with the English letter S, for organizing the PLC program operation or entering the corresponding program step.
⑤ Data block DB/DI (only S7-300/400) : In the S7-300/400 series PLC, you can use data block DB to store the processing results or data of the program, and the data block can be specified in the decimal form under the address of the DB (or DI).
