The storage areas in Siemens PLC are classified into I, Q, M, SM, T, C, V, S, L, AI, AQ
1. I storage area (input image register)
It is an input contact that can receive an external level signal
It is an external input, and all kinds of buttons, switches, sensors, etc. that send signals to the PLC are called inputs.
S7-200 PLC addressing range (I0.0~I15.7). The input image register (this area can be operated bit by bit is also called the input relay). The input relay coil is driven by an external signal, and the normally open and normally closed contacts are for user programming.
2. Q storage area (output image register)
is the output contact, used to provide level signal,
It is the output terminal, and other components controlled by the PLC output signal are all connected to Q, such as indicator lights, relays, etc.
S7-200 PLC addressing range (Q0.0~Q15.7). The output image register (also known as output relay) is used to transmit the output signal of PLC to the load, and the coil is driven by program instructions. Every I/O point of PLC is a definite physical point. The CPU 224 host has 14 digital input terminals of I0.0～I0.7, I1.0～I1.5, and 10 digital output terminals of Q0.0～Q0.7, Q1.0, Q1.1.
3. Internal flag bit (M) storage area
M is used as a control relay (also known as intermediate relay, general auxiliary relay ) to store intermediate operands or other control information.
It is an auxiliary relay inside the PLC, which only exists inside the PLC, and its main function is to connect each program segment internally
S7-200 PLC addressing range