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The programming soft components of the PLC are essentially memory units, each with a unique address.
To facilitate different programming functions, the memory units are partitioned, resulting in various types of programming components. Soft components are devices within the PLC that have certain functions, composed of electronic circuits, registers, and memory units; they are virtual components within the PLC, invisible and intangible; they are the components required for executing PLC programs.
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There are 13 symbols for all soft components of the Siemens S7-200 SMART PLC:
I (Input Relay), Q (Output Relay), M (Auxiliary Relay), SM (Special Auxiliary Relay), V (Variable Memory), L (Local Variable Memory), S (Sequential Control Relay), T (Timer), C (Counter), HC (), AI (Analog Input Image Register), AQ (Analog Output Image Register), AC (Accumulator)
1. Soft Component I
The input relay, also known as the input image register. On the PLC’s input unit, each digital input point corresponds to a bit of the input image register, primarily used to receive external switch signals.
When the external switch signal of the PLC is closed, the coil of the input relay is energized, and its normally open contact closes while the normally closed contact opens in the program. The input relay contacts can be used multiple times in the program.
2. Soft Component Q
The output relay, also known as the output image register. On the PLC’s output unit, each digital output point corresponds to a bit of the output image register, primarily used to drive external loads.
An output relay has only one normally open contact (also known as a hard contact) connected to the output terminal, while there can be many internal normally open and normally closed contacts.
3. Soft Component M
In PLC program design, soft intermediate relays are often needed, mainly serving as transitional or auxiliary functions. Its function is the same as that of intermediate relays in traditional relay control circuits.
The address range for intermediate relays provided by S7-200 is: M0.0~M31.7, totaling 256, divided into two categories:
1. Ordinary intermediate relays (non-retentive) (M0.0~M13.7), which turn OFF after the PLC power is interrupted. When power is restored, the program turns it ON, while others remain OFF.
2. Retentive intermediate relays (M14.0~M31.7) can retain the instantaneous state before power interruption and continue with that state after power restoration.
Intermediate relay coils are driven by program instructions, and each intermediate relay has an unlimited number of normally open and normally closed contacts available for programming use.
4. Soft Component SM
It provides a method for passing information between the CPU and user programs and can be used to store system state variables, control parameters, and other information. Users can use these bits to select and control some special functions of the CPU.
SM0.0 is always “1” or in the ON state when the PLC is in RUN state.
SM0.1 is the initialization pulse. When the PLC starts running, SM0.1 activates for one scan cycle.
SM0.5 provides a clock pulse, which is OFF (disconnected) for 0.5 seconds and ON (connected) for 0.5 seconds within a one-second cycle.
5. Soft Component V
The variable memory is used to store variables. It can hold intermediate results of control logic operations during program execution, and variable memory can also be used to save other data related to processes or tasks.
6. Soft Component L
The local variable memory is used to store local variables. Local variables are very similar to global variables stored in variable memory, with the main difference being that global variables are globally valid, while local variables are locally valid.
7. Soft Component S
The status relay, also known as the sequential control relay, is an important device for compiling sequential control programs. It is usually used with sequential control instructions to achieve sequential control functions.
8. Soft Component HC
The high-speed counter. The counting frequency of ordinary counters is affected by the scan cycle and cannot be too high, while high-speed counters can accumulate events faster than the CPU’s scan cycle. High-speed counters are generally used in motion control fields!
The Siemens S7-200 Smart series PLC can have up to 6 high-speed counters, numbered from HSC0 to HSC5, where “HSC” is the identifier for high-speed counters.
9. Soft Components AI and AQ
The analog input image register and the analog output image register. The CPU of the Siemens S7-200 Smart series PLC allocates a storage area for analog signal input/output. The addresses for the analog input image register and the analog output image register of the Siemens S7-200 Smart series PLC are AIW0 to AIW110; AQW0 to AQW110, totaling 56 words, and the addresses can only be addressed in word format, and the word addresses can only be even numbers.
AIW0, AIW2, AIW4, AIW6, AIW8, …… , AIW110
AQW0, AQW2, AQW4, AQW6, AQW8, …… , AQW110
Note: The analog input values are read-only data, while the analog output values are write-only data!
10. Soft Component AC
The accumulator is a register used to temporarily store data. It can hold operational data, intermediate data, and results! The Siemens S7-200 Smart series PLC provides 4 accumulators, each 32 bits long, with address ranges of AC0 to AC3, which are read-write units.
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