xLadder user manual-V2.0.0.6

5.4 Programming concepts

5.4.1 How the program works

The program is run by the loop, PLC reads and writes data continuously.When you

download the program to PLC and make PLC in the run mode, the PLC' s central

processing unit (CPU) executes the program in the following order:

A:PLC read input status.

B:The PLC program uses input values for logic control.

C:When the program is running and writes the results to the output image register.

D:At the end of the program, the output value of the output image register.

E: repeat the above steps.

PLC performs a series of tasks repeatedly. The cycle execution task is called the scan

cycle.PLC performs most or all of the following tasks during the scan cycle:

A: PLC read input status.

B:PLC executes the program's instructions, and stores the data in different memory

areas.

C: performs all communication requests.

D:PLC performs CPU self test diagnostic program.PLC ensure that the hardware,

program memory and all expansion modules are normal operation.

E:The values stored in the output image register are written to the actual output.

Attention: the execution of the scan cycle depends on the PLC that is set in the STOP

(stop) mode or the RUN (run) mode. In the RUN (run) mode, the program is executed;

in the STOP (stop) mode, the program is not executed.

5.4.2 Addressing overview

Identifying absolute and symbolic address

You can use absolute or symbol to identify the instructions in the program.Absolute

reference use memory area and bit or byte location to identify the address.symbolic

reference uses letters, numbers, and characters to identify addresses or values.

How to display the address of the program editor:

The absolute address is made up of the memory area and the number

I0.0

54

Made with FlippingBook - Online magazine maker