Delta DVP-ES2/EX2/SS2/SA2/SX2/SE manual PLC Scan Method

DVP-ES2/EX2/SS2/SA2/SX2/SE Operation Manual - Programming

1.1PLC Scan Method

PLC utilizes a standard scan method when evaluating user program.

Scanning process:

Scan input status	Read the physical input status and store the data in internal
	memory.
	Evaluate the user program with data stored in internal memory.
Evaluate user program	Program scanning starts from up to down and left to right until
	reaching the end of the program.
Refresh the outputs	Write the evaluated data to the physical outputs

Input signal				Input X
		Input terminal
					Store to memory
Input signal memory
			Read X0 status from memory
Program
					Write Y0 state into
	X0						Device
					Y0
	Y0			Read Y0 state from memory			Memory
					M0
					Write M0 state into
						Output
Output					Output latched memory

Output terminal

Output Y

Scan time

Input signal:

PLC reads the ON/OFF status of each input and stores the status into memory before evaluating the user program.

Once the external input status is stored into internal memory, any change at the external inputs will not be updated until next scan cycle starts.

Program:

PLC executes instructions in user program from top to down and left to right then stores the evaluated data into internal memory. Some of this memory is latched.

Output:

When END command is reached the program evaluation is complete. The output memory is transferred to the external physical outputs.

The duration of the full scan cycle (read, evaluate, write) is called “scan time.” With more I/O or longer program, scan time becomes longer.

Read	PLC measures its own scan time and stores the value (0.1ms) in register
	D1010, minimum scan time in register D1011, and maximum scan time in
scan time
	register D1012.
Measure	Scan time can also be measured by toggling an output every scan and then
scan time	measuring the pulse width on the output being toggled.
Calculate	Scan time can be calculated by adding the known time required for each
	instruction in the user program. For scan time information of individual
scan time	instruction please refer to Ch3 in this manual.

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