DVP-ES2/EX2/SS2/SA2/SX2/SE&TP Operation Manual - Programming 1.1 PLC Scan Method PLC utilizes a standard scan method when evaluating user program. Scanning process: Input signal Input X 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. Output Output Y Scan time 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. Scan time exception PLC can process certain items faster than the scan time. Some of these items interrupts and halt the scan time to process the interrupt subroutine program. A direct I/O refresh instruction REF allows the PLC to access I/O immediately during user program evaluation instead of waiting until the next scan cycle.
1-2 Scan input status Read the physical input status and store the data in internal memory. Evaluate user program Evaluate the user program with data stored in internal memory. 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 terminal Store to m Input terminal Store to m Input signal memory am Read X0 status fr 0 Write Y0 state into Device Y0 Read Y0 state from memory Memory 0 M0 Write M0 state into Output Output latched memory Output terminal Device Memory Output latched memory Output terminal Read scan time 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 register D1012. Measure scan time Scan time can also be measured by toggling an output every scan and then measuring the pulse width on the output being toggled.
Calculate scan time Scan time can be calculated by adding the known time required for each instruction in the user program. For scan time information of individual instruction please refer to Ch3 in this manual.