The PLC systems use microcomputer technology to achieve the purpose of simulating traditional relay control panel.
The microcomputer rst reads the external inputs and then sequentially scans and executes the user program, so as to
calculate the control result that the user wants to achieve. Finally, it outputs the result to drive the external loads and
performs the real control action. The PLC execution order is shown in Fig. 1-2.
The PLC will sequentially execute the cycle ( ① receive external inputs; ② process the user program; ③ output the
computed results). Then, the time spent for one cycle is called a “Scan Time”.
Fig. 1-2 PLC operation order and scan time
Loads the status (ON/OFF) form external input components via
the input interface and stores that into the data memory.
One cycle of
“Scan Time”
The CPU uses the status in the data memory and the rule of user
program to compute the results, then stores all the results into the
data memory.
User program
Transmits the operation result from the data memory to the
output part, via the output interface to control external loads.
The PLC processes user program by the Sequential Scan (in the Ladder Diagram scans from left to right then from top
to bottom), as shown in Fig. 1-3. Therefore, must pay extra attention to the procedural order of the program.
Fig. 1-3 PLC makes the sequential scan at the Ladder Diagram
The “Scan Time” and “Sequential Scan” are the basic and the most important concepts. A PLC user must fully
understand those meanings, and pay attention about the inuences when designing the PLC program.
Y0
Y25
X0
Y0
Y20
M0
M100
M105
X1
˙˙˙˙˙˙˙˙
3
1-1-3 PLC Operation and Scan Time
① To receive the
external inputs
② To process the
user program
③ To output the
computed results
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