When a PLC executes a program, it must know, in real time, when external devices that control a process are changing. During each operating cycle, the processor reads all the inputs, takes these values and energizes or de-energizes the outputs according to the user’s program. This process is known as program scan.
The process scan will occur as long as the PLC is in RUN mode because the input can change at any time.
The process time is the time to complete a process scan and indicates how fast the controller is. The time required to make a single scan can vary from about 1 millisecond to 20 milliseconds.
The actual scan time is calculated and stored in the PLC’s memory. The PLC computes the scan time each time the END instruction is executed.Scan time data can be monitored via the PLC programming. Typical scan time data include the maximum scan time and the last scan time.
The scan time is a function of the following:
- The speed of the processor module
- The length of the ladder program
- The type of instructions executed
- The actual ladder true/false conditions
Process scan execution:
The processor completes the process scan in three steps:
Step1: Update the input image table by sensing the voltage of the input terminals. Based on the absence or presence of a voltage, a 0 or a 1 is stored into the memory bit location designated for a particular input terminal.
Step 2: Solve the ladder logic in order to determine logical continuity. The processor scans the ladder program and evaluates the logical continuity of each rung by referring to the input image. If the condition for output are met, processor writes logic 1 and Logic 0 if the conditions are not met.
Step 3: The fi nal step of the scan process is to update the actual states of the output devices by transferring the output table results to the output module, thereby switching the connected output devices ON (1) or OFF (0).
Example for process scan for a rung is shown below:
Examine the status of the input image table bits.
Solve the ladder logic in order to determine logical continuity
Update the appropriate output image table bits, if necessary.
Copy the output image table status to all of the output terminals. Power is applied to the output device if the output image table bit has been previously set to a 1.
Copy the status of all of the input terminals to the input image table. If an input is active (i.e., there is electrical continuity), the corresponding bit in the input image table will be set to a 1.
This is done for all the rungs in a program.
Also read An introduction to PLC timer’s