The Omron CV1000-DM151 is a base unit (backplane) designed for use with CV-series programmable logic controllers (PLCs). It serves as the main rack, providing a total of five I/O slots for accommodating various input, output, and special function modules. This backplane ensures stable communication between the CPU and connected modules through the system bus.

In the input sampling phase of a typical PLC scan cycle, the controller sequentially reads the status of all digital inputs and the data from analog or special function modules. This process is executed in a systematic scanning manner, and the acquired information is immediately stored in the corresponding locations within the I/O image memory area. This memory area is a dedicated portion of the CPU’s data memory that holds a snapshot of the physical inputs at the moment of sampling.

Once the input sampling phase is complete, the PLC proceeds to execute the user program. During program execution, the CPU reads the input statuses from the I/O image memory rather than directly from the physical input terminals. This approach ensures consistency and stability of the logic evaluation, as the input data remains unchanged throughout the program execution, regardless of any real-world changes in input signals.

After the program execution phase, the PLC enters the output refresh stage. In this stage, the calculated output values stored in the I/O image memory are written to the physical output modules simultaneously. This process updates the actual output devices, such as relays, solenoids, or indicator lights, based on the results of the logic program.

It is important to note that during both the program execution and output refresh stages, any changes in the physical input status or data are ignored. The I/O image memory remains static throughout these phases, only being updated again at the beginning of the next scan cycle during the input sampling phase. This design principle is fundamental to the deterministic behavior of the PLC.

Because of this scanning mechanism, special attention is required when handling pulse signals as inputs. For a pulse signal to be reliably detected by the PLC, its duration must be longer than the entire scan cycle time. If the pulse width is shorter than the scan period, there is a risk that the PLC may miss the signal entirely, as the input sampling may occur before or after the pulse has occurred. Therefore, to guarantee that the input is captured under all operating conditions, the pulse width must exceed the maximum possible scan time of the PLC. This requirement is critical for applications involving fast transient signals, such as encoder pulses, limit switch triggers, or momentary push-button inputs.