Mitsubishi PLC timing analysis - Solutions - Huaqiang Electronic Network

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The timers in the Mitsubishi FX series PLC are categorized into two main types: general-purpose timers, which do not retain their count when power is lost, and retentive timers, which maintain their accumulated value even after a power interruption. These timers can operate with different time bases, such as 1ms, 10ms, or 100ms, depending on the application requirements. In terms of operation, when the timer’s condition is met and its coil is activated, the timer begins to count the clock pulses. Once the preset time is reached, the corresponding contact changes state. Since the PLC runs in a cyclic scan mode, each cycle consists of five stages: internal processing, communication services, input refresh, program execution, and output refresh. These stages are executed sequentially, which may affect the timing accuracy of the PLC. The precision of the timer depends on how the program is structured, the timing signal (the condition that triggers the timer), and the exact moment when the timer starts counting. Below is an analysis of the timer’s behavior under extreme conditions. 1.1 The Timer Contact Acts After the Coil Suppose the timer’s input (as shown in Figure 1) turns ON just after the PLC has completed the input refresh phase. Since the PLC only scans inputs once per cycle, the timer will not activate during the current program execution phase. It will have to wait until the next scan cycle, introducing a delay of one full scan cycle. This delay can impact timing accuracy in applications requiring high precision. Additionally, if the timer completes exactly after the ladder logic coil instruction has been executed—such as after the OUT instruction—the timing may not be accurate due to the sequential nature of the scan process. Understanding these limitations helps in designing more reliable and precise control systems using the Mitsubishi FX series PLCs.