Mitsubishi PLC timing analysis - Solutions - Huaqiang Electronic Network

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The timers in the Mitsubishi FX series PLC are categorized into two types: general-purpose timers that do not retain their state after power loss, and retentive timers that maintain their count even when power is turned off. These timers can operate with different clock pulse intervals, such as 1ms, 10ms, or 100ms. The basic operation of a timer involves starting the timing process once the input condition is met and the coil is activated. Once the set time has elapsed, the corresponding contact changes state.

PLCs function based on a cyclic scan mode, which typically consists of five stages: internal processing, communication services, input refresh, program execution, and output refresh. These stages are executed sequentially during each scan cycle while the PLC is in RUN mode. This scanning method can affect the accuracy of the timer, especially in real-time applications. The precision of the timer depends on how the program is structured, the timing signal it receives, and the exact moment when the timer begins its counting process.

Let’s analyze the timer’s behavior in a worst-case scenario. Suppose the input to the timer 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 scan cycle and will instead wait until the next one. This results in a delay of one full scan cycle. Another situation arises when the timer completes its timing after the coil instruction has already been executed—such as after an OUT instruction. In this case, the timer may not trigger the expected contact action, leading to potential inaccuracies in timing control.

Understanding these limitations is crucial when designing PLC programs that require precise timing. Engineers should account for the scan cycle time and the timing conditions to ensure that the system performs as intended. In some cases, using high-speed timers or adjusting the program logic can help reduce the impact of these delays and improve overall performance.