Many small partners are confused about the difference between relay interlock control and PLC control, as they share several similarities. Today, we’ll break down the key differences and explore how these two systems operate.
If you compare a relay-based interlock control circuit with a PLC-based one, you'll notice that they function in fundamentally different ways.
Figure 1 illustrates a traditional relay interlock control system. In this setup, if there are multiple control loops, pressing any start or stop button will immediately trigger the corresponding contactor to either close or open. For simplicity, we're ignoring the slight delay caused by the contactor's operation. In essence, this system operates in parallel—each input is processed instantly, similar to how a computer handles interrupts, where any input action is responded to immediately.
Now let’s look at a PLC control system, as shown in Figure 2. This system runs a program like Program 1. The process works step by step: it first reads the inputs, latches them, then scans the program from top to bottom and left to right, latches the output results, and finally updates the outputs. This is done in a sequential manner, controlled by a clock signal. So, the PLC continuously cycles through the process of input, processing, and output. While the scan speed is extremely fast, it's still sequential, and most users don’t perceive the delay. However, in some cases, such as high-speed counting, the PLC may not capture all input signals accurately unless it uses a specialized high-speed counter interface.
It's important to understand that while both systems have their own advantages, they operate in completely different ways. Not all input signals can be reliably captured by a standard PLC. For example, high-frequency signals like those from a high-speed counter require specific hardware interfaces. However, for regular control signals, such as start/stop buttons, a standard PLC is more than capable. Take the FX2 PLC, for instance. If the program is around 1K in size and the start button is from a brand like IDEC, even a very short press is detected and processed correctly. This shows that despite the serial nature of PLC processing, its fast CPU speeds ensure that normal control operations work seamlessly. Only in special cases, such as high-speed counting, do you need to use dedicated hardware or instructions. For everyday applications, the PLC is fully reliable and efficient.
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