Soft PLC cold start, warm start, hot start - Solutions - Huaqiang Electronic Network

Soft PLC startup methods are typically categorized into three types: cold start, warm start, and hot start. While the official documentation provides some guidance, it's often translated directly without much explanation. This can lead to confusion for users who are not familiar with the exact differences between these modes.

In general:

• Cold Start / Cold Initialization: All data is reset, including retain variables. This is like starting from scratch.
• Warm Start / Warm Initialization: Only non-retain data is initialized. Retain variables keep their previous values.
• Hot Start / Hot Start: No initialization occurs. The program runs as it was, preserving all variable states.

Although these concepts may seem straightforward to experienced PLC engineers, it's important to understand them clearly, especially when working with real-time systems or critical applications. To make it more tangible, let’s look at a simple example.

Imagine a basic PLC program that controls three lights using three variables. Initially, all the lights are off. One of the variables is "retained," meaning its value persists even after a restart. The program turns on all three lights during operation.

Now, suppose you want to add control for one additional light. After updating the program and downloading it via the soft PLC’s remote download feature, you need to restart the system. Depending on the startup mode, the behavior of the original three lights will vary:

• Cold Start: All variables are reset. The retained variable keeps its value, but the others are cleared. So, only the "privileged" light might stay on, while the others turn off.
• Warm Start: Non-retained variables are reset, but the retained ones remain. Again, the privileged light may stay on, while the rest are turned off.
• Hot Start: No variables are reset. All lights continue in their previous state, as if nothing changed.

This example illustrates how different startup modes can affect the behavior of your system. Understanding these differences helps ensure smooth transitions and avoids unexpected results during program updates or system reboots.