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 mentions these terms, they are often translated directly without much elaboration. Here’s a clearer breakdown:

Cold Start / Cold Initialization Resets all data, including retain variables. Warm Start / Warm Initialization Initializes only non-retain data, keeping retain variables intact. Hot Start / Hot Start Runs the program without initializing any data, preserving the current state.

Although these concepts may seem straightforward for experienced PLC engineers, it's important to understand them clearly, as even small details can have big impacts. To make this more tangible, let’s take a simple example.

Imagine a basic PLC program that controls three lights using three variables. Initially, all lights are off. One of them is "privileged" because its controlling variable is of the retain type. The program turns on all three lights during normal operation.

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

For instance, if we perform a cold start, all retain variables are reset, so the originally privileged light might turn off. A warm start would preserve the retain values, keeping that light on, while the others may be reset. A hot start would leave everything in its previous state, maintaining the same behavior as before the update.

This example shows how different startup modes can affect the system’s behavior, especially when retain variables are involved. Understanding these differences is crucial for ensuring smooth operation after program updates or system restarts.