In this lesson, we are going to familiarize you with a process called a watchdog. In all computers, including PLC’s, there is a background process that ensures that the logic solve or processing of all of the needed functions occurs in a timely manner. We describe exactly how this background process functions and what happens if the watchdog determines that the logic solve is taking longer than is expected. We are also going to talk about scan cycle times. While connected, you can take advantage of one of the PLC’s diagnostics to determine how long it takes for the CPU to process your code from the beginning to the end. Let’s jump in a discuss watchdogs.
In a previous lesson we explained the purpose of the watchdog timer. In this lesson, we will expand that lesson a little further and demonstrate the function of the watchdog timer. We are going to modify a previous program that contains a properly executed jump instruction such that it creates a ...
In the previous lesson, we had a program that contained a comparator instruction and it properly executed our jump instruction. We then changed that instruction to a field input. When we forced that input to true in the simulator, we managed to put our program in an infinite loop, causing a Stop ...
In some cases you may need to flash a bit in your PLC program. Let’s say that you have an HMI that is not capable of flashing an alarm notification. In that case, you may use the PLC program and one of a number of ways to flash a bit. You can then use that bit on the HMI to indicate a flashing al...
