The first thing the PLC will do is to check the status of all the inputs. When the PLC is powered up it will run as follows: Your PLC program will decide what outputs to set with the inputs as decisions. The program of logic that we save into the PLC is the main function of the PLC. The scan time is how the PLC program works So, the EEPROM is in most cases the optimal place for the program. The data in the EEPROM will still be there even after the power has been removed.
Most PLC’s have a backup battery, that prevents the RAM from being deleted, but there is a better place to save your program more permanently – the EEPROM. It can be risky because the RAM will be cleared when the power is off. Normally the program is saved in the RAM of the PLC, but as we know from the previous chapter the RAM can be a risky place to save your program. The program that reads the inputs and sets the outputs.īefore learning more about the program of logic we need to know is where in the PLC the program is placed. It is the program of logic that we put inside the PLC. The other piece of software is actually the software that we develop for the PLC. The firmware is installed by the manufacturer of the PLC. Many other tasks are handled by the firmware, and you will almost never have to worry about it. Some of the functions of the firmware are to make the microprocessor and the RAM communicate and to make the PLC compatible with the PLC programming software.
The firmware is responsible for running your program commands, managing communications, and some other tasks. The firmware is the operating system in the PLC. A little piece of software called the firmware and another piece of software called the program. PLC Firmwareįirst, let us divide the software inside the PLC into two parts. Some of them are pre-installed, and some of them will be your software. The accuracy of the timers will often depend on the length of the scan cycle.īesides the PLC hardware, the PLC contains some software. Especially if you are working with plc timers and delays. It is crucial to know about the PLC scan time and even to know how long the scan cycle is. The amount of time it takes for the PLC to make one scan cycle is called the scan time of the PLC. This will take some amount of time often measured in milliseconds (ms). The scan cycle is the cycle in which the PLC gathers the inputs, runs your PLC program, and then updates the outputs.
This is how the PLC and the software inside the PLC works. Now, that we know what hardware is inside the PLC, the next step will be to take a look at the software inside the PLC and how it runs.Įvery PLC has a scan time and a scan cycle.
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