hardware firmware

can anyone explain what is hardware firmware.We are using it in our siemens plc and they call it dongle.how does it work?

Hardware is metal, plastic, motors, coils, chips, etc. Software is code that you can read/write. PLC program, programs stored on hard drive, etc. Firmware is software that is stored on an EPROM, EEPROM, or some other device that you typically would not reprogram.
A dongle has a preprogrammed Microprocessor. It is set up in such a way as to make it impossible to 'crack'. Some on the older ones were easily copied by hackers, but the new ones have the software embedded to a point where it is impossible to access. The Microprocessor is hardware, the embedded code is firmware (software that you can't change).
A Dongle connects to the parallel port, USB port, etc. The program reads it to get a serial number that must match a code in your program. The software won't run unless the Dongle makes it happy.

firmware

dear keithkyll
thanks for information.Now i am thinking , if during running system if something happens to the dongle then the system will collapse and we will have to call siemens.But if siemens expert is not availabe at that time we will unnecessary stop our plant.

But if siemens expert is not availabe at that time we will unnecessary stop our plant.

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No way! You'll only be unable to program or monitor the PLC. The PLC itself will continue to work. You don't need the programming device to let the PLC do its job. So, you won't need the programming software nor the dongle neither.

Kind regards,