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"The PLC is fixed hardware, you can't simply patch that."

Oh yes you can patch it - that's how I got the 20 year old laminator working where I work. If the hardware is still good you can absolutely change firmware if you have the PLC control software (like RockWell ControlFlash) or know how to manipulate the ladder logic directly.

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You can change firmware aka how it behaves but you can't change it's capabilities.

Sure you can. Code is code. I took a machine without brush cleaning for transfer sheets and hand-tapped, mounted motors and controllers and brush assemblies, ran the connections into the PLC, install and connect the power relays, reflashed the PLC firmware with custom code, BAM it works.

As long as you have room in memory, you can add functionality.

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