Payload
Learn to build multiple payloads for your Cyber Weapon. Your main payload would attack the PLCs to disrupt it's default operation, while another payload performs a Man-in-the-Middle attack to fake the inputs coming from the PLC's I/O modules and tell it's output to do what it wants, not what the stock hard-coded logic says.
At the same time, program the payload to replay previously recorded input data to make the PLC logic and the operator think that all is well.