The HMI software resides on a PCMCIA Flash card (MMC) behind the operator panel. Corruption here can cause "Waiting for NCK" even if the NCK is fine.
If the NCK 7-segment shows a stable "6" but the HMI still waits: sinumerik 810d waiting for nck connection
She said nothing. She was staring at the Tadiran battery she had removed from the NCK module. Written on it in black marker, in her own handwriting, was a date: "Replace by: March 2018." The HMI software resides on a PCMCIA Flash