She’d found it. An edge case. At 11:45 PM, an operator had toggled the filler to manual mode just as a batch completed. The script tried to write to a null object. The HMI didn’t just ignore it—the unhandled exception corrupted the local tag memory, which sent a malformed "stop" telegram to the PLC’s communication processor. The PLC, confused and safety-conscious, halted.
“Okay, Carl,” she said, clicking into the Project tree . “Walk me to the cabinet. What do the diagnostics LEDs on the PLC say?” plc and hmi development with siemens tia portal read online
She was right.
Line 47. She opened the HMI script editor. There it was: a line of VB that wrote a timestamp to a custom property on a graphic IO field. A field that was conditionally visible. If the filler was in manual mode, the field didn’t exist. And if the field didn’t exist… She’d found it