Steve Bailey
Lifetime Supporting Member + Moderator
Today I was called in to a client to diagnose and fix a problem. Without getting into too much detail, I found that in one rung of the PLC program a normally open contact had been changed to normally closed. That was all. No address changes, no changes in timer presets or anything else. The machine was running fine before they shut it down for the night but when they started it up in the morning they had this problem.
This is a small outfit and there is nobody there that knows anything about PLCs. All they know is that when they have a problem with this machine that can't be explained by a blown fuse, a bent limit switch, or a broken wire, they call me. I can't picture any of this company's employees making the change. Their copy of the PLC programming software is on the same computer that runs their HMI software. The HMI software uses the only available serial port on the PC. That means that in order to use the PLC programming software to make any changes to the PLC program you must first shut down the HMI application before going online with the PLC programming software.
Furthermore, the stored version of the PLC on the computer was correct. That means that whoever changed it used a different computer or else went online to change the program in the PLC and then went offline and changed the PC's copy back to the way it was originally. It implies a better knowledge of how to work with the PLC than I'm prepared to attribute to any of the employees.
In the course of my career I've run into cases where a PLC program has become corrupted due to electrical spikes or a failed battery, but whenever that's happened the PLC doesn't keep running. I've never heard of a spontaneous change to a PLC program that resulted in a valid program.
I think I may entered the Twilight Zone.
This is a small outfit and there is nobody there that knows anything about PLCs. All they know is that when they have a problem with this machine that can't be explained by a blown fuse, a bent limit switch, or a broken wire, they call me. I can't picture any of this company's employees making the change. Their copy of the PLC programming software is on the same computer that runs their HMI software. The HMI software uses the only available serial port on the PC. That means that in order to use the PLC programming software to make any changes to the PLC program you must first shut down the HMI application before going online with the PLC programming software.
Furthermore, the stored version of the PLC on the computer was correct. That means that whoever changed it used a different computer or else went online to change the program in the PLC and then went offline and changed the PC's copy back to the way it was originally. It implies a better knowledge of how to work with the PLC than I'm prepared to attribute to any of the employees.
In the course of my career I've run into cases where a PLC program has become corrupted due to electrical spikes or a failed battery, but whenever that's happened the PLC doesn't keep running. I've never heard of a spontaneous change to a PLC program that resulted in a valid program.
I think I may entered the Twilight Zone.