We get a call at least once a week asking about “UL error” and why it isn’t in the manual. There are only 2 problems with those questions:
1) It isn’t an error, and
2) It’s in the manual (under Operational Status).
What does UL mean?
UL is about the same as Microsoft’s last warning “Format Hard Drive C: Yes/No”. It’s a chance to back up and decide if this is REALLY what you wanted to do. It’s the drive saying “I think my humans did something foolish, and I want them to tell me that that they are sure that’s what they want me to do”.
The cause of UL is simple. The drive powered down with one type of motor attached, and it powered up with a different type. When the drive powers up, after it passes the power up self-test, it goes out and asks the motor just what kind of motor it is.
If it gets an invalid answer (either from cable failure or from feedback failure on the motor) it throws a C204 “Motor Type Incorrect” error. If it gets a valid answer, it then compares that answer to the one in its memory, and will halt on UL if the two don’t match.
Your machinery taking care of you
This happens for two reasons. First, the drive wants to tell you that if you have replaced the motor, you need to check it, because it’s not the same type as the one you removed. This is a good final check on the guy who replaced the motor; he may have grabbed the wrong one.
It’s also a good check on the motor being from a third party repair house. We have seen MKD041’s that swore to the drive they were MKD112’s.
Second, the drive wants your permission to overwrite the previous motor parameters with the parameters stored in the current motor’s memory. If you have done everything else right, you then merely push the S1 button to tell the drive “Yes, a real human says to do this” and the drive writes the new parameters into the memory and is ready to proceed.
Unless, of course, you have absolute feedback, in which case now you need to re-reference the motor to the machine. Choose Wisely…