HP 8921A Programmer's Manual page 569

Text Only Error Messages
Text only error messages are generally associated with manual operation of the Test Set.
Refer to the HP 8920 or HP 8921 User's Guide for information on text only error
messages.
Text only error messages can also be displayed while running the Test Set's built-in
diagnostic or calibration utility programs. Refer to the HP 8920, HP 8921 Assembly Level
Repair manual for information on text only error messages displayed while running the
Test Set's built-in diagnostic or calibration utility programs.
Text only error messages take the following form: This is an error message.
For example
Input value out of range.
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