HP 80000 User Manual page 51

System Clock
10 MHz Reference:
2
Press to toggle bet,ween Int,ernal and Ext,ernal 10 MHz
reference for the internal syst,em clock. The External
Reference Input is located at the rear of the Mainframe.
Use an external 10 MHz reference if you need better
accuracy than that offered by the int.ernal crySal
reference.
System Operating Reference