HP 83751A Programming Manual page 148

Programming Commands
Marker Subsystem
MARKer[n]:REFerence
MARKer[n]:REFerence
MARKer[n]:REFerence?
This sets and queries which marker is the reference marker for use in the
for all the markers.
MARKerl:REFerence 5; and
MARKer:REFerence <n> is turned on and cannot be turned off if the
marker's current mode is DELZa mode.
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