HP 54721A User Reference page 5

Getting the best performance from the plug-in
To obtain the 1 percent dc accuracy specifications, you must perform
a best accuracy calibration. Also, when you move a plug-in from one
slot to another slot, you must repeat the best accuracy calibration to
retain the 1 percent accuracy specifications. See "Calibrate to Best
Accuracy" on page 17.
The best accuracy calibration also performs the sampler interleave
calibration, which must be performed for the plug-in to function
correctly at 4 GSa/s.
Installing the plug-in
You do not need to turn off the mainframe to install or remove any
plug-ins, and you can install a plug-in into any slot on the HP 54720
mainframe. However, on the HP 54710 mainframe only slots 1 and 2
have channel acquisition hardware, but you can still use slots 3 and 4
as trigger sources.
You can install this plug-in in slots 1 and 2, 2 and 3, or 3 and 4.
To make sure that the oscilloscope meets all of the published
specifications, there must be a good ground connection from the
plug-in to the mainframe. The RF connectors on the rear of the
plug-in are spring loaded, so finger tighten the knurled screw on the
front panel of the plug-in to make sure that the plug-in is securely
seated in the mainframe.
Trigger menu
You may notice a couple of differences in the trigger menu on the
mainframe when you are using this plug-in. The external trigger level
range for this plug-in is ±1 V. Also,
the
trigger source selection follows
the slots that the plug-in is installed in. For example, if the plug-in is
installed in slots 2 and 3, then the internal trigger source is listed as
channel 2, and the external trigger source is listed as trigger 3.