HP 54721A User Reference page 4

Rear panel of the plug-in
The rear panel has both internal and external trigger output signals.
One digital interface connector that has the control signals for the
attenuator ranges, channel setup hardkey, softkey selections, and
power supply lines.
You may notice that there are two channel output connectors. The
4-GSa/s sample rate is achieved by interleaving two sampler hybrids
inside of the mainframe. There is a channel output for each of the two
hybrids, and that is why the plug-in takes up two slots on the
mainframe.
ijcternal trigger output
Digital interface and
power supplies
Channel output
Internal trigger output
Channel output