BNC connector with readout ring
Most 7000 series plugins have rings around the BNC input sockets that allow attached probes to interface with the readout system.
The probe connector includes a contact pin connecting this ring. A resistor connected to ground encodes the probe attenuation, e.g. 11 kΩ indicates a ×10 probe and 6.8 kΩ a ×100 probe.
If the probe includes an Identify switch at the tip, this connects the readout pin directly to ground. The plug-in circuit uses the shift instructions in time slot 1 to implement the change of range.
The 7A14 plugin uses the same BNC connector with a ring but the ring serves a different purpose.