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, which is to detect the difference between the P6021 and P6022 probes, and switching in appropriate gain and frequency compensation. The P6021 shorts this ring to ground, directly switching three relays connected to -15 V internally.
Readout-less 5000-series plug-ins and some standalone scopes of the same era, e.g. many of the 400-series scopes such as 485, use the ring to detect a ×10 (but not a ×100) probe, and indicate the correct scale factor using LEDs.