Backshells are an accessory to a connector. Backshells can be round or rectangular based on the shape of the connector and typically are attached to the connector by screw threads or separate screws through connector flanges. Backshells make the transition from a connector to a long length of cable. They can have special features such as the ability to connect a braided metallic shield to provide electromagnetic field protection. They often provide a way to pot (fill with material to seal) and secure in a mold tool that will over-mold a strain relief.
