EIA-232 interface cables should have the shield grounded at both ends of the cable. Digi recommends Chassis Ground, available on the shell of Digi's DB-25 connectors, and pin 4 of a 10-pin RJ-45 connector.
EIA-422 interface cables must provide a ground path between the devices to be connected. This insures the integrity of data transfers and control signals. This should be connected to the Chassis Ground of each device. Digi recommends using the cable shield for this purpose.