Don't use master/slave. Use primary/replica or alternatives such as primary/secondary, principal/agent, controller/worker, or other appropriate terms depending on the context.
Use primary/subordinate as an adjective. Refer to arrangements in which one device controls another as a primary/subordinate arrangement, or refer to the controlling device as the primary server and the controlled device as the subordinate server.
Don't use primary/subordinate as a synonym for parent/child.
Examples Each subordinate device has a unique 7-bit or 10-bit address. The architecture uses a standard primary/subordinate design to replicate data from one server to many.
See also parent/child