The protocol and basic parameters of a Class B or Class C multiplex data transmission system are outlined. The protocol is based on the token passing ring principle. The system design is open allowing expansion of the network without major changes to the node controller software. A set of 31 network control functions are allowed, up to 255 nodes can be supported and message packets up to 13 bytes can be handled. A node can send several message packets while it possesses the token. Data rates for average transmission lengths and update periods are derived.