Abstract: A ring network system (1) has at least one node M which acts as a master or sender node to transmit an outgoing signal (O.S.1) having a relative address determined according to the relative location of the addressee node in the system. As the outgoing signal passes through each node in turn, it is changed and a node recognizes that it is the addressee node if the relative address has a predetermined common addressee value. The addressee node generates a return signal (R.S.1) with a node identifier which is transmitted in the opposite direction to the outgoing signal and the sender node ensures that this is received for a valid poll. The master node (M) may automatically configure itself by retrieving the identifier of all nodes in response to outgoing signals. This allows nodes to be easily added or deleted. Each node on the return path adds data which it has captured together with its identifier so that data may be captured from all nodes between the sender node and the addressee nodes.