Switched Ethernet in Synchronized Distributed Control Systems Using RTnet

We adopt a model of a real-time distributed network where the synchronization is done by an external timing network that triggers the acquisitions. A Switched Ethernet is used to send the synchronously sampled data from nodes to nodes to perform the distributed control algorithm. In order to guarantee low latencies and jitter, the traffic on the Switched Ethernet must be carefully analyzed to be able to correctly model closed-loop systems and potentially identify events that might lead to large delays or even packet losses. In this paper we discuss the most relevant results from a testbed control network with synchronous data acquisition nodes. A switched Gigabit Ethernet is adopted and each node uses RTnet running in a RTAI/Linux Operating System. To resolve the problem of timing uncertainties at the data link layer, we propose a new custom solution based on a TDMA discipline.