The R CT 5021 is a basic Rack Controller for the Series 5000 system. This provides for the remote monitoring and control of all modules installed in a LYNX rack chassis. Up to seven R CT 5010 Bus Expander controllers can be connected to the host rack with the R CT 5021 controller for system expansion. The R CT 5021 is also an integral part of the expansion path under the RCT 5031 Master Controller.