The UR90 is the IR receiver unit. The UR90 also looks similar to a UP5 and is mounted in the fascia of the layout. The UR90 is simply an addition to your existing LocoNet so no additional wiring is needed. The UR90 will support approximately 10 Infra-Ready throttles (DT100, DT300 and DT400 series). Most layouts will need more than one UR90 because IR is line of sight and the receivers must be able to "see" the IR signal. UR90 & UR91 can be used together on your layout to provide both wireless methods at the same time. If you already have DT300's or DT400’s the UR90 is the only thing you have to add to get infra red throttle operation.