The bus connection module RER 103 acts as an interfacing unit between an RE_ 500 device and the fibre-optic LON® or SPA bus. The bus connection module converts incoming optical signals from the LON® /SPA bus to electrical RS 485 signals for the RE_ 500 devices and vice versa. The module is plugged into the D-type subminiature connector on the rear plate of the host device. It can be used together with any RE_ 500 device provided with a 9-pin D-type connector. The bus connection module is powered from the D-type connector of the host device. RER 103 contains a Service Pin for LON® nodes.