The OBT has 2 separate functions. The first is a 5 channel modem which, on the bus side, provides the digital waveforms necessary to operate the Litton Bus drivers, and receives the outputs of the Litton bus detectors. On the user side, it provides an input / output at Digital Bus Interface level. The second function, internally coupled to the first, provides a multiplexing / demultiplexing function of the DBI signals down to Internal User Bus levels and vectored 16 bit serial register read and write commands (see section 7.2 of ESA standard TTC-B-01). In effect, the second function of the OBT provides the core of an RTU.