I know of several very different PDP-11 BBS's using very disparate architectures. Some were run on RT-11 or RSTS-11 entirely inside a BASIC program that managed every element of call answering, logging in, and disconnection. And others took advantage of TSX-11, RSX-11 and RSTS-11 login security and "captive accounts" that were either entirely menu-driven or had restricted command sets, with the menu options or command sets oriented strongly towards typical BBS functions. I know Billy Youdelman's TSX-11 BBS in LA was operating in the 1980's and 1990's and may have gone on longer than that. Tim N3QE