About a year ago the Research telephone switch came up on this list. Rob Pike wrote: "But the PBX story is correct. To demonstrate how message passing was a good model for a switching system, in particular to make a point to the switching systems division of Bell Labs/AT&T, Ken and Joe bought a commercial PBX and swapped out its processor for a PDP-11/23 (I think), and programmed it up. It was just before I arrived there but I was given the impression it had the desired strategic influence on Indian Hill. The feature we all loved it for was that instead of ringing the phone in the Unix room when you got a call, it would announce your name through the voice synthesizer: "Phone call for Ken." "Phone call for Joe". One rapidly stopped even hearing the announcement if it didn't end with your name.” I’ve been having an off list discussion with Bill Marshall and this PBX was influential in another way as well. First of all, Bill can confirm that it indeed was a 11/23, the same racks were used for Datakit switches. He also remembered that the software for this PDP-11 went by the nickname of “TPC” - for Tiny Phone Company. Lee McMahon was on the team writing TPC. The first software for the Datakit switch was written by Greg Chesson and was called “CMC” (for ‘Common Control’). There are still some references to CMC in the 8th Edition source code. This first software was later replaced by new code designed by Lee McMahon that was modelled after TPC. This new code was named “TDK”. This, too, can be seen in the 8th Edition source. The TDK protocols for building and releasing a Datakit virtual circuit appear to have been in use into the 1990’s.