Telephone centers or more precisely, branch exchange systems, are electrical and electronical systems that are responsible for establishing telephony communications. Their behavior can be explained by making conversations between connected phones. Private Branch eXchanges (PBXs) are some kind of such systems that are mainly used in companies, organizations and other private complexes.
 In this project, I've designed and implemented a PBX system that is capable of switching 4 phones (lines) and an external connection (trunk). A microcontroller from MCS-51TM family is used for handling and controlling the whole system. The switching unit is realized via 3 analog conversation circuits (one for each conversation connection). The hardware is implemented on 8 breadboards and the software is in the MCS-51TM assembly language.