A private branch exchange, and a method for selectively communicating a central processing unit with respective modules connected to the private branch exchange

A PBX (1) comprises a central processing unit (2) for controlling the PBX (1). A plurality of modules (6) for facilitating expansion of the PBX (1) at respective selectively addressable primary locations (4) through plus and socket connectors (7,5) and communicate with the central processing unit (2) through a primary bus (8) using SPI communication protocol. Primary address registers (14) at the respective primary locations (4) store a primary address code of each primary location which identifies the module (6) connected at that primary location (4). A plurality of secondary locations (15) in each module (6) are addressable through a programmable logic device (17) by the central processing unit (2). A comparator (19) compares a primary address on chip select lines (13) of the primary bus (8) with the stored primary address code in the primary address register (14) for determining if the module (6) is being addressed. A read/write circuit (20) reads the first byte of data of the communication after selection of a module (6) for determining the secondary location 915) which is to be addressed by the central processing unit (2). A main switch circuit (23) switches the selected secondary location (15) to the central processing (2), and a switch (27) switches a chip enable line (12) of the primary bus (8) to the main switch circuit (23) for switching to the selected secondary location (15).