A serial communications interface is described that enables the extension of an internal Communications Bus Architecture (CBA) bus segment to one or more external devices. The interface accomplishes this function by serializing bus transactions in one device, transferring the serial transaction between devices via one interface port, and de-serializing the transaction in the external device. The general features include low pin count (as few as three signals), simple packet based transfer protocol for memory mapped access, symmetric operation, simple block code formatting, supports both host to peripheral and peer to peer transactions, and support multiple outstanding transactions.