1. An integrated circuit comprising: a plurality of data handling devices interconnected to exchange data by an interconnection bus during a first mode of operation, said data handling devices comprising one or more bus master data handling devices and one or more bus slave data handling devices; and diagnostic control means, responsive to a diagnostic device external to said integrated circuit, comprising a diagnostic bus master data handling device for controlling one or more of said data handling devices to exchange diagnostic data with said diagnostic device via said interconnection bus during a second, diagnostic, mode of operation.