An arrangement is described comprising a diagnostic computer (11) connected to a host computer (12) for the purpose of troubleshooting the host computer's hardware and software. The arrangement includes an interface unit (13) comprising specific controls (31, 33, 35) with transfer and mask registers between the diagnostic computer and the host, and control units (22, 23) in the diagnostic computer, to perform functions required by the user of the system. The arrangement is specifically designed for use with a host computer comprising a level-sensitive scan (LSSD) register.