Operation on data cable ( bus SCSI ) is controlled by the host adapter . On a parallel SCSI bus , a device ( e.g. host adapter , disk drive ) is identified by a `` SCSI ID '' , which is a number in the range 0-7 on a narrow bus and in the range 0 â `` 15 on a wide bus . On earlier models a physical jumper or switch controls the SCSI ID of the initiator ( host adapter ) .