SCSI Reset OperationsThe SCSI Reset condition is defined in the SCSI Standard, X3.131-1986, Section5.2.2. A SCSI Reset condition may be forced by any SCSI device on the bus. The con-dition is forced by the assertion of the SCSI Reset signal.The AHA-1740A/1742A/1744 has four mechanisms which may force a SCSI Reset con-dition. The SCSI Reset condition may be invoked from the host software if the soft-ware sets the SCRST bit (Bit 4 of the Host Adapter Control Port). In this case, thenormal SCSI Reset operations will be performed by the host adapter and the SCSIReset signal will be asserted on the SCSI bus. In addition, the SCRD bit (InterruptFlag Port bit 3) will not be set, since the host itself caused the reset.The SCSI Reset is invoked if a Hard Reset is generated, either by the system boardRESET signal or by the setting of the HRST bit. This reset is described in the HardReset Operations section.The SCSI Reset condition may be invoked by the AHA-1740A/1742A/1744 as part ofthe recovery mechanism for a bus phase error. Bus phase errors may include detec-tion of an invalid information transfer phase or detection of an impossible phase se-quence (Command Phase after a Data Phase in the same command). In this case, thenormal SCSI Reset operations will be performed and the SCSI Reset signal will be as-serted on the SCSI bus. In addition, the SCRD bit (Interrupt Flag Port bit 3) will beset to indicate to the host computer that a SCSI Reset condition occurred. The settingof the SCRD bit also causes the Any Interrupt bit (Interrupt Flag Port bit 7) and theappropriate interrupt signal to be presented.The SCSI Reset condition may be invoked by another SCSI device attached to theAHA-1740A/1742A/1744 as part of the other device’s recovery mechanism or initiali-zation procedure. In this case, the normal SCSI Reset operations will be performed.In addition, the SCRD bit (Interrupt Flag Port bit 3) will be set to indicate to thehost computer that a SCSI Reset condition occurred. The setting of the SCRD bit alsocauses the Any Interrupt bit (Interrupt Flag Port bit 7) and the appropriate ISA in-terrupt signal to be presented.SCSI Soft Reset OptionThe SCSI standard indicates two optional methods of handling the normal SCSI Re-set operations. The AHA-1740A/1742A/1744 implements the Soft Reset option, de-scribed in Section 5.2.2.2 of the SCSI specification. The Soft Reset option is designedto allow a SCSI Reset signal to correctly clear the SCSI bus, but to allow ongoing sys-tem operations to continue without any major interruptions. When the SCSI Reset oc-curs, any activity on the SCSI bus is immediately halted and all bus lines are clearedfrom the bus. After the Reset condition ends, any operations in progress are again al-lowed to start up in the normal manner. No status or pointer information is de-stroyed. All disconnected commands are allowed to reselect and continue in thenormal manner. This Soft Reset function allows a multiple initiator system to use re-set to clear certain types of bus failures without damaging ongoing tasks from anyinitiator.EISA-to-Fast SCSI Host Adapter Hardware Functional Description4-9
