Echo Command Data (Operation Code 1F)This command is used to test the Command/Data Out Port, the Data In Port, and theassociated control bits in the other I/O Ports. After receiving this command, the hostadapter expects one byte of outbound information to be transferred through the Com-mand/Data Out Port. The host adapter then sends (Echos) the same data value backto the host through the Data In Port. After the host has read the data value providedon the Data In Port, the host adapter will generate the HACC interrupt to indicatenormal command completion.Byte 0 Out Outbound echo valueByte 0 In Returned echo valueAdapter Diagnostic (Operation Code 20)Unlike other Adaptec host adapters, this command does not cause the host adapter toperform its internal self-diagnostic. A host adapter Hard Reset is not executed, nor isa SCSI bus reset. The host adapter need not be reinitialized after this command be-fore resuming normal operation. No actual diagnostics are performed, although inter-nal flags are reset to the default state.Set Host Adapter Options (Operation Code 21)Unlike other Adaptec host adapters, this command does not provide the host a mecha-nism for specifying certain host adapter configuration options. Configuration parame-ters sent to the host adapter via a parameter list following the command OP Codeare ignored.Mailbox OverviewThe AHA-1740A/1742A/1744 uses a mailbox architecture for task communication be-tween the host and the host adapter when executing SCSI commands. This allowsthe host adapter to perform multithreaded operations with a minimum of host inter-vention. The mailboxes are located in main system memory. Each mailbox entry isfour bytes long. At power-up, the host issues an initialization command to inform thehost of the mailbox location. There are always an equal number of Outgoing Mail-boxes (MBO) and Incoming Mailboxes (MBI). The MBIs are located immediately af-ter the MBOs. Initiator operations and target operations use the same mailboxes.Both initiator and target operations may be in process at the same time. A typicalmailbox structure follows.This command is provided only for compatibility. Refer to the section titled InitializeSCSI Subsystem for a discussion of operation code 10, which provides these functions.adaptec AHA-1740A/1742A/17445-14
