Bit 2 - Data In Port Full (DF)The host uses the DF bit to synchronize transfers of data from the host adapter to thehost. When the DF bit is set to one, the host adapter has placed a byte in the Data InPort for the host to remove and process. When the host performs a read to the DataIn Port address, the DF bit is set back to zero automatically and not set to one againuntil a new data byte has been placed in the Data In Port by the host adapter for thehost.Bit 1 - ReservedThis bit is zero.Bit 0 - Invalid Host Adapter Command (INVDCMD)The Invalid Host Adapter Command bit is set to one if an invalid command or pa-rameter byte was received in the Command/Data Out Port. After sending a commandbyte or data byte, the host software determines that the next byte is ready to send bywaiting for the CDF bit to be reset. If the command byte or parameter byte is notvalid, the command sequence will instead be terminated by the host adapter. Thehost adapter always terminates a command by raising the Host Adapter CommandComplete (HACC) interrupt. If the HACC interrupt is set to one and the INVDCMDbit is not set, the command terminated normally. If the INVDCMD bit is also set toone, the command or parameter bytes were determined to be invalid and the com-mand terminated abnormally. The INVDCMD bit is only valid from the time theHACC interrupt is set until the HACC interrupt is reset. The bit’s value is not pre-dictable until a new HACC interrupt is set for a new adapter command.Command/Data Out and Data In PortThe second I/O port address is used by the host to write adapter command bytes andaccompanying parameter data bytes to the host adapter. It is also used by the hostadapter to send parameters back to the host to be read. The Command/Data Out Portis used by the host to send host adapter initialization and management commandsand parameters that cannot be sent by the standard mailbox protocol. Information re-quested by the adapter commands placed in the Command/Data Out Port is returnedthrough the Data In Port. The host should understand the format and number ofbytes to be transmitted for each command so that extra invalid bytes are not passedacross the interface. Bytes in addition to those required by the particular commandare likely to be interpreted as invalid, although they may instead cause the executionof valid commands that were not supposed to be performed.The host should only write to the Command/Data Out Port when CDF is zero. This al-lows time for the host adapter to process a previously written command or parameterbyte. CDF is automatically set to one when the host writes to the port and is reset tozero after the host adapter removes the byte from the port.If an adapter command needs additional data bytes, the host waits until CDF is zerobefore writing the additional bytes to the Command/Data Out Port. Just as in thecommand transfer case, each parameter byte written will set the CDF. The host canwrite additional data bytes only after CDF is again zero. The HACC interrupt will in-dicate when the command has terminated, normally or abnormally. If INVDCMD isalso set, the host adapter found either the command or data bytes to be invalid andEISA-to-Fast SCSI Host Adapter Hardware Functional Description4-5
