Set Bus Off Time (Operation Code 08)NoteThis command is not used by the AHA-1740A/1742A/1744 but is supported in orderto be compatible with software written for earlier host adapters, such as the AHA-1542B.This command sets the time that the host adapter will spend off the bus during adata transfer. The bus off duration is adjustable from one to 64 microseconds. The de-fault setting is four microseconds. After receiving this command, the host adapter ex-pects one byte of data which specifies the bus off time in microseconds.Byte 0 Bus Off Time ( 1 to 64 microseconds)After completing this command, HACC will be asserted indicating normal comple-tion. INVDCMD will be asserted, indicating an invalid command, if the data byte isgreater than 64. The valid range is four to 64 decimal. The actual time implementedby the host adapter is rounded down to the next 4-microsecond step at or below thespecified value. The minimum value is approximately one microsecond. Due to the in-ternal differences between the AHA-1740A/1742A/1744 and earlier host adapters, thedata is not actually used but is saved and returned in the Return Setup Data com-mand. The command is accepted only for compatibility.Set Transfer Speed (Operation Code 09)NoteThis command is not used by the AHA-1740A/1742A/1744 but is supported in orderto be compatible with software written for earlier host adapters, such as the AHA-1542B.This command adjusts the Bus Master DMA circuitry to a specified maximum ATbus transfer speed to and from the host memory. Several speeds may be selected bysetting the jumper configuration as explained in the System Configuration section inChapter Three, Installation. The selected jumper speeds are overridden if the SetTransfer Speed command is executed. The single data byte transmitted after the com-mand byte either sets the read and write speed together, or establishes separate val-ues for each. Appendix A gives the timings that result from the selected values. Thedefault setting is the value set by the jumpers. The I/O Channel Ready signal auto-matically slows the system further if required by the host memory.Data Byte 0 AT Bus Transfer Speed00 5.0 MBytes/second01 6.7 MBytes/second02 8.0 MBytes/second03 10 MBytes/second04 5.7 MBytes/secondadaptec AHA-1740A/1742A/17445-6
