Set Bus Off Time (Operation Code 08); Set Transfer Speed (Operation Code 09) - Adaptec AHA-1740A Technical Reference Manual

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Set Bus Off Time (Operation Code 08)

This command is not used by the AHA-1740A/1742A/1744 but is supported in order
to 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 a
data 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
After completing this command, HACC will be asserted indicating normal comple-
tion. INVDCMD will be asserted, indicating an invalid command, if the data byte is
greater than 64. The valid range is four to 64 decimal. The actual time implemented
by the host adapter is rounded down to the next 4-microsecond step at or below the
specified value. The minimum value is approximately one microsecond. Due to the in-
ternal differences between the AHA-1740A/1742A/1744 and earlier host adapters, the
data 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)

This command is not used by the AHA-1740A/1742A/1744 but is supported in order
to 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 AT
bus transfer speed to and from the host memory. Several speeds may be selected by
setting the jumper configuration as explained in the System Configuration section in
Chapter Three, Installation. The selected jumper speeds are overridden if the Set
Transfer 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. The
default 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
00
01
02
03
04
5-6
Note
Bus Off Time ( 1 to 64 microseconds)
Note
AT Bus Transfer Speed
5.0 MBytes/second
6.7 MBytes/second
8.0 MBytes/second
10 MBytes/second
5.7 MBytes/second
AHA-1740A/1742A/1744