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4.1.2
Hints for Software Driver writers
Software driver writers may find the following hints useful when
trying to increase data throughput.
1.
At the beginning of a MESSAGE phase, a target asserts SCSI
bus signals -C/D, -I/O, and -MSG at staggered times.
Some
hosts detect a change in bus phase and latch data as soon
as the signals are received, although the ANSI standard
requires that the target confirm the phase change by assert-
ing -REQ.
In the Q250/Q280, the -C/D, -I/O, and -MSG out-
puts of the 5080 are tristated (not used), and are replaced
by OR gates U24 and U26, so that -C/D, -I/O, and -MSG are
asserted almost simultaneously.
Software writers may use the simultaneous assertion of the
three signals to indicate that valid MESSAGE data is on the
bus and avoid the wait for -REQ to be asserted.
Note, how-
ever, that this is a violation of the ANSI standard, and
involves some risk of receiving an invalid MESSAGE from
devices other than Q200 Series drives.
2.
Set the Buffer Full Ratio and Buffer Empty Ratio (see the
Q200 Series Programmers Manual for details) to improve bus
utilization by optimizing disconnect/reconnect phases.
In
the following examples, a rough rule of thumb is that the
drive can transfer data to and from the 14 KByte buffer
memory at a maximum rate of about 14 KBytes/14 ms and at a
sustained rate, including wedges, of about 14 KBytes/19 ms.
When the drive is seeking, transfers to and from the disk
are
de~ayed.
See Figure 4-2 for a diagrammatic representa-
tion of the situation.
The transfer rate on the SCSI bus depends upon how fast the
host can "-ACK," and on the block sizes it requests.
This
data must be known to correctly set the Buffer Full and
Empty Ratios.
DISK
SCSI
DISK
(F AST) ~
,
(SLOW)
(SLOW)_,
/
"
I
\
lj
!
r
FF
r-+-
__
~_-+---.,
__
RECONNECT
----- 00
~
BUFFER FULL RATIO
DISK READ
SCSI
(FAST)
DISK
(FAST)
SCSI
DISK
___ (SLOW)
(SLOW),
~
/
"
I
\
r-+-
__ --+----,i--
FF
~
LJI
1
RECONNECT
~oo
-----
BUFFER EMPTY RATIO
DISK WRITE
Figure 4-2: Buffer Full and Empty Ratios
4-3
SCSI
(FAST)
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