Problems with SCSI scanner

[peer001 at TAT.PHYSIK.UNI-TUEBINGEN.DE]

We are trying to connect a SCSI scanner to a Personal Iris using the
generic SCSI driver ds/dslib of Irix 3.3.1.
The scanner works fine on a VAXstation 3100 (only asnc. transfer!) with
the generic SCSI driver GKDRIVER of VMS 5.3.
When we hook up the scanner as the only external device with SCSI id 4
and turn on the Iris, we get
>sc0: Unexpected transfer phase state=4A phase=10
>scsi(0,4,0) transfer aborted (Hardware error)
during the hardware diagnostics.
(The scanner does not implement a senddiag command!)
After setting sentenv bootmode C in the monitor and performing autoboot
we get 
>sc0: Unexpected info phase, state=4A phase=36
>sc0,4 reseting SCSI bus stray interrupt error
three times right after the Irix System V 3.3.1 line.

We have included ds in /usr/sysgen/master.d/scsi and rebuilt the kernel.
We have compiled and linked /usr/people/4Dgifts/examples/devices/inquire.c.
Doing inquire -d /dev/scsi/sc0d4l0 usually results in
...
cmdbuf = 12 00 00 00 06 00
databuf datalen   ... 96
>sc0: Unexpected info phase, state=4A phase=36
>sc0,4 reseting SCSI bus stray interrupt error
cmdlen after ioctl = 6
ioctl=0 ret=43    (which means recoverable bus error)
retry transient error cmd sent =6 data sent =0
sbyte =FF
.....
Only sometimes  (after turning the scanner of and on, because
it does not handle bus resets correctly) the inquiry12 command is 
performed correctly. The following testunitready01, however, produces
the same error again.
We already tried to set scsi_syncenable = 0xee in master.d/scsi, but it did
not improve things.

Since the scanner people are willing to modify their SCSI implementation,
we need to know:
1. How does a SCSI device pass the Personal Iris hardware tests?
2. How can we deal with those timing problems?
  i.e. The scanner is a slooow SCSI device with async. transfer only.
  Looking at dslib.c, it seems that inquiry12 in inquire.c is performed
  in async. mode since DSRQ_SYNXFR is never set. How else would we disable
  sync. transfers?

Maybe, someone can give us some hints. 
Thomas Ertl                               Tel.: 49-7071-292043
Lehr- und Forschungsbereich               Telex: 726 2867 UTNA D
Theoretische Astrophysik                  Telefax: 49-7071-295400
Universitaet Tuebingen                    ertl at tat.physik.uni-tuebingen.de
Auf der Morgenstelle 10                   PSI%45050260314::ERTL
D-7400 Tuebingen                          West Germany