Re: Technical SCSI question

[David.Empson@bbs.actrix.gen.nz]

In article <1992Nov24.141342.15785@slab.slip.uiuc.edu> derek@slab.slip.uiuc.edu (Derek Taubert) writes:
> Looking through Apple's Tech. Ref. on the SCSI card, I can't figure something
> out.  All of the calls involve REQUESTING a device on the SCSI bus to do some
> action.  By what mechanism does a device on the bus get the host to do
> something?
> 
> Example: disk is ejected from a button on the drive, icon gets dimmed.

Dead simple: in the case of removable drives, Finder (or whatever) is
polling the drive continuously, and it dims the icon if it gets a disk
offline indication from the driver.  There is nothing fancy going on.

The same mechanism is used for floppies and for removable SCSI devices
(e.g. SyQuest drives).

I don't think the SCSI card can act as a SCSI responder without some
major low-level programming.

I think the High Speed SCSI card supports "disconnection", where the
drive goes away and performs the read/write operation, reconnecting to
the computer when the operation is complete (so the SCSI bus could be
used to talk to another device while the first is performing I/O).
-- 
David Empson

Internet: David.Empson@bbs.actrix.gen.nz    EMPSON_D@kosmos.wcc.govt.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand