Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!mit-eddie!bloom-beacon!eru!hagbard!sunic!mcsun!hp4nl!phigate!philica!adrie
From: adrie@philica.ica.philips.nl (Adrie Koolen)
Newsgroups: comp.os.minix
Subject: Re: SCSI drives & ST01 interface
Keywords: SCSI ST01 AT
Message-ID: <715@philica.ica.philips.nl>
Date: 27 Nov 90 13:05:04 GMT
References: <1990Nov26.163451.21348@gapos.bt.co.uk>
Reply-To: adrie@beitel.ica.philips.nl (Adrie Koolen)
Organization: Philips TDS, Innovation Centre Aachen
Lines: 39

In article <1990Nov26.163451.21348@gapos.bt.co.uk> davep@gapos.bt.co.uk (Dave Parkinson) writes:
>With SCSI:
>* Whilst the Host initiates a SCSI activity/transfer, all the transfers
>  across the SCSI bus are under the control of the TARGET, not the HOST.
>  (It is the target that controls all the lines other than ACK.  The
>   target sets the speed of the transfer, and the time of the transfer).

Some clarification. The SCSI bus can be in one of 10 phases: Bus Free,
Arbitration, Selection, Reselection, Command, Data In, Data Out, Status,
Message In and Message Out. The current phase is determined by some (5)
SCSI signals. If noone drives the signals, the bus is in the Bus Free
phase. The Initiator drives the bus into the Arbitration and Selection
phases, the other phases (in which bytes are transfered from Initiator to
Target or vice versa, as Dave correctly stated) are driven by the Target.
The REQ and ACK lines are used to transfer one byte. The ACK is driven by
the Initiator, The REQ by the Target. In asynchronous transfers the speed
of the transfer is dependant on BOTH the Initiator AND the Target. In
synchronous mode, the Initiator and the Target have to agree on the
minimum time between two consecutive bytes (REQ or ACK pulses) and the
number of unacknowledged bytes.

>(My Adaptec controller does not support a 'disconnect & please signal me
>when you've completed the command' mode of operation.  Other drives/
>controllers might).

The Disconnect/Reselect feature is only useful if you have multiple
Initiators on one SCSI bus or more than one Target, which you want to use
concurrently. Minix can't handle this case because of the single-threaded
FS!

>(Having asserted REQ, the Target expects an ACK from the host
>within a certain time-period - 250ms in the case of the Adaptec).

250ms is the time recommended to wait for the assertion of the BSY signal
after selecting a Target (or reselecting the Initiator). I think that the
maximum REQ --> ACK time would be much shorter than 250ms.

Adrie Koolen (adrie@ica.philips.nl)
Philips Innovation Centre Aachen