Path: utzoo!attcan!uunet!ogicse!mintaka!olivea!olivey!dwc
From: dwc@olivey.olivetti.com (Dan Crocker)
Newsgroups: comp.periphs.scsi
Subject: Re: synchronous negot. and number of disks
Summary: some answers...
Message-ID: <49699@olivea.atc.olivetti.com>
Date: 25 Oct 90 22:26:55 GMT
References: <62.UUL1.3#1@vortex.COM>
Sender: news@olivea.atc.olivetti.com
Lines: 43

In article <62.UUL1.3#1@vortex.COM>, lauren@vortex.COM (Lauren Weinstein) writes:
> Here are a few SCSI questions of my own for the collected wisdom:
> 
> 1) What is "synchronous negotiation" mode?  Does this refer to negotiating
>    whether or not synchronous transfers will be done?  By default, I
>    assume the AHA-1540B does synchronous transfers, even with the 
>    "synchronous negotiation" mode disabled?  (As it is by default on the
>    1540B.)

Synchonous negotiation is the message passing sequence for two devices 
to agree on the rate at which synchronous xfers can take place.  Basically, 
one side might say: "I can do 5 MB/SEC" and the other might answer: 
"I can do 3 MB/SEC".  The final rate will always be the lower of the two.
As for the adaptec HA, I seem to remember that, if synchonous xfers are
disabled, it will respond to a synchronous negotiation but it will not
initiate it.  All the devices that I've seen (mostly disks) will use
asynchronous mode unless the initiator asks for synchronous mode (even if
it can support sync mode).  If you have a device that can do sync xfers,
I think you need to enable them on the HA.

> 2) Is the 5.0 default DMA transfer rate for the 1540B correct for 
>    most situations?

This should be fine since, if you connect a device that can't go that fast,
it will tell the HA and transfers will take place at the lower rate.
> 
> 3) How many disks with embedded SCSI controllers will the 1540B support?
>    The 1540B documentation says that SCSI disks should be set to to
>    ID 0 and 1.  Why not ID 2 and 3 as well?  Is the limit 2 or 4?

The ID settings are for accessing the disks using standard INT13 calls
(drive 80 = ID 0, and drive 81 = ID 1).  If you set the ID's to anything else,
DOS will not recognize them.  If, however, you have a DOS device driver
or you are using another operating system with the right driver, you should
be able to connect the SCSI maximum of 7.

One comment about sync. xfers.  I have seen drives whose async and sync
transfer rates are the same (i.e. 3 MB/SEC).  The only point here is that
you don't necessarily gain anything by using sync xfers.  Consult your
drive docs to find out for sure.

Hope this helps...
dan