Path: utzoo!attcan!uunet!cos!hqda-ai!media!csense!bote
From: bote@csense.uucp (John Boteler)
Newsgroups: comp.periphs.scsi
Subject: Re: synchronous negot. and number of disks
Summary: From the horse's mouth
Message-ID: <1990Oct27.050856.9410@csense.uucp>
Date: 27 Oct 90 05:08:56 GMT
References: <62.UUL1.3#1@vortex.COM> <49699@olivea.atc.olivetti.com>
Organization: Common Sense Computing, McLean, VA.
Lines: 70

dwc@olivey.olivetti.com (Dan Crocker) claimed:
>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?
>
>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.

I think the question referred to the DMA xfer rate to memory,
not across the SCSI bus itself. Read on...


Here it is from the horse's mouth:

>From: neese@adaptex.UUCP
Newsgroups: comp.unix.sysv386
Date: 10 Sep 90 16:13:00 GMT

The bus speed has nothing to do with the DMA rate.  It's more important to
consider the memory bus rate, which is not driven by the bus clock speed.
As the card is a bus master, it does ignore the bus clock speed and uses
the programmed DMA rate to do all data transfers.  The only signal that can
affect the actual transfer rate is the IOCHRDY signal.

The DMA speed is completely independent of the BUSON time.  If anything, you
would want to set the DMA speed as high as the machine will take it.  I always
recommend 6.7MBytes/sec for the DMA speed as this is much faster than the
SCSI bus can run and will help to minimize any disconnects that may occur
due to the adapter starving the device buffer it is connected to.

[ editor's note: I had problems on a 'G2' 286 motherboard at 6.7;
I had to switch to 5.7MB/sec for reliable performance. ]

>>I've installed the Sync option jumper on the controller. 
>that jumper doesn't matter - it's setting is overridden by ISC's scsi
>driver.

Software cannot override this jumper.  This jumper simply indicates who will
attempt to negotiate for synchronous protocol.  If the jumper is installed the
AHA-154x board will negotiate for it, if the jumper is removed the adapter
expects the device to negotiate for it.  I would allow a device to do the
negotiation as some devices will do the negotiation on each data transfer.
If the 154x does the negotiation, it will only be done once.

>>With BUSON 5, BUSOFF 9 and DMASPEED 0; the system seems to be
>>performing just fine. 
>
>sure, these are the 'works 100%' parameters. not the 'works 100% and
>performs 100%' paramters.

The system will perform with these paramters, but not they are not the
optimum values.  It actually takes some experimentation to find what is
best for any given environment.  The variables that come into play are
the device(s) connected, the size of any given data request, and the
actual speed at which data can be transfered across the AT bus.

			Roy Neese
			Adaptec Senior SCSI Applications Engineer
			UUCP @  uunet!swbatl!
				  {nominil,merch,cpe,mlite}!adaptex!neese
-- 
John Boteler   bote@csense                {uunet | ka3ovk}!media!csense!bote
SkinnyDipper's Hotline: 703 241 BARE | VOICE only, Touch-Tone(TM) signalling