Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!iuvax!mailrus!bcm!krebs.bcm.tmc.edu!dinah
From: dinah@krebs.bcm.tmc.edu
Newsgroups: comp.periphs
Subject: Summary of Diff. between synch. and asynch. SCSI
Message-ID: <1637@gazette.bcm.tmc.edu>
Date: 2 Aug 89 12:45:58 GMT
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Organization: Baylor College of Medicine, Houston, Tx
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Here is the summary of the responses I received from the question on the
difference between synchronous and asynchronous SCSI. Thanks for your
help. Sorry for the slow response on my part - I changed jobs between the time
I posted and now.

>From sun!aeras!tneale Thu Jul 13 15:34:25 1989

The trade off is speed.  Sync runs faster under almost all conditions.
Sync timing is accomplished by a negotiated (drive to controller and vise
versa) set of timing parameters, async works on handshake lines.
The catch is that both your controller (host adapter) and target (disk
drive, etc.) must be synchronous capable - not all drives are.

The SCSI spec says that the minimum data transfer rates are:

	async:	1.5 MB/sec
	 sync:	4.0 MB/sec

Obviously, higher data rates can mean high performance for your system.
There are other disk drive specs that will have a larger impact on your
system.


========================================================================

From: kong!ncratl.Atlanta.NCR.COM!mclauss
The SCSI interface has several modes. There are modes for 
arbitration, selection, bus idle, data in, data out, command in, command out
and several others. The asynchronous SCSI bus puts out a request (REQ) signal 
along with a data byte and waits for an acknowledge (ACK) signal before 
sending out the next byte with the next REQ.

This means that the cable must be traversed 2 times for each byte. That makes
the transfer time vary with cable length. The longer the cable the slower
the data transfer.

For synchronous SCSI, the initiator and target device do some negotiation
up front and set up some hardware parameters. Once this is done, anytime
data is transfered (see below), a REQ and data are sent, then after a
parameter delay another REQ and data are sent. This is done for a 
parameterized number of times up to (I believe) 10. The ACK pulses for 
each REQ may begin whenever they get there and are expected at certain
time intervals after that. The ACK pulses are counted to make sure the 
correct number are recieved. The following phases transfer data:

	data in
	data out
	command in
	command out


Synchronous makes SCSI less sensitive to cable length and provides fast 
data transfer. The problem with that is that the data transfer time
is sometimes insignificant when compared to the total overhead to take
care of other things. SCSI 2 addresses some of these problems to
reduce the other overhead so that increasing the data transfer speed
will actually make a big difference in the total speed of data. 

Until that time, going to synchronous SCSI may only increase total
throughput by about 25% to 50%. Think hard before you pay a lot 
of extra money for a synchronous drive. Also keep in mind that a 
synchronous drive will be asynchronous unless the host is synchronous
also.

Hope this helps

============================================================================

From rutgers!ssd.harris.com!garyb@gazette.bcm.tmc.edu Wed Jul 19 16:01:55 1989

===> Turn on technical SCSI mode

	Asynchronous SCSI uses a REQ/ACK handshake for every byte that is
	transferred.  So in the case of an input request, the target drives
	the data bus to it's correct value, waits a standard delay, and then
	drives REQ to true.  The initiator then reads the data bus when it
	sees REQ transition to true.  After having read the data bus, the
	initiator drives ACK to true, which tells the target we received the
	data, and it may continue.  Synchronous	SCSI uses a similar handshake
	with the exception that the transferer need not wait for receipt of
	an ACK before setting up another data item.  Instead, the data values
	are held for some known delay, and then the next value is setup.
	Asynchronous transfers are the default, and in order to use sync
	transfers, the targets and initiators capable of synchronous transfers
	(this is an optional feature after all) negotiate for synchronous
	transfers via send a SYNCHRONOUS DATA TRANSFER REQUEST message.  Using
	this message, target and initiator negotiate the parameters of the
	synchronous transfers.  Rejection of the message by either target or
	initiator will mean that async transfers must be used.  FYI, this
	description is taken largely from the SCSI spec, ANSI X3.131-1986.
	

===> Turn off technical SCSI mode

Anyway, what this all means is that devices which support SCSI
synchronous transfers have a higher theoretical data transfer rate.
When you plug in all the appropriate delays, the maximum transfer rate
for async SCSI transfers works out to around 1.5 Mbytes/sec.  In a
similar fashion, the maximum xfer rate of synchronous SCSI can be
calculated to ~4 Mbytes/sec.  So for SCSI busses where data phases
dominate (ie: lots of large transfers), using synchronous transfers
can be a big win, especially if there are multiple targets, and bus
bandwidth is at a premium.  However, be forewarned that many
peripherals that come with embedded SCSI controllers do not support a
data transfer rate greater than that of the theoretical async limit.
In these cases, synchronous SCSI buys you nothing because the device
cannot sustain a transfer rate fast enough to take advantage of the
higher maximiums.  As an example there are several disk drives that we
use here that support sync SCSI transfers.  Listed below are the avg
and peak transfer rates.  As you can see, only the last one has avg
transfer rates significantly above the 1.5 Mb/s async limit, and it
just noe beginning to ship in production quantities.

Disk Drive		  Avg	        Peak(sync)  Peak(async)
===============================================================
FUJI MX2246SA 171 Mbyte	  1.25 Mb/s   	2.5 Mb/s    1.5 Mb/s
FUJI MX2249SA 389 Mbyte	  1.25 Mb/s   	2.5 Mb/s    1.5 Mb/s
FUJI MX2250SA 760 Mbyte	  3 Mb/s        4 Mb/s      1.5 Mb/s

So unless SCSI bus bandwidth is at a preminim in your configuration,
and you have some pretty fast peripherals, chances are you don't
really need synchronous SCSI.  I guess it would still be advantageous
to have, but it would not be strictly necessary.

=========================================================================

Thanks again
Dinah Anderson					 internet: dinah@bcm.tmc.edu 
Baylor College of Medicine	           uucp: {rutgers,mailrus}!bcm!dinah