Path: utzoo!news-server.csri.toronto.edu!rutgers!cbmvax!daveh From: daveh@cbmvax.commodore.com (Dave Haynie) Newsgroups: comp.sys.amiga.hardware Subject: Re: SCSI-1 vs. SCSI-2 Message-ID: <19745@cbmvax.commodore.com> Date: 11 Mar 91 19:22:10 GMT References: <1991Mar8.200331.5111@javelin.es.com> Reply-To: daveh@cbmvax.commodore.com (Dave Haynie) Organization: Commodore, West Chester, PA Lines: 56 In article andrewsr@u2.rutgers.edu (Rich Andrews) writes: >Hello All! >What is the difference between SCSI and SCSI-2? (My guess: SCSI is >16bit and SCSI is 32bit). Not quite. SCSI-1 defines an 8 bit data path. SCSI-2 generally uses the same 8 bit data path, but does define 16 and 32 bit versions. Most Amiga hard disk controllers, even modern PIO controllers, work transfers between SCSI bus and main memory in 16 bit chunks. The A3000 funnels all SCSI transfers into 32 bit chunks. >Does the A3000 have a SCSI-2 port? Yes and no. At present, the A3000 comes with a SCSI-1 port. The SCSI chip we use on it defines this "SCSI-1"-ness, and there is apparently a pin-compatible SCSI-2 part that'll drop right in and work, at least hardware-wise. The other differences, which I know less about, are software. Specifically, the SCSI command set. Since it pretty much evolved over time, and many companies that uses SCSI disk drives didn't use all the features, some of the SCSI-1 command set is effectively "optional", and other pieces aren't as standard as one would like. One of the main ideas of SCSI-2 was to very strictly define the command set, so that all SCSI-2 drives, as well as good SCSI-1 drives, would play properly on a SCSI-2 bus. >How much better is the performance of a SCSI-2 over a SCSI? There are several SCSI-1 transfer modes. Every SCSI-1 controller implements the standard asynchronous mode, which goes around 1.5 MB/s. Some have an improved asynchronous mode which goes 2.5 MB/s; I don't know much about it, other than the SCSI chip used in A3000 and A2091 supports this. Then there's the synchronous mode, which, depending on the clock used, runs up to about 5 MB/s (I think the A3000 runs it at around 4 MB/s). SCSI-2 adds a high speed synchronous mode, which kicks up to a maximum of around 10 MB/s. If you could find a 32 bit SCSI-2 drive, you could expect a theoretical maximum transfer rate of 40 MB/s. Now, none of these transfer rates really attack the question of drive speed. Good SCSI drives manage bursts of 1.25-1.50 MB/s, straight from the disk. Until any seeking is necessary. The higher transfer rates help when you have multiple devices on the SCSI bus, and also to a degree with many of the modern drives which, like the Quantum series, have some on-drive caching and won't always need to burst data straight from the disk. But you don't expect in a single drive system to double your effective disk performance going from asynchronous to synchronous or from synchronous to high speed synchronous transfers. > // Rich | "have you ever noticed that opinion without knowledge is >\X/ Andrews | always a shabby sort of thing?" - Socrates -- Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: hazy BIX: hazy "What works for me might work for you" -Jimmy Buffett