Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site redwood.UUCP Path: utzoo!utcs!lsuc!pesnta!hplabs!hpda!fortune!redwood!rpw3 From: rpw3@redwood.UUCP (Rob Warnock) Newsgroups: net.periphs,net.micro,net.arch Subject: Re: simple SASI/SCSI interfaces Message-ID: <162@redwood.UUCP> Date: Fri, 15-Feb-85 22:36:33 EST Article-I.D.: redwood.162 Posted: Fri Feb 15 22:36:33 1985 Date-Received: Sun, 17-Feb-85 08:43:10 EST References: <5043@utzoo.UUCP> Organization: [Consultant], Foster City, CA Lines: 77 Xref: utcs net.periphs:611 net.micro:9013 net.arch:810 Henry Spencer writes in <5043@utzoo.UUCP>: +--------------- | The context is a desire to make it possible to connect "fast" devices | to a small computer, while not incurring much expense in the basic | system. The system has no expansion-oriented bus... | An interesting-looking possibility is a very simple host adapter for the | SASI/SCSI bus, not much more than some parallel ports with a few hooks | for interrupts and such. The question is, how well does this meet the | objectives? +--------------- Probably pretty well, if the interface is carefully matched to the use of it (as with the 68000 DTACK method you mentioned later). Early prototypes of the Fortune System used Xebec SASI-to-ST506 controllers and a "dumb" parallel port for the UNIX filesystem (polling for each character, etc.). Worked fine. There were throughput problems, but it was more due to improper disk layout than to the PIO method. Don't let the performance of early Lisas scare you... even with very crude hardware you can get good performance (relative to the disk latency) with careful analysis and tuning of the "bit-twiddling" code. +--------------- | Is it possible to find SCSI disk and tape controllers with reasonable | performance? (Defined as delivering a useful fraction of the potential | speed of the device, i.e. not requiring 5:1 interleaving on the disk | because the stupid controller can't keep up.)... | Is the idea of a very simple host adapter, with the cpu doing most of the | work except for the bare-bones handshaking, reasonable? Or is decent | performance impossible without smarts in the adapter? +--------------- Surpisingly, 1:1 is possible. While earlier SASI/SCSI controllers were all of the single-buffer type, newer (and still low-cost) boards from Adaptec, OMTI, and (maybe) Western Digital are all capable of 1:1 operation (they look like FIFOs). A simple "loop mode" byte-copy on a 68010 at 10 MHz moves data at 1.4 us/byte or over 5.7 megabits/sec transfer rate, well in excess of ST-506 rates. Your parallel port needs to designed very carefully to achieve such synchronized operation, but the cost would be low. You need to be careful with your interrupt-level assignments, since you don't want disk activity to cause you to lose serial data interrupts. And even with 1:1 transfers for swapping, your user filesystems will probably want 1:2 or 1:3 interlace ("system" interlace in mkfs) to match delays in the UNIX buffer handling code. Note that by using a small state machine to gather bytes into words, and by using a move-long in loop mode on a 10Mhz 68010, you can get over 14 Mbit/sec transfer rates from "programmed I/O". (Makes one think about SCSI & SMD, eh?) +--------------- | Is anybody making an SCSI Ethernet interface? Planning to? +--------------- Er... Ummmm.. I've thought about it, myself... but I don't know of anyone else who is planning to. (Further discussion of this should probably be via mail.) +--------------- | ... Investing some cpu time | in exchange for the simple adapter is acceptable, but having the cpu | spending all of its time managing the SCSI interactions is not. +--------------- I think it's perfectly reasonable. So does a guy by the name of John Bass <...!ihnp4!fortune!dmsd!bass>, who is currently building a single-board computer (68008-based) with just such a "bus" on it. (He gave a talk recently at a local computer club. Look for a product announcement soon.) He's using the NCR 5380 chip that handles most of the low-level handshake of SCSI (and includes the bus driver/receivers), and is talking to it directly with the 68k (although he was thinking about using DMA). Rob Warnock Systems Architecture Consultant UUCP: {ihnp4,ucbvax!dual}!fortune!redwood!rpw3 DDD: (415)572-2607 USPS: 510 Trinidad Lane, Foster City, CA 94404