robert@eowyn.cm.deakin.oz.au (12/02/89)
I would like to find out what happens when you intermix synchronous and asynchronous SCSI devices on the one bus? Does the bus drop to the lowest common denominator, or does it work at sync speeds to sync devices etc.? What happens when both types of devices are doing intermixed transfers? Does the speed drop significantly for the sync devices? I am thinking here of high speed SCSI disks and slow speed cartridge tapes or 8mm tapes. Thanks for any information. Robert Ruge Department of Computing & Mathematics Deakin University Victoria, 3217 ACSNET: robert@aragorn.cm.deakin.oz.au PHONE: 052 471319 Robert Ruge robert@aragorn
olson@anchor.sgi.com (Dave Olson) (12/06/89)
I tried to reply by E-mail, but it got bounced somewhere in oz... In comp.arch you write: >I would like to find out what happens when you intermix synchronous >and asynchronous SCSI devices on the one bus? Does the bus drop to >the lowest common denominator, or does it work at sync speeds to sync >devices etc.? What happens when both types of devices are doing >intermixed transfers? Does the speed drop significantly for the sync >devices? I am thinking here of high speed SCSI disks and slow speed >cartridge tapes or 8mm tapes. Thanks for any information. Typically negotions for doing sync mode transfers happen only at boot time or on opens, since they are relatively expensive (on the order of 5-10 ms). Since all SCSI transfer are only between pairs of devices, the sync transfers happen at full sync speed (limited by the negotiated offset and transfer period, but typically somewhat slower). The async transfers also happen at their full rated/negotiated speed. Almost all SCSI chips that can do sync mode transfers switch from sync to async just by re-programming 1 or 2 registers, so it's not hard (or expensive) to switch back and forth between modes when both types of devices are on the same SCSI bus. So far I haven't run into any SCSI tapes devices that do sync mode transfers (QIC, 8mm, or 9 track). I have heard claims that some of the DAT drives will do sync transfers, but haven't yet actually seen it demonstrated. In a mixed environment (sync and async), a major advantage of sync (aside from higher sustained transfer rates for fast devices) is better SCSI bus utilization when several devices want to use the bus at the 'same' time. With the DAT's being advertised as only 185 Kbytes/sec (sustained), and with large (512Kbyte to 1Mbyte) buffers, the increased bus utilization would be the only advantage of sync mode; even slow async SCSI devices have no problems running 185Kbytes/sec over the SCSI bus (as opposed to sustained transfer rate). Dave Olson It's important to keep an open mind, but not so open that your brains fall out. -- Stephen A. Kallis, Jr.
gerry@zds-ux.UUCP (Gerry Gleason) (12/07/89)
In article <7909@charlie.OZ> robert@eowyn.cm.deakin.oz.au () writes: >I would like to find out what happens when you intermix synchronous >and asynchronous SCSI devices on the one bus? Does the bus drop to >the lowest common denominator, . . . The mode of information transfer (sync/async) is negotiated between each target and initiator, so in order to use it, both ends of the connection must be capable of it. Note also that only data transfers can use sync transfers, not commands, messages or status. Gerry Gleason
stevew@wyse.wyse.com (Steve Wilson xttemp dept303) (12/09/89)
In article <7909@charlie.OZ> robert@eowyn.cm.deakin.oz.au () writes: >I would like to find out what happens when you intermix synchronous >and asynchronous SCSI devices on the one bus? Does the bus drop to >the lowest common denominator, or does it work at sync speeds to sync >devices etc.? What happens when both types of devices are doing >intermixed transfers? Does the speed drop significantly for the sync >devices? I am thinking here of high speed SCSI disks and slow speed >cartridge tapes or 8mm tapes. Thanks for any information. > >Robert Ruge Robert, SCSI will transfer in SYNCH mode to a device that is capable of synch operations, and to ASYNCH devices that can only transfer in asych mode. This is a situation where ASYNCH is the lowest common denominator, but if both the initiator and the target are capable of SYNCH, then they can use that mode of transfer. The above implies that if your system is completely SYNCH capable it will take full advantage of the improved speed. If you have a mix, you'll still get an advantage when talking to SYNCH devices. Steve Wilson