bill@unixland.uucp (Bill Heiser) (02/24/91)
Does having a SCSI drive external to a 386 system, on a cable
several feet long, affect the data transfer rate? Is it significantly
better to have the drive mounted internally in the system?
Bill
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other: heiser@world.std.comthad@public.BTR.COM (Thaddeus P. Floryan) (02/24/91)
In article <1991Feb23.194013.13400@unixland.uucp> bill@unixland.uucp (Bill Heiser) writes: > >Does having a SCSI drive external to a 386 system, on a cable >several feet long, affect the data transfer rate? Is it significantly >better to have the drive mounted internally in the system? No (re: data transfer rate). Assuming a drive fits in your system's box, the advantage of internal mounting is transportability of your entire system; a possible disadvantage of internal mounting "may" be additional stress on your system's power supply if the drive in question has large startup-current requirements (such as the large Maxtor drives). The system I'm sitting at right now (an Amiga) has a 20 foot single-ended SCSI bus, properly configured, and sporting three large Maxtor drives, one Quantum 80S, one Fujitsu tape drive and one Archive tape drive. All SCSI cabling is PROPERLY daisy-chained and terminated at both ends and is 100% reliable; the last boot time was July 7, 1990 ... it's been up since then (to be fair, the entire system is on a UPS). Using the "DiskPerf" program, even the furthest drive (the Quantum) easily achieves over 700Kbytes/S for reads and writes. The additional several nS delay at the end of the bus matters not one whit at SCSI speeds. Thad Floryan [ thad@btr.com (OR) {decwrl, mips, fernwood}!btr!thad ]
gkendall@ncr-mpd.FtCollins.NCR.COM (Guy Kendall) (03/01/91)
In article <1991Feb23.194013.13400@unixland.uucp> bill@unixland.uucp (Bill Heiser) writes: > >Does having a SCSI drive external to a 386 system, on a cable >several feet long, affect the data transfer rate? Is it significantly >better to have the drive mounted internally in the system? > If you are using asynchronous SCSI, then the cable length does affect the transfer rate. If the external cable is only several feet long, then it should not make any noticable difference. I once heard that the SCSI signals travel at around 5.25 nS/meter and if you are only talking several feet then this will not add much at all to the transfer period. If you are using synchronous SCSI, then it is not supposed to make any difference at all. -- Guy W. Kendall - Applications Engineer gkendall@ncr-mpd.FtCollins.NCR.COM NCR Microelectronic Products Division Voice: 719-596-5795 1635 Aeroplaza Drive Fax: 719-570-6045 Colorado Springs, CO 80916 BBS: 719-596-1649