On Monday 08 August 2005 13:05, BRUCE STANLEY wrote: >--- Gene Heskett <gene.heskett@xxxxxxxxxxx> wrote: >> On Sunday 07 August 2005 15:53, Tony Nelson wrote: >> >At 3:19 PM -0400 8/7/05, Gene Heskett wrote: >> >>Nice idea Claude, but can you tell us how to tell the difference >> >>between the cables so that we can properly identify them? >> > >> >Just google on "cable select" (with the quotes) and you'll find >> > that a Cable Select cable has pin 28 connected to a ground wire >> > at one device connector (master) and not connected at the other >> > (slave); normally this is done by punching out a little bit of >> > wire 28 (on a 40 wire cable) just past the middle connector, so >> > you'll see a little hole in the cable. You'd also find that, as >> > normally a single device should be at the end of the cable >> > (slave) and a single device should be master, so using a Cable >> > Select cable with only one device is, umm, problematical. >> > >> >If one must use a Cable Select cable for only one device and it >> > doesn't seem to work quite right at either position, I suppose >> > one could just cut the cable after the middle connector, which >> > would make it the end connector. Cables are cheap. >> >> That solution has much to recommend it in terms of signalling >> integrity. As a general rule, there should <b>never</b> be any >> cable beyond the last connected device, and this is particularly >> dangerous in a scsi environment where as little as 3" of >> unterminated cable can and will cause data error problems from the >> open end echo's. I've been looking at udma100/133 cabling for >> years trying to figure out just how the hell they get away with >> the sloppiness and still make it work. But that sloppiness in >> termination also restricts the maximum length (18" absolute max) >> to something thats not always reachable in a big tower case. >> >> -- >> Cheers, Gene > >18" max? > >I just orded two new UDMA 66/100/133 ROUND (inclosed) cables for my >system that are 24" long. They also had 36" lengths.. Uh-huh, but will they guarantee them to work? The lists I'm on have bad feelings about round cables in general. File systems trashed beyond e2fsk's ability to fix seem to be connected to round cables at a higher than random association level. The only thing they are good for is improved air flow in the case. Hook one of them up to a sacrifical drive, set it to run at udma133 if it will, partition it with one big partition, then write a script that does a mke2fs of the filesystem, then mounts it and copies your working drive to it, then umounts it and does an e2fsck on it, then loops and does the mke2fs & everything else on it again. I've got a bottle of suds that says it won't be e2error free in less than 5 cycles of the script. At the data speeds being moved on these cables, echo's from unterminated ends, something thats totally disallowed for scsi setups, can and will create problems. These things are transmission lines even if atapi/ide do not treat them that way. I've not figured out how the drives and interfaces manage to ignore the echo's as well as they do. For background, I am a broadcast engineer who has been trying to keep the echos at bay for the last 40 some years, because echos=vswr, and high vswr=transmission line burnouts at the power levels we deal with. For us, the burnout repair bill can well exceed 20,000 dollars. All because the ice detector on the antenna silently failed and didn't turn on the heaters. This cable, the common ribbon, with crimped on idc connectors, has a characteristic impedance in the 124 ohm area. When used in a scsi setup, where the allowed cable lengths assume good terminations, the max allowed length is many meters, 39 IIRC, but the terms are so fussy you wind up sacrificing virgins to make it work. And those are rather hard to find... -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) 99.35% setiathome rank, not too shabby for a WV hillbilly Yahoo.com and AOL/TW attorneys please note, additions to the above message by Gene Heskett are: Copyright 2005 by Maurice Eugene Heskett, all rights reserved.
