mcconnel@b11.ingr.com (Guy McConnell) (02/21/90)
In article <8395@cg-atla.UUCP>, weber@cg-atla.UUCP (Jeff Weber) writes: > In article <11052@stag.math.lsa.umich.edu> derrick@cs.AthabascaU.CA (Derrick Rowlandson) writes: > >Original-posting-by: derrick@cs.AthabascaU.CA (Derrick Rowlandson) > >Original-subject: SCSI cabling > >Reposted-by: emv@math.lsa.umich.edu (Edward Vielmetti) > > > >After spending a couple hours trying to cable up some SCSI peripherals > >to a SUN i would like to pose the following questions. > > > >A SCSI bus consists of a 50 pin bus running from a SCSI controller. > >Bus length can be up to 15 feet. > UPTO 20 feet and that includes the bus lenght used *inside* the > enclosure of choice. Actually, 6 meters or just over 19.5 feet is the specified maximum length for SINGLE-ENDED SCSI. > >Where must a SCSI bus be terminated? Only once, and at the last device on > >the bus would seem to be the logical choise. > Yes. It is the last physical device not the highest id's device. The SCSI bus must be terminated on BOTH ends. In a typical configuration this would be done at the host (usually at one end of the bus) and at the last physical device on the bus as mentioned above. If, however, the host is somewhere in the middle of the bus (not an illegal configuration) the bus must be terminated at the devices on each physical end of the bus. > >When taping into a SCSI bus, the tap or stub lenght can only be a few > >inches in lenght i guess? > (LENGTH) This stub can only be 5inches long. I believe the maximum stub length is 4 inches but I don't have my SCSI spec handy to verify this. > >But can you get away with a longer stub lenght (LENGTH) > >if you also terminate that specific device? > No. Agreed. You are asking for strange and potentially data destructive problems if you do this. Guy McConnell Intergraph Corp. Mass Storage Peripheral Evaluation
terryk@pinocchio (Terence Kelleher) (02/21/90)
A further note on SCSI busses. Avoid mixing cable types. If you have flat ribbon cable, stick with it. Likewise for twisted pairs. Try not to have both twisted pairs and flat ribbon on the same bus. The errors you MAY see will be to bizzare to understand. Never use a round cable that has 50 seperate wires. Use flat ribbon, round ribbon (ribbon cable curled up in a round jacket) or twisted pairs. Try to use shielded cable. -- Terence Kelleher Encore Computer Corporation terryk@encore.com