Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!mailrus!wasatch!uplherc!esunix!jsnow From: jsnow@esunix.UUCP (John Snow) Newsgroups: comp.sys.apple Subject: Homebrew SCSI pitfalls Keywords: SCSI Termination Message-ID: <1431@esunix.UUCP> Date: 9 Aug 89 17:08:26 GMT Organization: Evans & Sutherland, Salt Lake City, Utah Lines: 27 Here is a word of warning to those of you building homebrew SCSI drives for the Apple II family. APPLE VIOLATES THE SCSI SPECIFICATIONS for terminating the SCSI bus! The SCSI spec requires that the SCSI bus be terminated by 220/330 resistors at BOTH ends of the bus. The Apple II SCSI interface card does NOT terminate the cable. Apple normally gets away with this violation by supplying a very short SCSI cable to connect the SCSI interface card to the first disk drive where the SCSI cable IS terminated. This is poor design practice but normally works. I purchased a SCSI cable from Tulin along with a Tulin A-hive case and power supply. The SCSI cable Tulin sent me was 6 feet long -- much too long to work when there is no termination at one end. Sure enough, I spent a week trying to figure out whether my drive or my interface card was bad because things just weren't working. After a really frustrating week of trying different computers, drives, and interface cards and hooking a SCSI bus analyzer up to my system, I decided that the long cable and lack of termination was the problem. I hacked the 6 foot cable down to 18 inches and wired on a new connector and what do you know, it works! What would it have cost Apple to terminate things properly, less than a buck to put in the resistor packs and sockets to allow them to be removed if the CPU was not at the end of the cable! -- ----------------------------------------------------------------------------- John F. Snow UUCP: {ihnp4,decvax}!decwrl!esunix!jsnow Evans & Sutherland Computer Corp. or: uunet!utah-cs!esunix!jsnow Salt Lake City, Utah AppleLink PE: JohnSnow GEnie: J.SNOW2