Xref: utzoo comp.periphs.scsi:1752 comp.sys.amiga.hardware:5830 Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!magnus.ircc.ohio-state.edu!tut.cis.ohio-state.edu!pacific.mps.ohio-state.edu!linac!att!pacbell.com!ucsd!mvb.saic.com!ncr-sd!ncrlnk!ncrwic!encad!entec!jlohmeye From: jlohmeye@entec.Wichita.NCR.COM (John Lohmeyer) Newsgroups: comp.periphs.scsi,comp.sys.amiga.hardware Subject: Re: SCSI cabling Message-ID: <778@entec.Wichita.NCR.COM> Date: 20 Jan 91 18:22:40 GMT References: <2251.AA2251@caleb> Reply-To: jlohmeye@entec.UUCP (John Lohmeyer) Organization: NCR Corporation, Wichita, KS Lines: 42 In article <2251.AA2251@caleb> jdp@caleb.UUCP (Jim Pritchett) writes: > >I am trying to make a cable to connect a Mac style DB-25 SCSI connector >(on my A2090 controller) to the standard 50 pin rectangular style connector >(NOT the Mac 50 pin Centronics style connector) on the bare SCSI drive. >Most of the connections are obvious, but I have a couple of questions. > >1. Is it ok to wire all the extra ground wires together? (The 50 pin >connector on the drive has lots more pins labelled ground.) Yes. Apple left a lot of grounds out on the Mac, which works okay for short cables and lower transfer rates, but there will likely be problems for high transfer rates over longer cables. >2. There is a pin on the DB-25 labelled 'TPWR'. What does this mean? >What should I connect it to? (Or, should I leave that pin open?) This is likely their shorthand for Terminator Power (or TERMPWR). It _should be_ a diode isolated, fused, 5 volt level for the express purpose of supplying power for the SCSI terminator resistors. If there is only one source of +5 volts on this line, you can get away without the isolation diode. If you use a diode on this line, it must be a low forward drop diode such as a schottky diode. The SCSI standard requires a terminator at each end of the SCSI bus. For single-ended systems (which are most common and you certain have on the DB-25 end), this termination consists of a 220 ohm pullup resistor and a 330 ohm pull down resistor on each signal line (18 total). On very short cables and slow transmission speeds, you can often get away with a single terminator, but I don't recommend this. How valuable is your data to you? There is a much improved terminator defined in the SCSI-2 standard. It uses a single 110 ohm resistor on each signal line attached to the output of a 2.85 volt voltage regulator. This terminator is much less sensitive to sagging TERMPWR voltages and it is a closer match to the characteristic impedance of most cables used for SCSI buses. -- John Lohmeyer John.Lohmeyer@Wichita.NCR.COM NCR Corp. uunet!ncrlnk!ncrwic!entec!jlohmeye 3718 N. Rock Rd. Voice: 316-636-8703 Wichita, KS 67226 SCSI BBS 316-636-8700 300/1200/2400 24 hours