Xref: utzoo comp.periphs.scsi:888 comp.sys.mac.hardware:4771 comp.sys.apple2:4445 Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!usc!snorkelwacker!ira.uka.de!smurf!urlichs From: urlichs@smurf.sub.org (Matthias Urlichs) Newsgroups: comp.periphs.scsi,comp.sys.mac.hardware,comp.sys.apple2 Subject: Re: SCSI cable pinouts wanted Summary: Beware of spiky supply lines! Message-ID: Date: 9 Aug 90 18:55:05 GMT References: <4460UD182050@NDSUVM1> <32474@cup.portal.com> Organization: University of Karlsruhe, FRG Lines: 42 In comp.periphs.scsi, article <32474@cup.portal.com>, thad@cup.portal.com (Thad P Floryan) writes: < < Another caveat with multiple devices on a given bus is to be SURE that only < one is supplying +5VDC on SCSI pin 26 for terminator power. Some mfrs provide < jumpers to enable/disable, some feed thru a diode, some feed the power out < "raw", and some do nothing. < Any drive which feeds out power raw (or which grounds pin 26 when powered down) is severely broken. One wonders why a simple diode is so expensive. :-( < What I do on my system is supply terminator power from the host adapter, and < use an external terminator at the END of the bus. < If you do this (and possibly even if you don't), you should insure a clean power supply for the terminator. The new terminator from Apple (easily recognizeable because it's black; its use on the IIfx SCSI bus ranges from strongly recommended to absolutely necessary) has, close to the terminating resistors, both a 2.2 uF condensator and a 10 nF ceramic condensator to prevent spikes on the -Req line when all data wires change simultaneously. See Technical Note 273, which was recently posted to comp.binaries.mac. < Another observation I'd like to make is that MANY of the 3rd party consumer- < grade mfrs of SCSI HDs wire their boxes INCORRECTLY. SCSI specs mandate no < more than a 10cm "stub" off the bus, [...] Correct, unfortunately. Consider that most of these drives also have terminators built-in, and in that case they're supposed to be at the end of the chain anyway, in which case the above is perfectly legal. (Apple strongly recommends against shipping drives with built-in terminators, partly because of the confusion when installing the beasts, partly because of the reason stated above -- these condensators are uniformly missing.) BTW, if you do it "right", these two ribbon cables are going to be darn close together. Apple's newer SCSI devices fold these tightly and lead them through a largish ferrite ring. Does anyone know the reason for this? (It can't be to minimize crosstalk because you have to fold the ribbon cables tightly together to get them through the ferrite ring, or can it?) -- Matthias Urlichs -- urlichs@smurf.sub.org -- urlichs@smurf.ira.uka.de Humboldtstrasse 7 - 7500 Karlsruhe 1 - FRG -- +49+721+621127(Voice)/621227(PEP)