Mark.Simmonds@bbs.oit.unc.edu (Mark Simmonds) (06/22/91)
I was wondering if somebody knew the pinouts for the external scsi cable
for a mac, the 25 to 50 pin cable.
Mark
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internet: bbs.oit.unc.edu or 128.109.157.30thad@public.BTR.COM (Thaddeus P. Floryan) (06/22/91)
In article <4193@beguine.UUCP> Mark.Simmonds@bbs.oit.unc.edu (Mark Simmonds) writes: >I was wondering if somebody knew the pinouts for the external scsi cable >for a mac, the 25 to 50 pin cable. Following is a repost of material I originally posted 29-Aug-1988. I have another diagram that details the signals, but cannot (quickly) find it online; the attached diagram uses abbreviations for the signals. Since originally posting the attachment, I've noted better conformance to the specs by new equipment, but I continue to abide my original suggestions and have experienced NO problems amongst considerably more systems than I originally indicated I use. The one issue that still gives me the willies is TERMPWR. What I've finally been doing for the past 2+ years is simply pulling pin 26 out of the 50-pin shell for interconnects before making cables to be SURE that there's no monkey business; this way the TERMPWR is supplied only by the host adapter and only seen by the termination pack at the end of the bus. The peace-of-mind achieved by this technique (re: TERMPWR) is worth it, and I no longer need worry about poorly-written configuration setups in mfr's manuals (even OEM manuals); I used to DEMAND the schematics so I could see for myself precisely how a given device interfaces to the bus; I don't (demand schematics) anymore. And note that I have no compunction about opening newly-purchased equipment and using a soldering iron to "fix" things! :-) In the following attachment, unless specifically stated otherwise, the mention of a given wire or pin refers to the 50-wire cable and not the D25 connector. And references to "SCSI specs" in the attachment refer to the original SCSI docs I got from John Lohmeyer's SCSI BBS (at NCR in Wichita KS; John is the Chair of the ANSI SCSI X3T9.2 committee, and provided the additional material at the end.) Hope this info proves useful to you, too. Thad Floryan [ thad@btr.com (OR) {decwrl, mips, fernwood}!btr!thad ] -------------------- begin originally-posted material Many systems supporting SCSI (Amiga, Mac, etc.) use a DB25 connector at the host-adapter end to save real-estate; the SCSI devices themselves use the "correct" 50-pin connectors (either Centronics 50-pin or the dual-25 row kind). The so-called "Mac System Cable" has a DB-25 on one end and a Centronics 50-pin connector on the other. SCSI interconnects are 50-wire. At the end of this posting are the pinouts of the DB25 and the 50-wire. One thing I've noticed (after adding over 1GB HD and a Fujitsu 190MB tape drive to my Amiga's SCSI bus) is that some device manufacturers take liberties with SCSI wires 20,22,24,28,30, and 34; some tie these to ground, some tie them to +5VDC, and some leave them (properly) unconnected. 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. What I do on my systems is supply terminator power from the host adapter, and use an external terminator at the END of the bus. From my experience, what I suggest is that when you're making the cabling, pull pins 20,22,24,26,28,30,34 from the connector that attaches to the embedded SCSI drive and/or SCSI translator. I've noticed that Apple's SCSI interconnect cables do NOT pass pins 20,22,24,28,30,34 (for the reason I noted above). 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, yet many Mac-type drives I've opened up (just gotta see what's in there! :-) have a 2" 50-wire connection joining the two Centronics connectors continuing with a 15" stub to the 50-wire connector on either an embedded SCSI drive or an Adaptec 4000A or 4070 translator. This mis-wiring would account for the problems people have running a SCSI bus out to 20 feet (I have no problems since I wired my systems correctly, and the last device is 19' from the host adapter). Remember: SCSI wiring is supposed to be DAISY-CHAINED, per: WRONG: CORRECT: ____________________ __________________ / \ / \ [] | [] | \ \_DISK \_DISK / / [] [] | \__________________/ The "[]" is a 50-pin Centronics connector SCSI Connector (DB-25) Pin Name ------------- 1 REQ 2 MSG 3 I/O 4 RST 5 ACK 6 BSY 7 GND 8 DB0 9 GND 10 DB3 11 DB5 12 DB6 13 DB7 14 GND 15 C/D 16 GND 17 ATN 18 GND 19 SEL 20 DBP 21 DB1 22 DB2 23 DB4 24 GND 25 N.C. (or) Terminator Power SCSI Connector (50 pin header) Pin Name Pin Name ----------------------------------- 1 GND 2 DB0 3 GND 4 DB1 5 GND 6 DB2 7 GND 8 DB3 9 GND 10 DB4 11 GND 12 DB5 13 GND 14 DB6 15 GND 16 DB7 17 GND 18 DBP 19 GND 20 GND <- caution: not always 21 GND 22 GND <- caution: not always 23 GND 24 GND <- caution: not always 25 N.C. 26 Terminator Power 27 GND 28 GND <- caution: not always 29 GND 30 GND <- caution: not always 31 GND 32 ATN 33 GND 34 GND <- caution: not always 35 GND 36 BSY 37 GND 38 ACK 39 GND 40 RST 41 GND 42 MSG 43 GND 44 SEL 45 GND 46 C/D 47 GND 48 REQ 49 GND 50 I/O -------------------- additional material from John Lohmeyer In article <8551@cup.portal.com> Thad Floryan writes: > ... text deleted... >One thing I've noticed (after adding over 1GB HD and a Fujitsu 190MB tape >drive to my Amiga's SCSI bus) is that some device manufacturers take liberties >with SCSI wires 20,22,24,28,30, and 34; some tie these to ground, some tie >them to +5VDC, and some leave them (properly) unconnected. ^^^^^^^^^^ These pins should properly be tied to ground. Unterminated signals can cause noise problems. Anyone who ties these lines to +5VDC not only violates the SCSI standard but will cook some fuses (or failing that, some power supplies). >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. The best thing to do is have all initiators (host adapters -- there may be more than one) provide TERMPWR through a diode and a fuse. This is what SCSI-2 requires. Targets may also provide TERMPWR through a diode and a fuse. Your comments on daisy-chaining are quite correct. I like your diagram. Apple and other vendors opted for the DB-25 connectors to save space. Obviously, something had to go so they eliminated some ground signals. They may have shot themselves in the foot. These grounds affect their ability to transfer data at high speeds. At least they kept the 50-pin connectors on their external boxes. If a future Apple processor needs really fast transfers, it will probably have to use all 50-pins. At least the peripheral boxes won't have to be scrapped. John Lohmeyer, X3T9.2 (SCSI) Chairman j.lohmeyer@Wichita.NCR.COM -------------------- end of attachments