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From: kaufman@polya.Stanford.EDU (Marc T. Kaufman)
Newsgroups: comp.sys.mac
Subject: Re: Multiple Devices on the SCSI chain
Keywords: SCSI hard disks terminators
Message-ID: <11004@polya.Stanford.EDU>
Date: 1 Aug 89 04:36:55 GMT
References: <11004@burdvax.PRC.Unisys.COM>
Sender: Marc T. Kaufman <kaufman@Polya.Stanford.EDU>
Reply-To: kaufman@Polya.Stanford.EDU (Marc T. Kaufman)
Organization: Stanford University
Lines: 43

In article <11004@burdvax.PRC.Unisys.COM> jjs@PRC.Unisys.COM (James Solderitsch) writes:

>I have spent a frustrating day trying to add a 45 meg cartridge drive
>to my Mac Plus system (2.5 megs, system 6.02) that already was using
>and FX-20 (GCC 20 meg SCSI drive).  I thought I knew how to do it -- it
>should be trivial, right?  Maybe some of you with multiple read/write
>SCSI devices on the SCSI port have walked this road before.

The Mac Plus does not provide terminator power on the SCSI connector, and
most devices that have internal terminators, cannot power the terminators from
outside the unit.  If ANY device has terminators on (in) it, they must be
powered up for the bus to operate (otherwise they look like 135 ohms to
ground, which keeps the bus from sending any signals).

You have two choices:  NO terminators at all, which usually works with short
cable runs, or a terminator on the far-end device, which (usually) must be
powered by that device -- which means the device must be turned on.

>I have tried various combinations of device positioning (which drive is
>first on the chain) and terminator locations.  I have settled on the
>following for now:  FX-20 (boot device, priority 6) first, cartridge
>drive second (priority < 6), no terminators on the FX-20, terminator switch ON
>on the cart. drive.  This works mostly ok except I can't run the
>FX-20 by itself with the cartridge drive off.

Right, since the cartridge drive provides power to the terminators within it,
but those terminators can't be powered by the FX-20 (even if it puts power
out on the bus, which is probable, given your other configuration).

>If I put in that terminator
>to let the FX-20 run by itself, I can no longer use the cartridge drive
>as a data drive when booting from a floppy (with the FX-20 OFF)

because the FX-20 is not powering the terminator, and neither is the cart.
drive.

>What I want
>is to run with either of the devices on, separately or together, or off.

Try NO terminators, and short cables.

good luck.
Marc Kaufman (kaufman@polya.stanford.edu)