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Newsgroups: comp.sys.apple2 Path: news.weeg.uiowa.edu!news.uiowa.edu!hobbes.physics.uiowa.edu!math.ohio-state.edu!howland.reston.ans.net!wupost!waikato!comp.vuw.ac.nz!actrix.gen.nz!dempson From: dempson@actrix.gen.nz (David Empson) Subject: Re: How many drives can a GS take? Message-ID: <CpGwMK.LsF@actrix.gen.nz> Organization: Actrix Information Exchange References: <2pu4pg$b5q@eis.calstate.edu> <CpA2vw.G3C@actrix.gen.nz> <2qc3an$4f3@tuba.cit.cornell.edu> Date: Sun, 8 May 1994 05:01:32 GMT Lines: 66 In article <2qc3an$4f3@tuba.cit.cornell.edu>, Jay Krell <jmk3@crux4.cit.cornell.edu> wrote: > What David said, plus, here's a quote from the IIGS Firmware Ref: > > "You can attach up to two Disk II drives, two Apple 3.5 drives, and > two or more UniDisk 3.5 drives to the Apple IIGS disk port, depending > on IWM output specifications. A maximum of six devices can be connected > at any one time." Thanks for the correction, Jay. > I wasn't aware of the two Apple 3.5 drive limit. Here are the technical reasons for the limits: The Apple 3.5 Drive uses the drive select line (as do the 5.25" drives), which only has two states. I'm not sure if it uses ENABLE2 (pin 9), which is used to enable 5.25" drives; I suspect that it does, because of the brief blink that is visible on the 5.25" drive select light whenever a 3.5" drive is enabled. The reason you can have two drives of both types is that there is an extra enable line (3.5DISK, pin 4) which is 0 for a 5.25" drive, or 1 for a 3.5" drive. An interesting point here is that pin 4 is a ground line for the Disk ][, and presumably for the UniDisk 5.25 and other SmartPort devices. The 3.5DISK output in the IIgshas a series resistor, which should protect against the output being shorted to ground if there is no Apple 3.5 drive connected. The Apple 3.5 drive uses this signal (on its daisy-chain connector) to sense whether a 5.25" drive (or SmartPort drive?) is connected at the end of the chain and doesn't pass the signal through (the second drive does this if there are two connected). The UniDisk 3.5 uses the SmartPort protocol to communicate with the computer (packets of data sent between two intelligent devices). Addressing is handled by a 7-bit address byte, so you theoretically have 127 drives (address 0 is reserved for the host). The reasons for the drive ordering: - The Apple 3.5 drive must go first, otherwise it cannot sense the 3.5DISK signal. It doesn't interfere with any signals intended for SmartPort devices or 5.25" drives. - SmartPort devices (e.g. UniDisk 3.5) must go next, so that they can disable 5.25" drives (via the ENABLE2 signal) when a SmartPort access occurs. - 5.25" drives go last. References: IIgs Firmware Reference IIgs Hardware Reference (2nd edition) Inside Macintosh Volume III IIc Technical Reference (1st edition, possibly 2nd as well) -- David Empson dempson@actrix.gen.nz Snail mail: P.O. Box 27-103, Wellington, New Zealand