Spectre GCR Manual Manuals - Atari Documentation Archive

Appendix D: SCSI Hard Disks
This "true" 50-pin SCSI is then routed to the disk drive controller.
The controller is sometimes built into the disk drive, sometimes not. If
you have multiple disk drive controllers, the 50-pin cable is routed to
each one (that means it connects to them one after another). You
absolutely cannot do things like ''Y'' cables or a circular cable. Note
that the common Adaptec 4000 controller can handle two drives by
itself, so a two drive system doesn't necessarily imply two controllers.
Also, Atari's MegaFile series of drives has merged together the
ACSI-SCSI board and the disk controller board, and doesn't use the 50pin
"true SCSI" at all. You can't get at true SCSI with that kind of hard
disk. (The older SH204 is fine).
Thus, in my system here (just as an example), the 19-pin cable
comes out of the Atari, and goes to an ICO board, thence to the
SLM804 1aser printer. The 50 pin cable starts at the ICO board's other
end. It's routed to an OMTI controller, then to an Adaptec controller,
then (finally) to a hard disk with built-in controller, where the
terminator is; this last hard disk happens to be a Mac-formatted drive.
I can unhook that drive and move it straight to the Mac, to move big
chunks of data around *fast*.
Check out Figure 24.
Apple uses their own SCSI. It comes out of the Mac on a 25-pin
connector, a OB-25, that looks identical to the ones on the back of the
ST (where you hook up a printer and modem).
It's fairly close to "true" SCSI, except Apple doesn't put any
"terminators" inside the Mac, like they should have. « - editorial
comment.) Now, you say, "true" SCSI is 50 pins ... how do they manage
with 25? Well, the 50-pin "standard" SCSI has odd pins as ground,
and some pins that aren't really used, so 25 is enough for the
essentials. NOTE: true SCSI pin 25 is NOT ground; every other odd
numbered pin is.
What you need to do is connect the Apple 25 pin connector to the
50-pin connector "true SCSI" ribbon cable inside the Atari drive. This
is a matter of connecting pin A to pin B for Apple's 25 wires, and is a
nice afternoon project with some Radio Shack style parts. I've included
the necessary pin numbers:
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