Re: Zip Drive- Apple High Speed SCSI Termination Power Mod

[Rubywand <rubywand@swbell.net>]

Matthew Pearce writes ...
> 
> Willie Yeo (yeo@pigeon.qut.edu.au) wrote:
> :
> :       If you are gonna use an Iomega Zip Drive on the Apple IIgs via an
> : Apple High Speed SCSI card, you must remember that the HS-SCSI card does
> : not supply termination power. Either you must have a SCSI device to supply
> 
> ?? Lots of people have told me this, but I haven't had to modify my Apple
> High Speed SCSI card at all... Did Apple fix this problem?
> 
> I bought the card in June of '91-if that makes any difference.
>

     It does not seem likely that Apple changed their design to supply
termination power. It was fairly common for SCSI hard drives to supply
termination power. If a user needs to have the Apple High Speed SCSI
card supply termination power, a simple mod will add this feature.

     SCSI normally specifies that termination power be present; but,
absence of termination power does not necessarily mean that a SCSI
device connected to a SCSI interface will not function correctly. The
Zip Drive uses a short cable and the Apple/Zip connection is not likely
to push the limits of SCSI I/O speed.

     Perhaps, your particular Zip Drive setup does not need termination
power. Or, maybe, someone has done the termination power mod to the
card.  


     The Apple High Speed SCSI termination power modification is not
difficult to do. It was referenced by Neil Parker not too long ago. It
is described in a .gif pic on Ground:

ftp or http://ground.isca.uiowa.edu/apple2/Docs/hispeed.gif

The pic shows a simple sketch of the back of the Apple High Speed SCSI
card near connectors 26-33. The directions say that you connect a 1N914
diode between two points:

the anode (non-banded end) of the diode goes to the *top* of L1.
the cathode (banded/striped end) of the diode goes to the >bottom< of
RP2


The pic shows the *top* of L1 to be a solder pad (just a solder pad with
no trace showing) a little ways up from a point between connectors 32
and 33.

The pic shows the >bottom< of RP2 to be just a bit up and to the left of
the *top* of L1. It is the lowest of several points (the pic shows 8)
arranged in a vertical column and should have a trace going off to the
left.     


Rubywand