Re: Apple II comp.sys.apple2 FAQs 4/16/98 ZIP DRIVE & TAPE

[Rubywand <rubywand@swbell.net>]

Willie Yeo writes ...
> 
> Rubywand (rubywand@swbell.net) wrote:
> 
> : 005- How do I install a Zip drive?
> 
> : Set Termination-  Another consideration is termination. If all you have
> : connected to your SCSI card is the Zip Drive, then you should set the
> : switch on the back to turn ON the termination. If you have other devices
> : that come after the Zip, then you should set it to OFF.  The last device
> : in the chain (the one farthest from the interface) should have
> : termination set ON.
> 
>         Don't forget that Apple High Speed SCSI does not supply
> termination power. A Zip drive connect directly to the GS by itself
> (that's what I have at home) via the Apple HS SCSI card requires either
> another device that can supply termination power, or requires a hardware
> hack on the Apple HS SCSI card to provide the termination power.
> 
>         Else, the Zip drive will not work properly with the GS.


     Thanks for mentioning this! A problem I ran into with the Zip Drive
& Tape FAQs is balancing SCSI coverage with Zip-specific questions.
(SCSI is covered in more depth in the Hard Disk & SCSI FAQs. See the
excerpts below.)

     The need for termination power should be mentioned in all SCSI
related FAQs. (It is noted in the CD-ROM FAQS.)  I'm planning to send an
up-dated version of the Revision 003 FAQs to Ground in about a week.
Your observation will be added to the Zip Drive & Tape FAQs.  


Rubywand 




____________________________


Excerpt from Hard Disk and SCSI FAQs

file F003HDNSCSI.TXT on Ground at ...

ftp://ground.isca.uiowa.edu/2/apple2/Faqs
http://ground.isca.uiowa.edu/2/apple2/Faqs


>>

From: David Empson

007- What is SCSI "termination power"?

     At least one device (SCSI card or any SCSI drive) must provide
power for the SCSI terminators by feeding 5 volts onto the TERMPWR line
on the SCSI bus.

     Usually, termination power is fed through a diode to prevent
backfeeding from a higher voltage source in case some other device is
also supplying termination power. A good implementation will have a fuse
to protect against shorts and a capacitor to cope with a sudden rise in
termination power drain.

     The Apple SCSI cards do not provide termination power (though some
recent Apple Hi-speed SCSI cards were modified by Apple to provide
termination power).  The RamFAST SCSI card can supply termination power.

     If a drive can supply termination power, I recommend letting it do
so. The TERMPWR line can, in some cases, represent a significant load on
the +5V rail going to the Slots. Both of my Quantum drive mechanisms
provide termination power to the SCSI bus, avoiding the need to supply
it from anywhere else.

----------------------------


From: Rubywand

     On the RamFAST SCSI RevC card, DIP switch #1 is set to ON to supply
termination power. On other RamFAST SCSI cards, a jumper is placed at
JP1 to supply termination power.

     According to RamFAST documentation, it is okay to have the card set
to supply termination power whether or not another device does with a
few notable exceptions. If a connected hard disk (e.g. a Sider drive)
has a sticker saying that the drive supplies termination power and that
the interface must not, then the RamFAST must be set to _not_ supply
termination power.

____________________________


 ....


Related FAQs Resources: R009HSSTMOD.GIF (GIF pic)

018- How do I modify my Apple Hi-Speed SCSI card to supply
     Termination Power?

     The Apple High Speed SCSI Termination Power modification consists
of adding a diode. It is shown in resource file R009HSSTMOD.GIF from
Harold Hislop and Dan Brown.

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 >bottom< of RP2 is 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.     

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