Subject: Re: Apple SCSI cards term power
From: [email-masked] (David Empson)
Date: Mon, 26 Jul 1999 22:39:44 +1200
Newsgroups: comp.sys.apple2

Jason Whorton <[email-masked]> wrote:

> Hello.  I have a couple of questions concerning the Apple SCSI cards and
> termination power.
>     I have one of the High Speed cards and it does not have a diode between
> L1 and RP2.  Would the diode between L1 and RP2 be the only way that the
> term power mod could be made?

I can't answer this part, as I've never looked at modifying the card
(I've always used a hard drive which supplies termination power).

> ... mine does not have a diode between L1 and RP2, but it does seem to
> work fine on my IIe.  My SCSI system set-up on my IIe was as follows:
> 
> Apple High-Speed SCSI Card
> Apple System Cable (25-pin to 50-pin)
> Apple Cable Terminator
> Apple HD80SC
> 
> Does anyone know if the hard drive inside the HD80SC case provides
> termination power?

It probably does.  Hard drives sold by Apple which were potentially used
as an external drive on the Mac Plus and Apple II would have to supply
termination power.

> I am going to buy another HD**SC case and a much bigger hard drive to
> replace whatever drive is inside it.  I am currently planning to buy an
> Apple labelled 500MB HD out of a Macintosh.  Does anyone know if
> these drives provide termination power?

Possibly not.  Most Mac models after the Plus supply termination power
from the motherboard, so there is no need for the drive to do it.

Some drive manufacturers arranged things so that their drives always
supplied termination power.  So far both my Quantum drives (LPS240 and
Trailblazer 850) have done.  Apple used Quantum for a fair number of
their internal drives, so you might be lucky.

>     Does anyone kave any information on the regular Apple SCSI card?  Since
> the FAQ mentions that it does not provide termination power, how can I
> modify it to do so?

I imagine there would be a similar modification, adding a diode from the
+5V rail to the SCSI TERMPWR signal line.

> The person I got the card from said that some modification had been made
> to it.  I don't know what it was.  I think a ROM chip was replaced,
> though.

That would probably be for upgrading to the latest firmware (revision C,
ROM labelled 341-0437-A).

> What problems do I need to look out for when using the regular Apple SCSI
> card and the HD80SC?  The HD80SC will be the only SCSI device to ever be
> on the SCSI chain.

Nothing in particular comes to mind, assuming you have the latest
firmware.

If you did add another drive, you need to add a second terminator, since
the original Apple card isn't terminated.

The recommended arrangement for the original Apple card is:

Card --- Pass-through - First Drive --- Second Drive - Terminator
         Terminator

Either drive can be terminated internally instead of using an external
terminator.

-- 
David Empson
[email-masked]
Snail mail: P.O. Box 27-103, Wellington, New Zealand