Jumper setting on external SCSI Hard Drive

"DJW" writes:
>
> I am installing an older IBM 50-pin SCSI hard drive in an external case
> on an older power Mac. ...
> I have confirmed that I need to do the following:
> - Enable parity checking
> - Enable auto start

You might be able to leave auto-start disabled for a Mac.

With auto-start disabled, the drive doesn't spin-up until the computer
tells it to. On most PCs, this never happens, but I think Mac firmware
(especially on those Macs with built-in SCSI) will send the command.

If it works, I consider this a good idea. The startup-draw of many
devices starting simultaneously can sometimes overload a circuit. With
auto-start disabled, it's startup will happen after the other devices.

> - Disable sync negotiation
> - Disable write protect
> - Set the SCSI ID internally to a number between 1 and 6 not already
> being used (i.e. I will set it to the number that this hard drive is
> replacing) I need to set it internally for the box has no external
> switch for ID setting.

Ugh. I can't stand SCSI cases without any ID selectors....

> The two settings I can not find what to do with are:
> - Unit attention
> - Termination power

I'd leave both of these set to the factory default. Hopefully, the
diagram that has identified these pins also says what the defaults are.

I don't know what Unit attention is, so I can't help with that one.

Termination power is what the name implies - it tells the drive to
provide power on the SCSI bus for the purpose of driving a terminator.

SCSI terminators (especially active ones) require a power supply to
work. Some device (any device) on the bus may supply this power, but
something has to (or you'll need a power brick for your terminator,
which is a pain in the neck.) A lot of computers and SCSI host adapters
provide this. Many drives do as well, but there's no guarantee.

If the drive is jumpered to provide termination power and you don't need
it, it probably won't damage anything. The only exception I can think
of goes back to the old Adaptec 1542 cards. Those cards had a fuse on
the termination power line. That fuse could blow if more than five
devices were providing termination power at once.

In general, I'd leave term power turned off unless you find that you
really need it (because no other device is.)

-- David
 >> Stay informed about: Jumper setting on external SCSI Hard Drive 

In article , David C. wrote:
> You might be able to leave auto-start disabled for a Mac.
>
> With auto-start disabled, the drive doesn't spin-up until the computer
> tells it to. On most PCs, this never happens, but I think Mac firmware
> (especially on those Macs with built-in SCSI) will send the command.
My PCs Adaptec AHA2940UW will send the command. I have my
devices start in sequence to give my PS a chance to catch up. (they
are FULL hight SCSI 5 1/4" disks here!)
I got a Seagate 1GB in my mac performa that also defaults to
this. I haven't had a problem

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B'ichela
 >> Stay informed about: Jumper setting on external SCSI Hard Drive 

In article , DJW wrote:
> By what I can remember the last time I was in my computer's box I saw
> that termination power was jumped (enabled on?) on the internal CD. I
> use an active terminator device on the last scsi peripheral so I guess
> you are saying if I need the power set on I will not see the led lamp
> on the terminator device lit?
If No light, you need term power.

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B'ichela
 >> Stay informed about: Jumper setting on external SCSI Hard Drive 

B'ichela writes:
>>
>> With auto-start disabled, the drive doesn't spin-up until the computer
>> tells it to. On most PCs, this never happens, but I think Mac firmware
>> (especially on those Macs with built-in SCSI) will send the command.
> My PCs Adaptec AHA2940UW will send the command. I have my
> devices start in sequence to give my PS a chance to catch up. (they
> are FULL hight SCSI 5 1/4" disks here!)

Many Adaptec cards will send the command, but you often have to
explicitly configure this using the card's setup screen (typically
accessed by pressing a hot-key during the boot sequence.)

I haven't used a 2940, but the cards I have used came with this disabled
from the factory.

I remember tracking down several "my system won't boot" problems to a
drive with auto-start disabled and a SCSI card that doesn't send the
startup command.

-- David
 >> Stay informed about: Jumper setting on external SCSI Hard Drive 

"DJW" writes:
>
> By what I can remember the last time I was in my computer's box I saw
> that termination power was jumped (enabled on?) on the internal CD. I
> use an active terminator device on the last scsi peripheral so I guess
> you are saying if I need the power set on I will not see the led lamp
> on the terminator device lit?

That would be right. If your terminator has a light, and the light is
on, then some device is sending termination power to the bus.

-- David
 >> Stay informed about: Jumper setting on external SCSI Hard Drive