The following article is adapted from a posting to The iMac List written by Sathe Dilip. It is
reprinted with his permission.
After ordering it in March, I finally received my Microtech XpressSCSI
USB-to-SCSI converter cable about three weeks back. I wanted to use it
with my SCSI Zip drive. I had avoided buying a USB Zip, thinking that I
would be also able to use any other SCSI devices that are supported.
The data sheet on USB XpressSCSI said it worked with a Zip, so I was
counting on that.
After I received it, I called Microtech to ask them a couple of
questions about connectors, etc. In that call, I was told that the Zip
drive does not work alone with the XpressSCSI. You have to use a second
SCSI device on the chain and terminate it, I was told. I have an old
Quantum SCSI drive, which has a termination switch. So I connected
everything as I was told and started everything, only to find that even
the LED on the adapter won't light-up. Loaded the driver software -
nothing.
I called Microtech and was told that XpressSCSI receives its power
from the termpower (termination power) connection of the SCSI chain,
and that I should look for the termpower jumper on the Quantum drive. I
opened the drive, only to find that the drive did not have this jumper.
(I exchanged some email in this regard with LaCie, but that's a
different story). I was told that I will need to use a compatible
device (listed on Microtech's web page) or a powered terminator
(somewhere near $60).
I borrowed a Jaz drive (listed as compatible) from a friend to test
the thing out and connected it at the end of the SCSI chain. This time
the LED on XpressSCSI lit up, but I did not get any drive on the
desktop. So I called Microtech again. This time the tech support guy
was a good one. He explained what I needed to get the thing to work.
The list included latest drivers for anything I connect on the SCSI
chain and the latest update for USB class devices from Apple.
Microtech's manual says you will be prompted (by their software?) if
the device you have connected needs a driver, but I never received a
prompt.
In the meantime, I had also researched the subject of termpower and
found that:
- It is just a 5 volt supply (actually they use a Schottky diode in
series with the 5 volts from the drive's power supply, so it is a bit
less, something like 4.8 volts)
- Apple's implementation of SCSI with the 25 pin connector is
not per the SCSI standard.
- Older Macs and some Apple SCSI devices did not use the termpower
connection over the cable.
- The newer Macs connect pin number 25 of the SCSI connector to
enable use of termpower.
- My Quantum drive does not support term power.
Armed with this knowledge, I decided to make the Zip and Quantum
drives work with the SCSI adapter. I downloaded the latest drivers for
the Zip, as well as Apple's latest update for USB class devices. I
downloaded the upgrade to Silverlining (drivers used by LaCie, makers
of Quantum drives). I installed these on the iMac and the drive,
respectively. Then I proceeded with creating a 25 pin adapter that had
pin no. 25 fed with an external 5 volt supply (pin 24 is ground). I
connected this at the end of the SCSI chain, over the second 25 pin
connector on the Quantum.
Now I connected everything - and sure enough, everything worked. I
had the Quantum and the Zip showing on my desktop. I observed that Zip
Tools from Iomega did not work properly with my drive, but that is
probably not a Microtech problem. (I can use the SCSI Zip drive alone
with the iMac when I use the 5 volt adapter I made.)
In short, the USB XpressSCSI works, but it needs certain
considerations (compatible devices, termpower, latest drivers).
Microtech manuals are not much help. Their tech support gives out
advice in bits and pieces. If you do not have SCSI devices from your
earlier Macs and you just need to connect a storage device to your
iMac, I say just buy a USB ready device.
As to why Microtech chose to get the device power from termpower and
not from the USB port (all active ports are supposed to supply up to
500 mA; this adapter needs about 250 mA), I don't understand. A similar
device manufactured by Newer Tech seems to have the same approach to
device power, so it may have something to do with the SCSI
standard.
I have been wondering why the manufacturers have chosen SCSI
termpower as the power source instead of taking the power from the USB
port. I am coming to believe that they were probably targeting notebook
users in doing so. They did not want to deplete the notebook batteries
faster by taking the power from the USB port. However, they could have
added a switch and left the choice to the end user.
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