[rescue] FC-AL T-Cards and Port Bypass Circuits.

Paul paul at techcenter3000.com
Fri Oct 3 07:14:47 CDT 2003

Ok, I've been looking into some FC-AL stuff. I've got a qlogic 2100 card, I'm
working on some cabling, and I'm looking into building my own t-cards.

Here's the interesting bit: Seems I can get PBC circuitry for ~$4.75 USD per
in qty , or a quad for ~$9.50.
(1.025 Gbps version, not much difference in the 2G version)

I'm thinking about going ahead and designing a board that would allow the use
of 4 drives per board, using more or less standard t-cards, but with one extra
pin. The second board would be a modified T-Card that would work on a
per-drive basis, but also be more expensive per drive.(9.50 < 4.75 * 4)

The four port version would have 6 DB-9's, 1 each per drive, 1 in, 1 out. A
simple loopback would 'terminate'  the out if not in use. It would also have
one molex using only the 5v line. A five pin external connector could be added
cheaply to have a remote fail LED panel to indicate a failed drive.

In case you're not familiar with this stuff, a big problem with FC-AL
(especially for hobbyists using straight T-cards) is that it's a loop that
goes THROUGH each device. So, if you have 32 HD's hanging off t-cards, and one
fails, the whole loop goes down. Bad news.

The PBC (Port Bypass Circuit) uses the 'Fail' pin built into FC-AL devices to
determine whether to bypass or 'route around' a failed drive. The Fail pin is
active, so if the drive is totally dead, it acts the same as a 'normal'

A really nice part about all this is it would be trivial to add hot-swap
capability. Possibly as trivial as a driver backed by a physical
pushbutton/switch. (I'm brainstorming details and need to do more research on
powerdown req's, and ID assignment. Can we reassign the same ID?
Automatically? What happens if I want to add a drive that is NEW, but have
pulled an 'old' one? How 'old' is 'old'? Like I said, I need to do more
research on this part!)

This would require RAID or OS support, you wouldn't want say /usr dropping
offline unexpectedly without some kind of error-recovery/report, but SW RAID
is pretty common.

So, the questions are, would anyone else here be interested in helping
develop/test this thing?
And, would anyone be interested, or know of people interested, in buying these
things if the price could be brought down? I'm guesstimating that that 4 port
addon could be built in small qty for somewhere between 50 to 75 bucks in
small qty. The T-Card could probably be built for about 30 to 40. These aren't
hard and fast prices, but guesstimates.

How'd ya like to be able to buy a four port enclosure with automatic bypass,
hotswap, failed drive indicators, and simple (easy to have fabricated) sleds
for (possibly) as low as $100.00?

I might include that the specs would be public. the would most definitely
include blueprints for the bloody drive sleds. (Ever wished you could get a
new drive sled MADE at your local plastic/metal shop?)

Please give feedback. Am I full of it, or do I have a good idea here?
Something I missed? Suggestions? (Donations/VC to get this thing moving
quickly? GRIN!)

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