Russell McMahon wrote:

>>Sounds like you need two controllers for sure. If you try to write 
>>them
>>in parallel without the full two controller setup, how will you know
>>when one fails?
> 
> The application is for photos. I'm a keen amateur photographer and in 
> some cases I'm extremely keen not to lose photos. And, the system 
> would be saleable if it worked well. [Build it before I do (and I 
> probably wont) and I'll buy one :-) ].

So you'd happily beta test a solution?

> If the system is good enough to write at least one good photo of the 
> pair with a suitably low error rate then the loss of an occasional A 
> or B copy is acceptable as long as both A & B are lost with only very 
> low probability. Say 1:1000 for A or B and 1:100,000 for A & B copies 
> of a single pghoto would be very acceptable. Better would be better.

Given the MTBF of CF cards, I'd expect better than 1:1E6 failure rate.

> What I am trying to avoid is the loss of a whole card full of data, as 
> has happened to me enough times so far as to make me nervous when I'm 
> doing critical photo sessions. Sometimes the card is recoverable by 
> formatting and somtimes its dead. 

This suggests that the problem is a SOFTWARE error, and so
having a redundant card would just mean that you have the SAME
error TWICE (e.g. on both cards). Perhaps your camera firmware
has a bug (and hopefully an update to fix it).

I am also surprised that you have had problems with cards
being 'dead'. Of course you are using high quality, brand name card,
like the SanDisk Extreme II and III series? Do you still have a dead
card around? Have you tried 'Image Rescue' software to recover?

Have you tried using a micro hard drive instead?

The only time I have had a bad card was when I was impatient and
pulled it out before the write completed. Even then, only the
last photo was garbage. But if the FAT is corrupted (file allocation
table, as used by the DCIM standard) you could have bit of a mess.

> Losing eg the middle  half of a 
> wedding service or similar would be 'unfortunate'. What I do now is 
> carry a number of CF cards and alternate amongst them whenever there 
> is a 'gap in the traffic'. That way, if one card of say 4 fails, you 
> lose a number of time stripes and not a whole block. 

A good plan, since it appears that the problem MAY be your camera
firmware.

 > I have a laptop setup to suck data when time allows.

> Next stage is to write to an 
> external drive, check the external and internal copies against the CF 
> card and only then delete it.

Presumably AFTER you have redundantly written the data to optical
media. Hard drives fail too.


In theory you could write to two absolutely identical cards in parallel, 
but only READ one (since the software needs to update the FAT by
reading it).
A small bit of logic would isolate the cards from the bus
so that a silicon failure in one wouldn't cause data corruption.

But would you be happy having your cards stick out an extra inch
or two to accommodate the needed connectors and logic?
(assuming the camera supports the extra thickness of two cards
at once.)

Given the form factor issues, would a CF to dual microSD card
be a solution? The penalty is much higher cost and
much smaller capacity, but at least it would completely
fit into a CF slot.

Robert

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist