> > The real upside is the digital hardware mostly goes away, just a clock
and
>
> You still need SOME buffer memory to get break-free continuous sampling.
> If you have to do the buffer anyway, you might as well make it big.
>

That would depend on several things, mostly the parts chosen. If the
interface is anything similar to the FT245 FIFO arrangement, you might be
able to get away from the memory altogether.

A system with a buffer needs several more things to make it effective.
First you add address generation, than a variable sample rate clock, of
course you also need a trigger. Then the trigger gets complicated because
you've got lots of different channels to trigger on (or combinations of all
of them), then there's multistage triggering. Of course then you (or I)
want to add pre and post triggering. Just the triggering alone becomes a
nontrivial exercise in digital logic, and forget about a PIC for that - if
I can't toggle a single pin faster than a couple MHz, I certainly can't do
any of this for a 10 to 100 Msps system.

A system with a sample rate capable datapipe gets rid of all those parts,
you can replicate them in software at the PC end. Yes, there's limitations
of the harddrive speed, but that only becomes an issue if you want to store
100's of meg of data. If all you'll need is a few meg (still larger than
any likely hardware buffer) the software just discards the data until the
trigger condition is met (might be a little more involved with a circular
buffer for pretrigger) then captures for a specified depth. Just because
the PC receives the data doesn't mean it needs to save it all :-)


> I'm hoping Bob uses standard DDR RAM and connectors, and reads the
> SID (serial ID) that tells the FPGA what timing to use on the RAM.
> Then you can use your cast-off RAM (from when you upgraded your PC).
> If physical size is an issue, we could look at SDRAM for laptops,
> with a top speed penalty. RAM is relatively cheap, so there
> is no reason to skimp on it.
>

I don't think PC RAM is a likely contender. Since it's dynamic, the refresh
generation would add significantly to the hardware complication factor.


> > a handful of logic to multiplex the data. Memory, address gen,
triggers,
> > pre & post trigger counters, all move to PC software. Wouldn't even
need
> > variable sample rates, just sample as fast as possible all the time and
> > have the PC software throw out anything you don't want.
>
> That does push a fair load onto the PC, so no using old laptops.
> That also gets around the aliasing problem since the LPF
> can be done in the PC.
>

Granted, But I guess that means we have to decide what is considered an
"old PC". We're definitely out of the realm of a 386 with a monochrome
screen. I would guess (and I invite discussion on this point) that we'd be
looking at maybe a PII @ 500+MHz, 128+Mb ram, win9x (or your favorite
flavor of Linux - just not win2k or XP)?

> > All depends on finding a suitable 1394 interface solution...
(suggestions
> > welcomed)
>
> Been looking. Not finding.
> Lots of nice chips from TI and others, but quite a bit of
> development time required to use. I'm now looking at the
> combo chips (FW/USB2) used in removable drive cases.
>
> Found a really cheap case with FW/USB2 c/w power supply
> for $80C. If you make the A/D subsystem look like an IDE drive,
> you basically issue a read command, and the data comes
> at you at ATA133 rates and magically appears on the PC hard drive.
>
> There is no reason that a PC program couldn't mount the
> 'drive' and the multianalyser firmware/driver make it look like
> a drive with continuously changing data for a given block.
> The read address would determine your sampling rate and
> channel gain. Simple enough that it could be platform independent.
>

That's actually an interesting possibility, don't know how practical it is
though. Is there any good info out there about implementing IDE connection
at ATA133 speed?

> If we want cheap, we need to use something that is mass manufactured
> for the consumer market.
> What could be cheaper than a drive case with FW/USB2 to ATA133?
> And you get a box to put the rest of the guts in.
>

With a P/S! (though I'm sure it'd need more regulation to work with high
end analog...)

-Denny

> Thoughts?
>
> Robert
>
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