Seeking recommendations for USB DAQ on Linux

Can be stacked. Works. Got one right here and some at clients. I do not use Linux but I believe there are drivers for it, just check the site. They also have a forum for Linux users.

On Fri, 11 Apr 2008 13:39:07 -0700, Julian Vrieslander rearranged some electrons to say:

National Instruments makes good-quality hardware, and excellent software support.

And it helps to have a bank account similar to Rockefeller's ...

Thanks to David and Joerg for the replies. I'll check into both NI and Labjack.

I was aware of the NI products, but a bit hesitant about buying from them. It's not their pricing. I do a lot of work with Macs, which they used to suppport quite well. But a more recent attempt to get assistance with an NI card in a Mac was less rewarding. Their attitude to Mac users now seems to be "Use LabView or don't call us...".

Maybe their support for Linux is better.

On Sat, 12 Apr 2008 09:27:19 -0700, Joerg rearranged some electrons to say:

If one doesn't have, or can't afford, a staff of programmers, then buying something that works out of the box and reducing your development costs has to be traded against buying cheap hardware with no software support. Most people aren't doing projects for free these days.

Well, the Labjack cost us $99 per device. Support is free and via a forum where (so far) I had the answers almost immediately. They seem to have that forum interface on their desktops all day long. There even is a special forum for Linux users.

Why USB? Ethernet has a lot of advantages..

Best regards, Spehro Pefhany

Gets a lot more expensive though. But it's a good point, you can even buy a LabJack for LAN connection. However, then they are north of $400.

Why is it that expensive? There are nice small modules with integrated ADCs and ethernet for $67 :

Mounted in a box, it shouldn't cost more than $100.

N.I. pricing is in the same ballpark as Agilent and Tektronix... even a little less in many cases.

But I would agree they're well out of the "hobbyist" price level.

Well, I guess they want to turn a nice profit :-)

The other reason may be that almost everyone (including me) buys the $99 USB LabJack so they have to amortize the NRE for the Ethernet version over a much smaller number of produced units. Also, it's not a raw module but it comes in a nice enclosure that can be bolted down to a panel. It has screw terminals for many the I/O and a D-Sub for some others. Those things are really practical.

Three USB parallel ports and a minihub? You'd have to check for glitches but it should work.

USB transfers packets in timeslots of 1 ms. I'm still learning the USB protocol, but I think if all three USB ports are on the same hub, each device will be accessed every 3 ms. And Linux is not a realtime system (without additional work), so if the OS thinks it is a good idea to process some network traffic, you might lost many milliseconds before the next update.

On a multitasking OS 2 latency and task switching interrupts will delay your signal much more then 2 ms, even on real-time Linux. If you only need to send every 2 ms that will work, but the data will arrive very irregular at the other end, both in case of par port, USB, or Ethernet,

As others mentioned, Ethernet has some advantages, I have a IIM7000A module (on headers) here that only cost me 25 Euro or so, including magnetics, it has a W3100A chip in it. Connect it to a 8052 (addressing) or perhaps PIC, and you have your solution. Source code is available free on the internet.

The simplest way (I do that also) is to connect a few PCF8574 IO expanders to 3 bits of the par port. I2C protocol is insensitive to long delays causes by task switching. Par ports still exist, and it is possible to unload the par port driver and do direct IO in Linux.

Yes, the lower msec range is the gray zone where a streaming mode may be required. The LabJack has that but I don't know if it can be used that slow. So far I have only used it at full speed.

The last i heard they have _no_ support for *nix.

On Sun, 20 Apr 2008 10:30:59 -0700, JosephKK rearranged some electrons to say:

Their web site says otherwise.

So it does. That is a welcome change.