Seeking recommendations for USB DAQ on Linux

[Julian Vrieslander]

I'm looking for a simple data acquisition interface that will run on a
Linux PC (Fedora Core 6). The requirements are very simple: 24 bits of
digital output. We need to be able to flip single bits or groups of
bits without causing glitches on unchanged bits. Data rate is slow -
maybe one write every 2 milliseconds.

Google shows that there are some simple USB-based DAQ interfaces that
might work for this application. I'm wondering if anyone here can
recommend a particular vendor. It would be nice if the product comes
with end-user support and easy-to-understand documentation, since the
people who will be installing and maintaining this system are
biologists, not computer geeks.

 

[Joerg]

I'm looking for a simple data acquisition interface that will run on a
Linux PC (Fedora Core 6). The requirements are very simple: 24 bits of
digital output. We need to be able to flip single bits or groups of
bits without causing glitches on unchanged bits. Data rate is slow -
maybe one write every 2 milliseconds.

Google shows that there are some simple USB-based DAQ interfaces that
might work for this application. I'm wondering if anyone here can
recommend a particular vendor. It would be nice if the product comes
with end-user support and easy-to-understand documentation, since the
people who will be installing and maintaining this system are
biologists, not computer geeks.

http://www.labjack.com/labjack_u3.php?prodId=52

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.

 

[Joerg]

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

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

 

[Joerg]

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.

 

[Joerg]

Why USB? Ethernet has a lot of advantages..

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.

 

[Frank Buss]

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 :

http://www.emacinc.com/som/somne64.htm

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

 

[Joerg]

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

http://www.emacinc.com/som/somne64.htm

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

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.

 

[przemek klosowski]

I'm looking for a simple data acquisition interface that will run on a
Linux PC (Fedora Core 6). The requirements are very simple: 24 bits of
digital output. We need to be able to flip single bits or groups of
bits without causing glitches on unchanged bits. Data rate is slow -
maybe one write every 2 milliseconds.

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

 

[Frank Buss]

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.

 

[[email protected]]

I'm looking for a simple data acquisition interface that will run on a
Linux PC (Fedora Core 6). The requirements are very simple: 24 bits of
digital output. We need to be able to flip single bits or groups of
bits without causing glitches on unchanged bits. Data rate is slow -
maybe one write every 2 milliseconds.

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.

 

[Joerg]

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.

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.

 

[JosephKK]

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.

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

 

[JosephKK]

Their web site says otherwise.

So it does. That is a welcome change.