For reading/operating lab instruments like multimeters and power supplies, the GPIB standard is used. This standard defines the type of cabling and connection.
However nowadays the interface cards for this type of card are awkward to get and are replaced by an USB-based GPIB adapter like this one:
In this weblog entry, I'm going to explain the situation where you have a Linux machine and a USB device, but as an application developer you don't know how to continue from here. This is all explained using a USB-based GPIB adapter.
These particular devices are handled on Linux using the default gpib_common.ko kernel module and a device-specific driver which the manufacturer distributes (in this case the ni_usb_gpib.ko module).
However that's not the end of it. When the device is plugged in, it has to be connected to a device file like /dev/gpib0. This is done by loading the modules, loading the firmware (with some particular devices), running the gpib_config utility and setting the correct permissions on the device file.
We want this done automatically on our Debian machines. We already have compiled and repackaged the gpib_common.ko and ni_usb_gpib.ko modules, and have installed these.
First we need to know if the module will be loaded when we connect the device. The kernel handles loading appropriate modules by calling modprobe. Typing the following command can tell you whether modprobe knows what to do when such a device is connected. Replace the 'grep gpib' with 'less' if you don't know what string you're looking for.
$ modprobe -c | grep gpib
If that's fine, then connecting the device should load the modules. Type the following command to check this, again replacing the 'grep gpib' with something more appropriate:
$ lsmod | grep gpib
Connect the device and check for changes. If a device driver is loaded, then a udev rule should be written. If not, re-examine the output of the modprobe -c command. An example extract:
$ modprobe -c | grep gpib ... alias usb:v3923p702Ad*dc*dsc*dp*ic*isc*ip* ni_usb_gpib alias usb:v3923p709Bd*dc*dsc*dp*ic*isc*ip* ni_usb_gpib ...
Observing one of those lines a bit closer:
alias usb:v3923p702Ad*dc*dsc*dp*ic*isc*ip* ni_usb_gpib
^^^^ ^^^^
The vendor ID The product IDIn my case, no driver got loaded, because the vendor and product numbers that are outputted by modprobe don't match with the USB device its vendor and product number.
To see what the vendor and product numbers of the USB device are, type:
$ lsusb ... Bus 007 Device 003: ID 3923:702b National Instruments Corp. ...
Check the string 1234:5678 just before the name of the device. The first part is the vendor number, the second part is the product number, both hexadecimal. As you can see, in this case the vendor number matches, but not the product number.
Add a new file in /etc/modprobe.d and then add a line in there which looks like the lines in the existing modprobe configuration. Adapt the vendor and product numbers as necessary. Then type lsmod and connect your device. Type lsmod again to see whether the appropriate driver loaded. If not, did you make a typo in the vendor or product numbers?
Now that the driver is loaded, we go on to writing udev rules. Check the Writing udev rules document for this.
Some tips for writing udev rules follow.
First you find the device details using:
$ find /sys -name dev | grep usb
Find the device its path in /sys and use it with udevinfo. Note that some devices require firmware to be loaded. udevinfo then will not show a lot of information. The device must then be identified purely by its product and vendor ID.
$ udevinfo -a -p /sys/class/usb_device/usbdev2.18/dev
Before you plug/unplug the device, run the udev monitor to see what events are fired off:
$ udevmonitor
To test whether your rule fires off, use logger in your udev rule:
SUBSYSTEM=="usb", DRIVERS=="usb", ATTRS{idVendor}=="3923",
ATTRS{idProduct}=="709b", ACTION=="add",
RUN+="/usr/bin/logger My rule fires off!"Note that the above line is actually ONE line, without any linefeeds except at the end.
When you want to load firmware upon connecting the USB device, you might need to pass the location of the device to the firmware loader. It's possible to use the value of sysfs attributes in your RUN or PROGRAM part of the udev rule:
SUBSYSTEM=="usb", DRIVERS=="usb", ATTRS{idVendor}=="3923",
ATTRS{idProduct}=="702b", ACTION=="add",
RUN+="/usr/bin/logger Running /usr/sbin/gpib_config for device
USB-GPIB-B on bus %s{busnum}, device %s{devnum}"The %s{...} is substituted for a particular sysfs attribute its value.