Advantech PCI-1736UP Spezifikationen PDF herunterladen (Seite 32)

Comedi

21 / 148

• TRIG_EXT: the ‘scan begin’ event occurs when an external trigger signal occurs; e.g., a rising edge of a digital line. scan_-

begin_arg chooses the particular digital line.

The scan_begin_arg used here may not be supported exactly by the device, but it will be adjusted to the nearest supported

value by comedi_command_test().

The timing between each sample in a scan is controlled by the convert_src events. The available options are:

• TRIG_TIMER: the conversion events occur periodically. The time between ‘convert’ events is convert_arg nanoseconds.

• TRIG_EXT: the conversion events occur when an external trigger signal occurs, e.g., a rising edge of a digital line. conver-

t_arg chooses the particular digital line.

• TRIG_NOW: All conversion events in a scan occur simultaneously.

The end of each scan is almost always speciﬁed by setting the scan_end_src event to TRIG_COUNT, with the argument being

the same as the number of channels in the chanlist. You could probably ﬁnd a device that allows something else, but it would

be strange.

The end of an acquisition is controlled by stop_src event. The available options are:

• TRIG_COUNT: stop the acquisition after stop_arg scans.

• TRIG_NONE: perform continuous acquisition, until stopped using comedi_cancel().

Its stop_arg argument is reserved and should be set to 0. (‘Reserved’ means that unspeciﬁed things could happen if it is set

to something else but 0.)

There are a couple of less usual or not yet implemented events:

• TRIG_TIME: cause an event to occur at a particular time.

(This event source is reserved for future use.)

• TRIG_OTHER: driver speciﬁc event trigger.

This event can be useful as any of the trigger sources. Its exact meaning is driver speciﬁc, because it implements a feature

that otherwise does not ﬁt into the generic Comedi command interface. Conﬁguration of TRIG_OTHER features are done by

INSN_CONFIG instructions.

The argument is reserved and should be set to 0.

Not all event sources are applicable to all events. Supported trigger sources for speciﬁc events depend signiﬁcantly on your

particular device, and even more on the current state of its device driver. The comedi_get_cmd_src_mask() function is

useful for determining what trigger sources a subdevice supports.

4.5.4 The command ﬂags

The flags ﬁeld in the command data structure is used to specify some ‘behaviour’ of the acquisitions in a command. The

meaning of the ﬁeld is as follows:

• TRIG_RT: ask the driver to use a hard real-time interrupt handler. This will reduce latency in handling interrupts from your

data aquisition hardware. It can be useful if you are sampling at high frequency, or if your hardware has a small onboard data

buffer. You must have a real-time kernel (RTAI or RTLinux/GPL) and must compile Comedi with real-time support, or this

ﬂag will do nothing.

• TRIG_WAKE_EOS: where ‘EOS’ stands for ‘End of Scan’. Some drivers will change their behaviour when this ﬂag is set,

trying to transfer data at the end of every scan (instead of, for example, passing data in chunks whenever the board’s hardware

data buffer is half full). This ﬂag may degrade a driver’s performance at high frequencies, because the end of a scan is, in

general, a much more frequent event than the ﬁlling up of the data buffer.

1 2 ... 27 28 29 30 31 32 33 34 35 36 37 ... 158 159

Keine Kommentare

Advantech PCI-1736UP Spezifikationen Seite 32