External trigger

[Fredrik Ragnarsson]

Hi!

I'm trying to use an external trig signal to control the acquisition.

I would like to start the acquisition on positive edge and stop on negative edge and after that write the data to a file.

I'm reading asynchronously with

while(1) {

    n = read(comedi_fileno(dev), buf, bufsz);

// alt 1

cmd->start_src = TRIG_EXT;

cmd->stop_src = TRIG_COUNT;

cmd->stop_arg = X;

This set-up starts the acquisition by the external trig signal and it stops after X acquisitions.

// alt 2

cmd->start_src = TRIG_EXT;

cmd->stop_src = TRIG_NONE;

This set-up starts the acquisition but then it runs for ever.

// alt 3

cmd->start_src = TRIG_NOW;

cmd->scan_begin_src = TRIG_EXT;

cmd->scan_end_src = TRIG_COUNT;
cmd->scan_end_arg = 0;
cmd->stop_src = TRIG_NONE;

This set up starts a scan on positive edge and stops the scan on negative edge (read is blocking until the trigger goes high again). 

I can't find out how to get a notification when the trigger goes zero so I can write the data to the file and close it.

The options for my card NI pci-6023e are:

start: now|ext|int
scan_begin: timer|ext
convert: timer|ext
scan_end: count
stop: none|count

Any suggestions?