continous acquisition

I use the board PCI-1714 from Advantech.
It has 4 Analog Inputs, i will use 2 of them.

I will use the command mode and configurate it so, that i receive samples on and on.
n_chan = 2;
subdevice = 0;

cmd->subdev = subdevice; // AI
cmd.flags = 0;
cmd->start_src = TRIG_NOW;
cmd->start_arg = 0;
cmd->scan_begin_src = TRIG_TIMER;
cmd->scan_begin_arg = 40000;
cmd->convert_src = TRIG_TIMER;
cmd->convert_arg = 40000;
cmd->scan_end_src = TRIG_COUNT;
cmd->scan_end_arg = n_chan;
cmd-> ;stop_src = TRIG_NONE;
cmd->stop_arg = 0;
cmd->chanlist = chanlist;
cmd! ->chanlist_len = n_chan;
cmd->data = data;
cmd->data_len = ???;

chanlist[0]=CR_PACK(chan+0,range,aref);
chanlist[1]=CR_PACK(chan+1,range,aref);

the goal is to receive every 40us a new sample of every channel

my questions are:
- if these sources and their arguments are correct?
- how should i specify the datalen, if i would have samples on and on?
- how should i specify the length of a sample?
- is it possible to write the ADC result in one variable and a interrupt or anything else is informating me about the new sample

thank you for any help!