This is a summary and conclusion of using a NI6259 with Xenomai/Analogy for 
digital data I/O. First of all, many thanks to Alexis whose Analogy branch 
really allowed us to succeed! We would just like share the final results of our 
implementation with the hope that this might help others.

We needed a 19 bit parallel I/O for bi-directional communication between a host 
computer and a robot controller. The 32bit DIO of the NI6259 seemed to well 
suited. The communication protocol needs about 300 DIO commands per ms, where 
every about 20 commands the DIO port needs to switch from write to read and 
then back.

Initially we tried a4l_sync_dio() (synchronous communication). We measured that 
one a4l_sync_dio() takes about 5us. Additionally, the switch from read to write 
mode also seems to take some additional time. Thus, the communication speed was 
not sufficient.

A 2nd attempt was to use instructions and instruction lists using 
a4l_snd_insnlist() and a4l_snd_insn(). We measure that this can bring down one 
data acquisition to about 3.5us when all commands are queued up in an 
instruction list, but due to the read/write switch we needed, we cannot use 
very long instruction lists, and the read/write switch takes too long. Thus, 
the communication speed did not improve a lot.

A 3rd attempt was to use CMD structures. Following comedi examples, we create a 
200ns clock and triggered the CMD streaming with this clock. This allows VERY 
fast DIO. But, again, the read/write switches that are needed frequently made 
CMD structures inefficient as we could not cue up a lot of DIO commands before 
the next read/write switch, and in order to do the read/write switch, the CMD 
has to be aborted. We also noted that starting a CMD has some delays.

We realized that none of these approaches was feasible.

We create a simple IC-based circuit that allowed to branch the read/write DIO 
protocol such that we could use one DIO channel of the NI6259 for write only 
(never switched to read), and the 2nd DIO channel on the NI6259 for read only 
(no switching). By avoiding the read/write switching, all three ideas from 
above are possible and run fast enough (after some optimization of our 
communication protocol). We ended up using a4l_sync_dio() as it is the easiest 
to use, and yielded sufficient speed. With the CMD approach, we would be able 
to go a factor 5-10 faster even.

For you info, our interface C-code is attached. This is not general purpose 
software, but should allow other to get the idea of what we do.

Best wishes, and thanks again to Alexis!


Attachment: ni6259_interface.c
Description: Binary data

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