Session based vs. Legacy interface in generating digital outputs via an NI device

Thread Starter


Joined Dec 4, 2014
Hi guys,
For my experiment, I transfer digital outputs from MATLAB via a National Instruments (NI) device (PCIe-6320), which has digital output ports having 8 channels each (e.g. port 0 has 8 channels: 0.0:0.7). Timing considerations are very important.

Before purchasing my current 64-bit MATLAB 2014 system, I used 32-bit MATLAB of an older version, with a different NI board - PCI-6224. For the 32-bit system I used "Legacy interface" code (putValue command) to generate outputs, and found that the average delay in execution time of a MATLAB line that updates a single channel from 0 to 1 (and vice versa) was 1.2 ms. Furthermore, the mean delay was proportional to the number of output channels simultaneously updated in the same MATLAB line, i.e. a MATLAB line that updated simultaneously 8 channels (0.0:0.7) from 0 to 1 would take 8X1.2 ms to execute.

Upon moving to the current system, A new computer with MATLAB 64-bit and PCIe-6320, I discovered that the Legacy interface set of commands (for generating and acquiring digital outputs and inputs) is not available for 64-bit, and that I must use the "Session-based" set of commands. I indeed used the outputSingleScan command to update the output lines. Surprisingly, the mean delay in executing a MATLAB line updating only one output channel was ~8 ms, and for updating 8 channels simultaneously - ~14 ms.

I have tested the new 6320 board through an external NI software and found that it is capable of updating the lines in a rate much higher than 1000 Hz (corresponding to the 1 ms delay I'd like to achieve), so I suppose that the difference between my old system and the current one has to do with the MATLAB interface - it's either the session-based vs. Legacy set of commands, which might be slower, or MATLAB 64-bit vs. 32-bit, regardless of the Digital Input/Output interface. Additionally, it could be due to heavier workload on my new current computer, although I did everything I could think of trying to reduce it.

I would really appreciate it if someone could enlighten me as to the difference between the Legacy and session-based interfaces, in a way that might explain the big delay and different behavior I see.

Thank you very much, Aviv.