Hi Jose,
The 9172 CompactDAQ chassis contains one System Timing Controller which has one Analog Input timing engine. This requires chassis containing different analog input modules to all be configured with one common analog input task which results in one sample clock rate. With slow sampling modules, if the sampling rate of a hardware-timed acquisition exceeds the maximum sampling rate of the module, they will return the same data repeatedly until a new conversion on the slower modules completes (no warning or error is generated). This provides for flexibility when mixing modules in chassis. More details regarding multi-module configurations can be found in the followings links (1,2).
The slower modules (DC modules) repeat their samples until a new conversion takes place. For example, if running an AI task at 1 kHz using a module with a maximum rate of 10 Hz, the slow module returns 100 samples of the first point, followed by 100 samples of the second point, etc. Other modules in the task will return 1,000 new data points per second, which is normal. These repeated samples are data points that are sent by the module, not merely duplicated by the driver in the software layer. This is important because each one of these repeated data points is actual data that passes through the 9172 FIFO and ultimately the Universal Serial Bus. Essentially this means that a slow sampling DC module will use, at a minimum, as much bus bandwidth as a higher speed module in the same task.
Since many of the slow sampling DC modules have 24bit ADC's, each sample is represented with 4 bytes as opposed to a 16bit module representing each sample with 2 bytes. This means that for a fast 4 channel 16bit module and a slow 4 channel 24bit module operating within the same task, the 4 channel 24bit module will take up more bandwidth even though it can only create new data at a fraction of the rate of the faster module.
The increased bandwidth the 24bit modules uses when paired with a fast 16bit modules needs to be considered because of USB 2.0's bandwidth specifications. It is possible to create a situation where the USB bandwidth is exceeded due to slow sampling modules running at a high rate. In this case, "error -200361: Onboard device memory overflow." is possible because the bus can't stream data from the onboard FIFO fast enough, causing the FIFO to overflow. Thus, I would suggest decreasing the sampling rate of DAQmx task to the level which is compatible with slowest module in your system, which is 9217.
Sev K.
Senior Systems R&D Engineer | Wireless | CLA
National Instruments
