USB-1808X streaming latency

[ScottB]

I am interested in a real-time control application that needs to stream data in with a latency below 1ms. Does the USB-1808X offer this capability?

 

Thanks!

[Mark Rivers]

The USB-1808X data sheet says that the data FIFO for analog input is 4 kS, i.e. 4,096 samples.  If the device is running at its maximum rate of 200 kHz on all 8 inputs, then the FIFO will fill in 4096/(200000*8) = 2.56 ms.  It seems that would be the worst-case latency under those conditions, and it is probably under 1 ms if they are emptying the FIFO when it is less than half full.

[JRys]

We did a few unofficial tests on the USB-1808X to see how fast a digital bit could be updated, and it turned out to be about 1000 times per second. Updating a value on an analog output was about the same. Both were asynchronous updates using a software loop with the Windows driver. Asynchronous analog input was faster, over 8000 times a second. Long story short, 100 Hz control is a more realistic number. 

 

[ScottB]

Thank you! I found this quite helpful. JRys, I intend to use the DAQ only for analog inputs. The outputs will be provided by another device. Do you concur with Mark's analysis of the latency for analog inputs? Is there a way for me to control the FIFO so that it empties more frequently?

Edited May 1, 2023 by ScottB

[JRys]

The device driver exclusively manages the FIFO; unfortunately, I don't have any performance data regarding latency. 

 

[ScottB]

JRys,

Do any products sold by MCC contain latency information that I can reference? This kind of thing is obviously vital to any DAQ application intended for control so I would be surprised if none of the MCC products have this information. Apparently the latency due to the USB connection itself is only about 100 microseconds so if the latency is too great for my application it must be due to either the FIFO or something else about how the data is processed before being sent over the USB connection. I don't need to know its exact value but I do need to know if it is vastly greater than the latency imposed by the USB connection itself.

[JRys]

Our products are not designed to be control devices, we don't consider them real-time, and there is no published latency information.