A-1-1 Stream Data Rates [T-Series Datasheet]

Streaming Data Rates

The fastest data rates on T-series devices occur when operating in stream mode.  Much of the command-response overhead is eliminated in stream mode because the device is responsible for initiating IO operations.  The device collects scans in its stream buffer, then the host application retrieves multiple scans at once.  The end result is a continuous data stream, sampled at regular intervals, collected with a minimum number of communication packets [2.].

There is an important distinction between scans and samples. Definitions are as follows:

• Address: Also called a channel.  An address usually returns the value of 1 input connection.

• Sample: A reading from one address.

• Scan: One reading from every address in the scan list.

• Scan list: The list of one or more addresses in a scan.

The scan rate is the rate at which scans are collected. It is a fraction of the sample rate, where the fraction is the inverse of the number of channels being read in a single scan.  The scan rate is defined as:

ScanRate = SampleRate / NumAddresses

The sample rate and scan rate are equal when the NumAddresses is 1.

Interchannel Delay - T4/T7

Interchannel delay is the time between each sample within a scan.  For example, say 3 channels are streamed at 1000 scans/second with ResolutionIndex=1 and Range=10.  That is a sample rate of 3000 samples/second, so from the table above the interchannel delay is 15 μs.  The stream interrupt will fire every 1000 μs, at which time it takes about 5 μs until the 1st channel is sampled, then 15 μs later the 2nd channel is sampled, then 15 μs later the 3rd channel is sampled, and then about 965 μs later the next scan starts. 

What if in the above example we wanted the 8 μs delay rather than 15 μs?  The sample rate must be greater than 60 ksamples/second for that, so the solution is increase sample rate by scanning more channels (channels can be repeated in the scan list) or scanning faster and discarding the extra data.

The interchannel delay is a fixed time with little jitter, so the known time can be accounted for in user software to adjust phase if those microseconds are important.  As an alternative to using the table above, the user can measure interchannel delay on their device by using a scope to look at the SPC timing output described in the Stream Mode section.

Notes

1. Various software issues need consideration when implementing a feedback loop that executes at the desired time interval.  Some considerations are: thread priority, logging to file, updating the screen, and other programs running on the machine.

1. The number of packets used to retrieve stream data depends on the number of data points allowed to accumulate in the stream buffer.

1. Setting the resolution index to 0 (default) in stream mode is equivalent to a resolution equal to 1.  The default resolution index in stream mode behaves different than command-response mode.

T4 Stream Rates

The T4 has a typical maximum sample rate of 50 ksamples/second.  This maximum is reflected in the first row of data in the following table (highlighted).  The scan rates reported are the maximum sample rates divided by the number of channels in the scan list (within ~10%).

The scan rates in the following tables are continuous over USB or Ethernet.

The scan rate is defined as (see "Streaming Data Rates" above):

ScanRate = SampleRate / NumAddresses

The minimum scan rate is 0.0157 Hz.

Table A.1.5. T4 Stream: Scan rates for different values of resolution index.  Applies to USB and Ethernet.  Applies to all streamable addresses including low-voltage and high-voltage analog inputs.

* Default stream Resolution index for the T4.

Table A.1.6. T4 Stream: Typical noise and interchannel delay values depending on resolution index.

* Default stream Resolution index for the T4.

** 40 µs for sample rate <= 20k.  13 µs for sample rate > 20k.

T7 Stream Rates

• Ethernet can usually maintain just under 120 ksamples/second.

• USB generally maxes out right around 100 ksamples/second.

• When using WiFi, the device can acquire data at the fastest rates, but transfer of data to the host is limited to about 1 ksamples/second, so the fastest stream rates cannot be maintained continuously. In this case, stream-burst can be used rather than continuous stream, where each stream is limited to a specified number of scans that fits in the device's stream buffer.  For high-speed wireless streaming, use the Ethernet connection with an external Ethernet-WiFi bridge.

The T7 has a typical maximum sample rate of 100 ksamples/second.  This maximum sample rate is achievable when a stream is configured with RANGE = ±10V and RESOLUTION_INDEX = 0 or 1 [3.].  This maximum is reflected in the first row of data in the following table (highlighted).  The scan rates reported are the maximum sample rates divided by the number of channels in the scan list (within ~10%).

The scan rates in the following tables are continuous over USB or Ethernet.

The scan rate is defined as (see "Streaming Data Rates" above):

ScanRate = SampleRate / NumAddresses

The minimum scan rate is 0.0157 Hz.

Table A.1.7. T7 Stream: Scan rates over various gain, resolution index, channel count combinations.  Applies to USB and Ethernet.

N.S. indicates settings not supported in stream mode.

* When streaming one channel, the maximum sample rate is not affected by range because the signal chain does not need to settle.

** The default stream resolution index for the T7.

The maximum scan rate decreases at higher resolution index and range settings simply because analog conversions take longer to complete. Single-channel stream is a slight exception—its maximum scan rate decreases at higher resolution indices but not according to range settings because it does not need to settle.

Table A.1.8. T7 Stream: Typical noise and interchannel delay values depending on range and resolution index.

N.S. (Not Supported) indicates settings not supported in stream mode.
* The default stream resolution index for the T7.
** 15 µs for sample rate <= 60k.  8 µs for sample rate > 60k.

T8 Stream Rates

The T8 has a maximum scan rate of 40 ksamples/second. Unlike other T-series devices, the scan rate does not depend on how many addresses are sampled per scan since all analog inputs are sampled simultaneously.

The scan rates in the following table are continuous over USB or Ethernet. For reliable streaming, refer to the T8's Maximum Stream Speed section in the Stream Mode documentation.

Table A.1.9. T8 Stream: Scan rates for different values of resolution index. Applies to USB and Ethernet, and all streamable addresses.

The maximum scan rate decreases at higher resolution index simply because analog conversions take longer to complete.

Typical noise values depend on range and resolution index. Command-response noise is the same as stream noise. The T8 does not have interchannel delay. Refer to Section A-3-3-2: T8 Noise and Resolution for details.