3.2.1 Stream-Out [T-Series Datasheet]

Performing Stream-Out Using LJM

LJM's stream-out functions simplify the use of stream-out. They are available in LJM 1.2100 and later.

• To output a periodic waveform from the device, use LJM_PeriodicStreamOut.
• To output an irregular waveform, use LJM_InitializeAperiodicStreamOut and LJM_WriteAperiodicStreamOut.

Performing Stream-Out Manually (Advanced)

For each waveform being streamed out:

1. Choose which target channel will output the waveform
2. Configure stream-out
   1. STREAM_OUT#(0:3)_TARGET
   2. STREAM_OUT#(0:3)_BUFFER_ALLOCATE_NUM_BYTES
   3. STREAM_OUT#(0:3)_ENABLE
3. Update the stream-out buffer
   1. STREAM_OUT#(0:3)_LOOP_NUM_VALUES
   2. STREAM_OUT#(0:3)_BUFFER_F32 or STREAM_OUT#(0:3)_BUFFER_U16
   3. STREAM_OUT#(0:3)_SET_LOOP
4. Start stream with STREAM_OUT#(0:3) in the scan list
5. Stream loop: read and update buffer as needed
6. Stop stream

Executing stream-out for multiple output waveforms is a matter of performing the above steps in the order above and using corresponding STREAM_OUT#(0:3) addresses in the scan list.

1. Target Selection

The following target list represents the I/O on the device that can be configured to output a waveform using stream-out. The list includes the analog and digital output lines.

• DAC0
• DAC1
• FIO_STATE
• FIO_DIRECTION
• EIO_STATE
• EIO_DIRECTION
• CIO_STATE
• CIO_DIRECTION
• MIO_STATE
• MIO_DIRECTION

The above listed digital IO registers use the higher byte as an inhibit mask. Bits set in the inhibit mask will prevent the corresponding DIO from being changed. For example, writing a value of 0xFAFF to FIO_STATE will set FIO0 and FIO2 to high. FIO1 and FIO3-7 will remain unchanged.

Digital IO registers (FIO_STATE, EIO_STATE, CIO_STATE, MIO_STATE) do not configure direction. Use the corresponding _DIRECTION register (FIO_DIRECTION, EIO_DIRECTION, CIO_DIRECTION, MIO_DIRECTION) to configure direction. This can be done either before or during stream.

2. Configure Stream-Out

Configuration will set the buffer size and target. The target specifies which physical I/O to use. Data in the buffer will be output onto the target I/O as a generated waveform.

Configuration can be done before or after stream has started.

3. Update Buffer

Each stream-out has its own buffer. Data is loaded into the buffer by writing to the appropriate buffer register. Output waveform data points are stored in the buffer as 16-bit values, so values greater than 16-bits will be converted automatically before being stored in the buffer. Use only one buffer per STREAM_OUT channel.

For outputting an analog waveform (DAC output), write an array of floating-point numbers to the STREAM_OUT#(0:3)_BUFFER_F32 register.

For outputting a digital waveform, pass an array of integer 0 or 1 values to the STREAM_OUT#(0:3)_BUFFER_U16 register.

Once the waveform data points are stored, configure STREAM_OUT#(0:3)_LOOP_NUM_VALUES and STREAM_OUT#(0:3)_SET_LOOP.

4. Start Stream

Next, start stream with STREAM_OUT#(0:3) in the scan list.

The order of STREAM_OUT#(0:3) in the scan list determines when the target updated. For example, if STREAM_OUT3 is before STREAM_OUT0 in the scan list, STREAM_OUT3_TARGET will be updated before STREAM_OUT0_TARGET.

5. Stream Loop

Read from stream, if there are stream-in channels.

Also, if the output waveform needs to be updated, read STREAM_OUT#(0:3)_BUFFER_STATUS to determine when to write new values to the buffer. When to write values depends on how large the buffer is and how many values need to be written.

For a more thorough description of how a Stream-Out buffer works, see 3.2.1.1 Stream-Out Description.

6. Stop Stream

Stopping a stream that streams out is no different from stopping stream-in.

Example

This example demonstrates how to configure DAC0 to output an analog waveform that resembles a triangle wave, and also quickly measure two analog inputs AIN0 and AIN2 in streaming context.

Configuration steps specific to stream-out

STREAM_OUT0_ENABLE = 0                       –> Turn off just in case it was already on.
STREAM_OUT0_TARGET = 1000                    –> Set the target to DAC0.
STREAM_OUT0_BUFFER_ALLOCATE_NUM_BYTES = 512  –> A buffer to hold up to 256 values.
STREAM_OUT0_ENABLE = 1                       –> Turn on Stream-Out0.

With the LJM library, write these registers with a call to eWriteNames or multiple calls to eWriteName.

General stream configuration

STREAM_SCANLIST_ADDRESS0= AIN0         –> Add AIN0 to the list of things to stream in.
STREAM_SCANLIST_ADDRESS1= STREAM_OUT0  –> Add STREAM_OUT0 (DAC0 is target) to the list of things to stream-out.
STREAM_SCANLIST_ADDRESS2= AIN2         –> Add AIN2 to the list of things to stream in.
STREAM_ENABLE = 1                      –> Start streaming. LJM_eStreamStart does this.

With the LJM library, this is all done with the call to eStreamStart.

Other settings related to streaming analog inputs have been omitted here but are covered under the section for stream mode.

Load the waveform data points

The following data points have been chosen to produce the triangle waveform: 0.5V, 1V, 1.5V, 1V, so the next step is to write these datum to the appropriate buffer. Because it is a DAC output (floating point number), use the STREAM_OUT0_BUFFER_F32 register.

STREAM_OUT0_BUFFER_F32 = [0.5, 1, 1.5, 1]  –> Write the four values one at a time or as an array.
STREAM_OUT0_LOOP_NUM_VALUES = 4            –> Loop four values.
STREAM_OUT0_SET_LOOP = 1                   –> Begin using new data set immediately.

With the LJM library, write the array using eWriteNameArray, and write the other 2 values with a call to eWriteNames or multiple calls to eWriteName.

Observe result with stream mode

Every time the stream is run, AIN0 is read, then DAC0 is updated with a data point from Stream-Out0's buffer, then AIN2 is read.  Thus, the streaming speed dictates the frequency of the output waveform. 

Sequential Data

Once a sequence of values has been set via the STREAM_OUT#_SET_LOOP register, that sequence of values will loop and only be interrupted at the end of the sequence. Therefore, to have stream-out continuously output a sequence of values that is larger than the size of one stream-out buffer, probably the easiest way to do so is to:

1. Start by dividing the stream-out buffer into 2 halves,

2. Write one half of the buffer with your sequential data,

3. In a loop, every time the STREAM_OUT#_BUFFER_STATUS reads as being half full/empty, write another half buffer-worth of values.

Note that the buffer is a circular array, so you could end up overwriting values if you're not careful.

Here's an example:

Stream-out buffer is 512 bytes, divide that by 2 to get the number of samples the buffer can hold => 256 samples

256 samples divided by 2 to get the "loop" size, AKA the set-of-data-to-be-written-at-a-time size => 128 samples

Write 128 samples:

Write 128 to STREAM_OUT0_LOOP_NUM_VALUES

Write 128 samples to STREAM_OUT0_BUFFER_F32 (This should probably be done by array write, which is much faster than writing values individually.)

Write 1 to STREAM_OUT0_SET_LOOP

Loop while you have more sequential data to write:

Read STREAM_OUT0_BUFFER_STATUS

If STREAM_OUT0_BUFFER_STATUS is 128 or greater, write the next 128 samples, along with STREAM_OUT0_LOOP_NUM_VALUES = 128 and STREAM_OUT0_SET_LOOP = 1

Sleep for something like 1 / scanRate seconds to prevent unnecessary work for the hardware

Maximum Speed Estimations

T4: With 1 channel and a looping waveform, stream-out can perform at 40 kHz.

T7: With 1 channel and a looping waveform, stream-out can perform at 100 kHz. When streaming out to DACs at high speeds, you may notice effects of the time constant, depending on the output waveform.

For more information, see maximum stream speeds.