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3.2.4 Low-Level Stream [T-Series Datasheet]

Stream mode is complicated but can easily be executed using the high-level LJM stream functions. LJM is recommend for all users, except users that need to integrate a T-series device into a system that cannot use LJM. The rest of this section is about manually executing stream protocol without LJM.

Executing stream mode involves the following:

  • Stream setup

  • Stream start

  • Stream-in data collection, if any stream includes stream-in channels

  • Stream-out buffer updates, if stream includes stream-out channels (See the Stream-Out section)

  • Stream stop

Spontaneous Stream vs. Command-Response Stream:

Data can be sent to the host in one of two data collection modes:

  • Spontaneous: In spontaneous mode, packets are automatically sent to the host as soon as there is enough data to fill a packet. The packet size is adjustable. See the register definitions below.

  • Command-Response (CR): In CR mode, the stream data is stored in the device‚Äôs buffer and must be read out using a command. CR mode is useful for when the connection is unreliable.

T-series devices connected via either USB and Ethernet are capable of both spontaneous stream and command-response stream.

T7-Pro only: T7-Pro devices connected via WiFi are capable of only command-response stream.

Setup

Manual stream setup requires configuration of the registers that LJM_eStreamStart automatically configures:

Additional Configuration Notes

Additionally, address 4018 (STREAM_DATATYPE) must be written with the value 0. Note that address 4018 (STREAM_DATATYPE) is not in ljm_constants.json and is not compatible with LJM_NameToAddress.

STREAM_ENABLE must be written last.

Stream Burst

Writing a non-zero value to the register STREAM_NUM_SCANS will cause the device to automatically stop sending stream data after a certain number of scans. We commonly refer to this noncontinuous stream-mode acquisition as a stream burst.

After STREAM_NUM_SCANS scans have been acquired, the device will no longer scan new data and the status/error STREAM_BURST_COMPLETE will be returned from the LabJack. Upon receiving STREAM_BURST_COMPLETE, stream should be fully disabled by writing 0 to STREAM_ENABLE.

In most cases, STREAM_NUM_SCANS should not be configured when using LJM. LJM will set STREAM_NUM_SCANS when using StreamBurst.

Data Collection

Spontaneous Stream: Once stream has been initiated with STREAM_ENABLE, the device sends data to the target indicated by STREAM_AUTO_TARGET until STREAM_ENABLE is written with the value 0. Stream-out streams that do not contain stream-in channels do not send data.

Modbus Feedback Spontaneous Packet Protocol:

Bytes 0-1: Transaction ID

Bytes 2-3: Protocol ID

Bytes 4-5: Length, MSB-LSB

Bytes 6: 1 (Unit ID)

Byte 7: 76 (Function #)

Byte 8: 16

Byte 9: Reserved

Bytes 10-11: Backlog Bytes

Bytes 12-13: Status Code

Byte 14-15: Additional status information

Byte 16+: Stream Data (raw sample = 2 bytes MSB-LSB)

Command-Response Stream: When collecting data using command-response stream mode, data must be read from STREAM_DATA_CR (address 4500). Data is automatically discarded as it is read. 

Modbus Feedback Command-Response Packet Protocol:

Bytes 0-1: Transaction ID

Bytes 2-3: Protocol ID

Bytes 4-5: Length, MSB-LSB

Bytes 6: 1 (Unit ID)

Byte 7: 76 (Function #)

Bytes 8-9: Number of samples in this read

Bytes 10-11: Backlog Bytes

Bytes 12-13: Status Code

Byte 14-15: Additional status information

Byte 16+: Stream Data (raw sample = 2 bytes MSB-LSB)

Backlog Bytes:

Backlog Bytes is the number bytes contained in the device stream buffer after reading. To convert BacklogBytes to the number of scans still on the device:

BacklogScans = BacklogBytes / (bytesPerSample * samplesPerScan)

Where bytesPerSample is 2 and samplesPerScan is the number of channels. 

Status Codes:

  • 2940: Auto-recovery Active.

  • 2941: Auto-recovery End. Additional Status Information is the number of scans skipped. A scan consisting of all 0xFFFF values indicates the separation between old data and new data.

  • 2942: Scan Overlap

  • 2943: Auto-recovery End Overflow

  • 2944: Stream Burst Complete

Stop

To stop stream, write 0 to STREAM_ENABLE. All stream modes expect to be stopped, except for burst stream (see STREAM_NUM_SCANS for more information on bust stream).

Code Example

A general low-level stream example written in C/C++ can be found here.

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