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3 - Operation [U3 Datasheet]

The following sections discuss command/response mode and stream mode.


Command Response

Command/response mode is where communication is initiated by a command from the host which is followed by a response from the LabJack.  Command/response is generally used at 1000 scans/second or slower and is generally simpler than stream mode. See Section 3.1 for details about command/response data rates.

Command/response mode is generally best for minimum-latency applications such as feedback control.  By latency here we mean the time from when a reading is acquired to when it is available in the host software.  A reading or group of readings can be acquired in times on the order of a millisecond.

Stream Mode

Stream mode is a continuous hardware-timed input mode where a list of channels is scanned at a specified scan rate. The scan rate specifies the interval between the beginning of each scan. The samples within each scan are acquired as fast as possible.  As samples are collected automatically by the LabJack, they are placed in a buffer on the LabJack, until retrieved by the host.  Stream mode is generally used at 10 scans/second or faster. See Section 3.2 for details about stream data rates.

Stream mode is generally best for maximum-throughput applications where latency is not so important.  Data is acquired very fast, but to sustain the fast rates it must be buffered and moved from the LabJack to the host in large chunks.  For example, a typical stream application might set up the LabJack to acquire a single analog input at 50,000 samples/second.  The LabJack moves this data to the host in chunks of 25 samples each.  The Windows UD driver moves data from the USB host memory to the UD driver memory in chunks of 2000 samples.  The user application might read data from the UD driver memory once a second in a chunk of 50,000 samples.  The computer has no problem retrieving, processing, and storing, 50k samples once per second, but it could not do that with a single sample 50k times per second.

The U3 supports 1 active stream at 1 scan rate.  Command-response calls can be used while a stream is active to read/write almost everything except analog inputs (which includes the internal temperature sensor), as stream mode takes over the analog input system.

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