LabJack U3 User Manual
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3.2 Stream Modetactntinuous hardware timedifiesrffer is emptied, and thenThe table below shows various stream performance parameters. Some systems might requirea USB high-high configuration to obtain the maximum speed in the last row of the table. A “USBhigh-high” configuration means the U3 is connected to a high-speed USB2 hub which is thenconnected to a high-speed USB2 host. Even though the U3 is not a high-speed USB device,such a configuration does often provide improved performance.Stream data rates over USB can also be limited by other factors such as speed of the PC andprogram design. One general technique for robust continuous streaming would be increasingthe priority of the stream process.A sample is defined as a single conversion of a single channel, while a scan is defined as asingle conversion of all channels being acquired. That means the maximum scan rate for astream of five channels is 50k/5 = 10 kscans/second.Table 3-4. Stream PerformanceFull resolution streaming is limited to 2500 samples/s, but higher speeds are possible at theexpense of reduced effective resolution (increased noise). The first column above is the indexpassed in the Resolution parameter to the low-level StreamConfig function, while the secondcolumn is the corresponding index for the Resolution parameter in the UD driver. In the UDdriver, the default Resolution index is 0, which corresponds to automatic selection. In this case,the driver will use the highest resolution for the specified sample rate.ENOB stands for effective number of bits. The first ENOB column is the commonly used“effective” resolution, and can be thought of as the resolution obtained by most readings. ThisThe highest input data rates are obtained in stream mode, which is supported with U3 hardwareversion 1.21 or higher. Hardware version 1.21 started shipping in late August of 2006. ConabJack for information about upgrading older U3s. Stream is a coLinput mode where a list of channels is scanned at a specified scan rate. The scan rate specthe interval between the beginning of each scan. The samples within each scan are acquiredas fast as possible.As samples are collected, they are placed in a small FIFO buffer on the U3, until retrieved bythe host. The buffer typically holds 984 samples, but the size ranges from 512 to 984depending on the number of samples per packet. Each data packet has various measures tonsure the integrity and completeness of the data received by the host.eSince the data buffer on the U3 is very small it uses a feature called auto-recovery. If the buffeoverflows, the U3 will continue streaming but discard data until the budata will be stored in the buffer again. The U3 keeps track of how many packets are discardedand reports that value. Based on the number of packets discarded, the UD driver adds theproper number of dummy samples (-9999.0) such that the correct timing is maintained.Low-Level UD Max Stream ENOB ENOB Noise InterchannelRes Index Res Index (Samples/s) (RMS) (Noise-Free) (Counts) Delay (μs)0 100 2500 12.8 10.0 ±2 3201 101 10000 11.9 9.0 ±4 822 102 20000 11.3 8.4 ±6 423 103 50000 10.5 7.5 ±11 12.542 |
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