Appendix D - Maximum Data Rates for the LabJack U12 [U12 Datasheet]
How fast is the U12?
Definitions
sample = 1 measurement from 1 channel
scan = 1 measurement from all selected channels
Short Answer
Command/Response - 50 scans/second (up to 4 AI)
Command/Response - 25 scans/second (up to 8 AI)
Stream - 1200 samples/second (up to 4 AI)
Burst - 8192 samples/second (up to 4 AI)
Longer Answer
The LabJack U12 has 3 basic modes of operation: command/response,
stream, and burst.
Command/Response
- This is a software timed mode where the application program
sends a command and the LabJack responds with data. This mode allows
the monitoring/control of all inputs/outputs on the device. All
the command/response functions take up to 20 milliseconds to execute.
A couple of key functions are:
AISample
- Returns readings from up to 4 analog inputs and writes/reads the
state of the 4 IO pins.
AOUpdate
- Sets the voltage of both analog outputs, controls/writes/reads
all 20 digital I/O, and returns the current counter reading.
So,
for example, if you are monitoring 4 analog inputs and the 4
IO pins, you could call AISample up to 50 times per second and get
a data rate of up to 50 Hz per channel. If you want to monitor 8
analog inputs, you would call AISample twice and get a maximum data
rate of 25 Hz per channel. If you are doing feedback control, you
need to make 1 call to AISample and 1 call to AOUpdate for a maximum
loop rate of 25 Hz.
Stream
- This is an input only hardware timed mode where the LabJack
collects data at a rate controlled by it's own crystal, and stores
that data in an onboard buffer until read by the PC. The data is
transferred from the LabJack buffer to the PC buffer, simultaneous
with data collection, allowing the data to be streamed to disk continuously.
You
can read 1-4 AI channels at up to 1200 samples/second (so 4 channels
would be up to 300 scans/second). Alternatively, you can read 1
AI channel and the counter at up to 300 scans/second. In both cases,
the states of the 4 IO pins are also returned.
The
LabJack U12 cannot process any other commands while in stream mode,
and stream mode only works on 1 LabJack at a time (since there is
no way to synchronize).
Burst
- This is an input only hardware timed mode where the LabJack
collects data at a rate controlled by it's own crystal, and stores
that data in an onboard buffer. The data is transferred from the
LabJack buffer to the PC buffer after data collection is finished.
You
can read 1-4 AI channels at up to 8192 samples/second (so 4 channels
would be up to 2048 scans/second). The LabJack buffer holds up to
4096 samples. The states of the 4 IO pins are also returned.
The
LabJack U12 cannot process any other commands until the burst is
done.
Channel-To-Channel Delay - Unlike the U3/U6/UE9, when using stream/burst mode on the U12 all samples are evenly spaced in time. So if you scan 1 channel at 1000 scans/second, that channel is sampled every 1 ms. If you scan 2 channels at 500 scans/second, there is 2 ms between each scan and 1 ms between each sample (Channel1, 1 ms, Channel2, 1 ms, Channel1, 1 ms, ...).