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)

LONG 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 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.