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Command(s) 4 to send the equations for any sensors that need special equations
(use only if needed)
Command 3 to start the sampling process
GET commands (one or more) to retrieve data from the CBL2.
It is important to notice that the channels get setup before sampling starts and that the
Command 3 starts the actual sampling. The GET command forces the host calculator to
wait until the data is ready and then transfer the data from the CBL 2 into the
calculator.
Only a few of the commands that can be sent during sampling are useful; many of
these commands will abort the sampling. For example, if you send a Command 1
during sampling, the sampling aborts, your data is lost, and the new channel is set up
according to the new command.
A more useful command to send during sampling is Command 7. If you send this
command, you can then do a GET and see the status of the CBL 2. The status will show
the sampling (ARMED – meaning that the unit has not gotten to the trigger condition
yet, or BUSY – meaning that sampling is in progress).
Another useful command to send while sampling is the Command 8, which reads back
the most recent data sample collected for a single channel. This allows a sophisticated
program to monitor data collection while the CBL 2 is collecting the data.
One of the new features of the CBL 2 is its ability to automatically work with many
digital sensors such as photogates. With a photogate, the timing of the data transitions
is the parameter of interest. Using the standard command order defined above, the
user would:
Send {1, 21, ...} to command a digital input channel
Send a command 3 to start sampling
Send a GET to retrieve the data.
In order to find the transition times, the data collection program must scan the data
looking for each transition and then subtract the time of the start and end points to
get the transition time. Using Command 12 automates this process (and makes it more
accurate).
There are a few things to be aware of, though. When using Command 12, the number
of samples collected is not the same as the number of samples taken. If the photogate
only transitions 12 times, there will be 12 transitions recorded. Even if there are 1000
analog data samples taken, there will still only be 12 transitions of the photogate data.
Therefore, you should command a SEND {12, 41, 0} to find out how many transitions
were recorded before requesting photogate data from the CBL 2. After getting the
number of transitions, you can retrieve the actual transition timing.
CBL 2 Technical Reference
35
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