Operating Modes
Before LabHelper can
acquire data, it must be given a sampling protocol which includes:
The initial input window of LabHelper (shown at right) offers a choice of 1 to 16 channels in four modes of operation:
You can also load a 'setup file' with a complete pre-recorded sampling protocol (setup files are stored from the OPTIONS window).
CHART MODE
gathers a fixed maximum amount of data from 1 to 16 channels (up to 32,670
cases); the operator can intervene at any time to stop acquisition before
the maximum number of samples is obtained. The computer automatically
selects the optimal gain on each channel to maximize resolution. Data
are plotted as line graphs, with a separate graph for each channel.
Data are stored either under manual supervision (i.e., the operator selects
a new file name for each set of data) or automatically under the autorepeat
option. In autorepeat mode, the operator provides a 'root' file name
before data acquisition starts. Subsequently the program automatically
saves data whenever its buffer is full, appending sequential suffixes (1,
2, 3, 4, etc.) to the file root name for each save. Sample rates for
single channels can be as fast as 1000/sec (or more) if you have a fast
CPU, but are slower (again depending on machine type) if the number of channels
is large.
OSCILLOSCOPE
MODE works like a typical oscilloscope. At low to moderate sampling
rates, data from a single channel are plotted from left to right.
Whenever the next point would 'fall off' the right edge of the screen, plot
position is swapped to the left edge and each new point 'erases' the previous
point at that x-coordinate. The operator provides a 'root' file name
before data acquisition starts. Whenever the TAB key is struck, the
existing screenfull of data is saved under the root name plus a sequential
suffix number (see detailed description in the SETTING
UP section). Unlike chart mode, oscilloscope mode uses a fixed
preset gain of 1, 2, 5, 10, 20, 50, or 100 when using a Lab-NB, PCI-1200,
or DAQCard-1200 (for inputs of 0 - 5, 0 - 2.5, 0 - 1, 0 - 0.5, 0 - 0.25,
0 - 0.1, or 0 - 0.05 volts, respectively). Negative only (i.e., -5
to 0 volts) and ± (i.e., -5 to 5 volts) ranges are also available.
For 16-bit cards, the gains are 1, 2, 10, and 100 on a plus or minus
10 V range. Use of a fixed gain maximizes sampling speed, and LabHelper
can record and display at rates of up to ~100 samples/sec on a Mac IIx or
IIcx (much higher rates are possible on faster machines). An oscilloscope
screen can contain up to 32,670 samples.
For most higher sampling rates (> 200 samples/sec), LabHelper shifts into discontinuous sampling. In this procedure, a screenfull of samples is obtained by the A-D card at a preset gain and rate. These data are displayed while the next screenfull is being obtained, with the cycle repeating continuously.
OSCILLOSCOPE
PLUS CHART MODE combines one oscilloscope channel plus one to four chart
channels. Note that sampling rates for oscilloscope and chart channels
are usually different. Because this mode is a combination, it is also
a compromise: The oscilloscope channel cannot sample as fast as in 'pure'
oscilloscope mode, sample averaging is keyed to the sample rate of the oscilloscope
channel, a total of only five channels is permitted, autorepeat is not available,
and sequential switching of external devices (see below) is not permitted.
You should also be aware that if the chart channel sample rate is high,
chart timing may be affected by the time lost when oscilloscope screens
are saved to disk. If you are saving to a hard disk, the latter problem
is usually not significant unless chart sampling intervals are less than
1 second or data files are very large. The TAB key saves oscilloscope
data, and the ESC (escape) key permits you to stop sampling to adjust sample
parameters or save chart channels. Large screens can contain up to
32,760 samples.
For higher scope sampling rates, LabHelper shifts into discontinuous sampling. In this procedure, a screenfull of samples is obtained by the A-D card at a preset gain and rate. These data are displayed while the next screenfull is being obtained, with the cycle repeating continuously.
MULTICHANNEL
OSCILLOSCOPE MODE works like a two- to four-channel storage oscilloscope.
The operator provides a 'root' file name before data acquisition starts.
Whenever the TAB key is struck, the existing screenfull of data is saved
under the root name plus a sequential suffix number (see the more detailed
description in the SETTING UP section).
Unlike chart mode, oscilloscope mode uses a fixed preset gain (1,
2, 5, 10, 20, 50, or 100). This yields input ranges of 0 - 5 volts,
0 - 2.5 volts, 0 - 1 volts, 0 - 0.5 volts, 0 - 0.25 volts, 0 - 0.1 volts,
or 0 - 0.05 volts, respectively; negative only (i.e., -5 to 0 volts) and
± (i.e., -5 to 5 volts) ranges are also available. For 16-bit
cards, the gains are 1, 2, 10, and 100 on a plus or minus 10 V range. Select
the gain and range settings from the pop-up menus in the SAMPLE
RATES window. Use of a fixed gain maximizes sampling speed
and is required for fast multichannel scanning. An oscilloscope screen
can contain up to 32,760 samples.
In Multichannel Oscilloscope mode LabHelper uses discontinuous sampling only: screenfulls of samples are obtained by the A-D card at a preset gain and rate. After the data are sampled, they are displayed while the next screenfull is being obtained, with the cycle repeating continuously. The lag time for updating the screen depends on the sample rate, the number of channels, the number of samples, and the overall speed of the computer. At low sample rates (and hence long sample periods), the response of the computer to keyboard commands is slowed. This is because the computer only checks for keyboard input between screenfulls of data, not when actually recording.
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