6
WAVEFORM STRUCTURE
INTRODUCTION
LOGICAL DATA BLOCKS
OF A WAVEFORM
This section discusses how to read and write waveforms and under-
stand their contents. Waveforms can be divided into two basic
entities, the basic data array (i.e. the raw data values from the
ADC’s in the acquisition) and the accompanying descriptive infor-
mation, such as vertical scale, horizontal scale, time of day, which
are necessary for a full understanding of the data.
The information in a waveform can be accessed using the IN-
SPECT? query which interprets it in an easily understood ASCII
text form. It can also be more rapidly transferred using the
WAVEFORM? query or written back into the instrument with the
WAVEFORM command. The oscilloscope contains a data struc-
ture called the template which is a detailed description of how the
waveform’s information is organized.
The template gives a detailed description of the form and contents
of the logical data blocks of a waveform. It is provided as a refer-
ence to be used by you and your programs. A sample template is
given in Appendix B although you are encouraged to use the
TEMPLATE? query to examine the actual template that your in-
strument is using. The template may change as the instrument’s
firmware is enhanced. The template will help provide backward
compatibility for the interpretation of waveforms.
Usually, a waveform will contain just a Waveform descriptor block
(1.) and a Data array block (5.). In more complicated cases
or more of the other blocks will be present. The data blocks are:
1.
Waveform descriptor block (WAVEDESC). This block in-
cludes all the information necessary to reconstitute the display
of the waveform from the data. This includes:
¯ hardware settings at the time of acquisition
¯
the exact time of the event
¯ the kinds of processing that have been performed
¯ the name and serial number of the instrument
¯ the encoding format used for the data blocks
¯ miscellaneous constants
2. An optlonal user-provided text (USERTEXT). The WFTX
command can be used to put a title or description of a wave-
form into this block. The WFTX? query command gives an
alternative way to read it. This text block can hold up to 400
characters. However, you should limit the length of each line to
about 45 characters, otherwise the text will be wrapped onto
the next line after 56 characters.
3. A block of sequence acquisition times (TRIGTIME). This
block is needed for sequence acquisitions to record the exact
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