GPIB
4-18 370B User Manual
The second part of data acquisition is to digitize and code the displayed curves.
This converts the curves to a series of binary numbers representing the horizontal
and vertical locations of a sequence of points along the curves. This part of the
data is called the curve. Together with the preamble it fully describes the
displayed curve.
Acquiring Data Within the 370B. To acquire data, set the display function to Store
mode . The command to do this over the bus is DISplay STOre. At this point the
current set of parameters are recorded and the curves being shown are digitized.
They can now be copied over the bus to the controller.
Data Structure. The preamble and curve are each a string of eight-bit bytes. The
preamble is a string of ASCII letters, numerals, and punctuation. Each character
is represented by one byte. The major part of a curve is a sequence of binary-
coded numbers, which is prefixed by a 25 character ASCII string identifying the
curve.
The curve data string represents the 370B display, where each data point in the
display is described as a 10-bit horizontal and 10-bit vertical position. The
horizontal and vertical positions are each coded as a pair of bytes in the data
string. Thus, there are four bytes for each data point. Since a display can have
from 1 to 1024 data points, the curve data can include from 4 to 4096 bytes. In
addition, there are two bytes at the start of the string for a byte count and one
byte at the end for a checksum value, for a maximum of 4099 bytes.
The numbers are coded in two’s-complement binary format. The low-order eight
bits are stored in the second byte while the two high-order bits are stored in the
low-order positions of the first byte. The sign bit fills the remaining bits in the
high-order byte.
Waveform data is transferred from the 370B to the controller in two parts, the
preamble and the curve data. As explained in the following discussion, these two
parts can be transferred together or separately.
Transferring the Preamble. To receive a waveform preamble from the 370B, send
the command query WFMpre? The response will be a string containing 17
parameters, each shown as a label and value pair. See the Table 4–10 in the
Command List portion of this section for details. Follow the command query
with an instruction to the controller to input the response, allowing for a string
about 410 characters long.
Hewlett Packard 200/300 Series:
300 DIM PREAM$(410)
310 OUTPUT 718;”WFM?”,END
320 ENTER 718;PREAM$
Moving Waveform Data to
the Controller
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