Specifications

The input and output SCP’s are configured using the SCPI programming language.
Analog SCP’s are measured with the VT1419A’s A/D. Configuring analog SCP’s
includes specifying what type of Engineering Unit (EU) conversion are desired for
each analog input channel. For example, one channel may require a type T
thermocouple conversion and another may be a resistance measurement. The
on-board Digital Signal Processor (DSP) converts the voltage read across the
analog input channel and applies a high-speed conversion which results in
temperature, resistance, etc. Digital input SCP’s perform their own conversions as
configured by the SCPI language.
When the Trigger System is configured and either generates its own trigger or
accepts a trigger from an external source, all digital input SCP’s latch their current
input state and the A/D starts scanning the analog channels. All measurement data
is represented as 32-bit real numbers even if the input channel is inherently integer.
The EU-converted numbers such as temperature, strain, resistance, volts, state,
frequency, etc. are stored in an Input Buffer and later accessed by ‘C’ programs
executing on the VT1419A card. Approximately 2,000 lines of user-written ‘C’
code can be downloaded into the VT1419A’s memory and can be split among up to
32 algorithms. VXI Technology refers to these as algorithms because an algorithm
is a step-by-step procedure for solving some problem or accomplishing some end.
Though the documentation continues to refer to the ‘C’ code as algorithms, they
may be thought of in traditional terms with each algorithm representing a ‘C’
function with a main() program which calls them.
The user-written ‘C’ algorithms execute after all analog/digital inputs have been
stored in the Input Buffer. The ‘C’ code accesses the measurement data like
constants with the names of I100-I163 representing the 32-bit real EU-converted
numbers. As seen in Figure 3-1, the algorithms have access to both local and global
variables and arrays. The I-variables are inherently global and accessible by any
algorithm. Local variables are only visible to the particular algorithm (just like in
‘C’ functions). Declared global variables can be shared by any algorithm.
Agilent's VEE can read or write any local or global variable in any algorithm by
using SCPI syntax that actually identifies the variable by name, but a more efficient
means of reading data is available through the VT1419A’s FIFO and Current Value
Table (CVT). As seen in Figure 3-1, any algorithm can write any expression or
constant to the FIFO/CVT. Agilent VEE can then read the FIFO/CVT to
characterize what’s happening inside the VT1419A and to provide an operator view
of any input/output channel, variable, or constant.
Output SCP’s derive their channel values from O-variables that are written by the
algorithms. O100-O163 are read/write global variables that are read after all
algorithms have finished executing. The 32-bit real values are converted to the
appropriate units as defined by the SCPI configuration commands and written to the
various output SCP’s by channel number.
Programming the VT1419A Multifunction
Plus
Overview of the VT1419A Multifunction
Plus
Chapter 3 49
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