HP VXI 75000 C Series, Руководство пользователя и программирования SCPI

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Creating and Running Custom Algorithms 123 Chapter 4
Channel Input Buffer (Input Phase) BEFORE algorithms are executed
during the Calculate Phase.
•
The execution of all defined algorithms (Calculate Phase) is complete
BEFORE output values from algorithms, stored in the Channel Output
Buffer, are used to update the output channel hardware during the
Output Phase.
In other words, algorithms don’t actually read inputs at the time they
reference input channels, and they don’t send values to outputs at the time
they reference output channels. Algorithms read channel values from an
input buffer, and write (and can read) output values to/from an output buffer.
Here are example algorithm statements to describe operation:
inp_val = I108;/* inp_val is assigned a value from input buffer element 8 */
O116 = 22.3;/* output buffer element 16 assigned the value 22.3 */
O125 = O124;/* output buffer [24] is read and assigned to output buffer [25] */
A Common
Error to Avoid
Since the buffered input, algorithm execution, buffered output sequence is
probably not a method many are familiar with, a programming mistake
associated with it is easy to make. Once you see it here, you won’t do this in
your programs. The following algorithm statements will help explain:
O124.B0 = 1;/* digital output bit on HP E1533 in SCP position 3 */
O124.B0 = 0;
Traditionally you expect the first of these two statements to set output
channel 24, bit 0 to a digital 1, then after the time it takes to execute the
second statement, the bit would return to a digital 0. Because both of these
statements are executed BEFORE any values are sent to the output
hardware, only the last statement has any effect. Even if these two
statements were in separate algorithms, the last one executed would
determine the output value. In this example the bit would never change. The
same applies to analog outputs.
Algorithm
Execution Order
The buffered I/O sequence explained previously can be used to advantage.
Multiple algorithms can access the very same buffered channel input value
without having to pass the value in a parameter. Any algorithm can read and
use as its input, the value that any other algorithm has sent to the output
buffer. In order for these features to be of use you must know the order in
which your algorithms will be executed. When you define your algorithms
you give them one of 32 pre-defined algorithm names. These range from
’ALG1’ to ALG32’. Your algorithms will execute in order of its name. For
instance if you define ’ALG5’, then ’ALG2’, then ’ALG8’, and finally ’ALG1’,
when you run them they will execute in the order ’ALG1’, ’ALG2’, ’ALG5’,
and 'ALG8'. For more on input and output value sharing, see “Algorithm to
Algorithm Communication” on page 130.