National Instruments NI-FBUS - Manual (Page 43)

Chapter 2 Using the NI-FBUS Configurator
NI-FBUS Configurator User Manual 2-20 ni.com
Determining Function Block Execution Time
Because function blocks must execute serially, there is a trade-off between
how many PID loops you can run and how fast you can run them. The
maximum execution time for each block (that is, the worst-case execution
time) is specified by the device manufacturer. To view the maximum
execution time for a block, complete the following steps.
1. If you have not already done so, select View»Preferences and click
the Block View tab.
2. Select the Show Block Information checkbox, and click OK.
Now when you double-click on a function block, a new tab called
Block Information will appear.
Look at the parameter EXECUTION_TIME on the Block Information tab
for the function block. If the number is a hexadecimal number, you can
change it to decimal by right-clicking and unchecking Hexadecimal Data.
The decimal value of this parameter is the execution time in 1/32 ms,
as required by the Foundation Fieldbus specification (that is, dividing this
number by 32 will give the maximum execution time, in milliseconds).
For example, the maximum execution time for a National Instruments
FP-3000 AI-110 block (with firmware revision 2) is listed as 192. Dividing
this by 32 gives 6 ms maximum execution time.
Changing the Block Execution Order
To change the execution order of the blocks, complete the following steps:
1. In the Function Block Application Editor, click the Execution Order
button. Numbers appear on the blocks, indicating their current order,
as shown in the following figure.