National Instruments FP-3000 - Manual (Page 128)

Appendix F Troubleshooting
FieldPoint FP-3000 User Manual F-10 www.ni.com
Note that if you want to monitor values with an HMI package on the host
machine, you may have to slow down the execution rate. This is because
HMI communications are considered unscheduled communications by the
specification. Scheduled, publisher-subscriber Fieldbus communications
are of higher priority than nonscheduled HMI communications. If you do
not leave enough unscheduled time in your macrocycle, the HMI
communications will never get through.
How Much Unscheduled Time Should I Allow?
Start with 30-40% of the total schedule (macrocycle) available for
unscheduled communications. With slow devices (NOT the FP-3000) you
can often include a large part of the time that function blocks are executing
in your calculation of this percentage. This is because on slow devices only
a small part of the execution time involves broadcasting information over
the bus. During the rest of the execution time, there is no scheduled
communication on the bus. With the FP-3000, the function block execution
times are so short that there is usually not enough time to send an
unscheduled message between the scheduled broadcasts of the function
blocks. In this case, you need to include extra time in your macrocycle for
unscheduled communication.
Problems Using Manufacturer-Defined Features
NI-FBUS uses identifying information in the actual device to locate the
device description for the device. The identifying information includes four
resource block parameters:
MANUFAC_ID , DEV_TYPE , DEV_REV , and
DD_REV . If the identifying information is incorrect, NI-FBUS will not be
able to locate the device description for the device. When it has located the
device description, NI-FBUS matches the block types in the device
description with the actual blocks in the device by using the Item ID of the
block characteristics record.
If the blocks in the device do not match the blocks in the description, or if
there is no appropriate device description for the manufacturer, device type,
device revision and device description revision being returned by the
device, then there is a device description mismatch. In either case,
NI-FBUS uses only the standard dictionary (
nifb.dct ) and you will be
unable to use any manufacturer-supplied functionality.
These parameters can be read from the device’s resource block.
The following procedure will help you troubleshoot a
DD_SIZE_MISMATCH_ERROR by finding out if there is a device description
available on your computer that matches what your device expects.