Appendix B
Troubleshooting and Common Questions
©
National Instruments Corporation
B-15
NI-FBUS Hardware and Software User Manual
In other words,
FirstUnpolledNode
tells the LAS the beginning of a
region of addresses to not probe, and
NumOfUnpolledNodes
tells the
LAS the length of that region. So if
FirstUnpolledNode
were 0x25, and
NumOfUnpolledNodes
were 0xba, then no addresses from 0x25 to 0xdf
would be probed. That means that if a device with an address of 0x25 were
placed on this bus, the LAS would not probe it, and it would never be able
to send or receive packets on the bus.
The reason to have a
NumOfUnpolledNodes
whose value is nonzero is
as follows. The LAS probes every address in the list, then starts over again
at the beginning. Because a device cannot come on the bus until its address
is probed, if the LAS is probing all 255 – 16 + 1 = 240 possible addresses
and each probe node request goes out every
T
milliseconds, it might take
240
T
milliseconds for a device to get on the bus. If, however, the LAS
probed only the first 16 addresses and the last 16 addresses, it might take
32
T
milliseconds for the device to get on the bus. This results in the
new device being recognized almost eight times faster.
These parameters also affect the Set Address protocol, because recognizing
a device at a new address is really the same as recognizing a completely
new device, as the new address must be probed for the device to come
online. In this way, the
NumOfUnpolledNodes
parameter can affect the
value of the Set Address protocol parameter
T3
, which is described in
the
section. For example, increasing the
NumOfUnpolledNodes
parameter might fix a SetAddress T3 problem
because it takes the device less time to be recognized at the new address.
Using Fieldbus with OPC
Starting with version 3.1, NI-FBUS has a separate OPC Data Access
Server, which is compliant with the
OPC Data Access 2.0 Specification
.
This OPC server supports VIEW-oriented I/O operations, and has better
performance.
NumOfUnpolledNodes
FirstUnpolledNode
Addresses
0x01
0xff
Address Polled for New Devices
