National Instruments NI-FBUS User Manual

Page 119

Appendix B

Troubleshooting and Common Questions

B-10

ni.com

The Fieldbus specification describes how to tell the LAS to skip probing
certain addresses in the range, to speed up how long it takes to detect new
devices on the bus (or devices that are having their addresses changed).
The two parameters involved in maintaining the list are called

FirstUnpolledNode

and

NumOfUnpolledNodes

, and they can be found

in the NI-FBUS Interface Configuration utility advanced settings for a port,
in the DLME Master Info section. The following diagram shows how the
LAS determines the list.

In other words,

FirstUnpolledNode

tells the LAS the beginning of a

region of addresses not to probe, and

NumOfUnpolledNodes

tells the LAS

the length of that region. So if

FirstUnpolledNode

was 0x25, and

NumOfUnpolledNodes

was 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.

Note

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 240T 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
32T 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

. For example, increasing

the

NumOfUnpolledNodes

parameter might fix a

SetAddress

NumOfUnpolledNodes

FirstUnpolledNode

Addresses

Address Polled for New Devices

0x10

0xff