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Appendix B
Troubleshooting and Common Questions
NI-FBUS Hardware and Software User Manual
B-14
ni.com
correct value for this parameter can be dependent on the number of devices
on the link. For example, if you are using a bus monitor, you might see
a
WHO_HAS_PD_TAG
request going to the device to start the Set Address
sequence, and an
IDENTIFY
response coming back, but with the host never
continuing on to the next step of the protocol (the
SET_ADDRESS
packet).
This probably means that your T1 value is too small and should be
increased.
T3 is a parameter that describes the expected time for the device to respond
at its new address. This parameter is highly dependent on the number of
devices on the link, and the number of addresses being polled. Refer to the
Setting Number of Polled Addresses
section for instructions on how to set
the number of polled addresses. If you are using a bus monitor, you may be
able to see the host identify a device (with the
IDENTIFY
packet) at the new
address, before the devices has sent its probe response (PR) packet to the
host. This is an error that is indicative of a T3 value that is too small; if this
occurs, increase your T3 value until the
IDENTIFY
to the new address
occurs after the PR.
All of the System Management Info timers are in units of 1/32 of a
millisecond; for instance, T3 = 32000 units means that T3 = 1 second.
Setting Number of Polled Addresses
The Fieldbus specification describes how a Link Active Scheduler device
(LAS device) probes a list of addresses to allow devices to come online
during normal operation. The LAS sends a Probe Node (PN on the bus
monitor) packet to each address in its list of addresses during operation, and
the length of time between Probe Nodes depends on the number of devices
on the link and the setting of the Link Maintenance Token Hold Time
parameter.
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.