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Chapter 4

Block Reference

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 National Instruments Corporation

4-9

over the Fieldbus using a host configuration tool, the block that is sending 
data to the other block is called the publisher. The block receiving the data 
is called the subscriber. When communication is established from a 
publisher to a subscriber, the subscriber takes on the value and the status of 
the published variable. If communication is not established between the 
publisher and the subscriber, the subscriber has a status that reflects the lack 
of communication. Statuses themselves are composed of three subfields: 
the quality, the substatus, and the limit.

Table 4-3 describes the 

Quality

 subfields.

The 

Substatus

 field is used to describe more specifically the cause of the 

given quality. For example, a status with the quality of 

Bad

 might have a 

substatus of 

Device Failure

, indicating that the value should not be 

trusted because it is from a device that has failed. Another common 
substatus is 

Non-specific

. The 

Non-specific

 substatus is used when 

no other substatus applies. There are too many substatuses to explicitly list.

Table 4-3.  Quality Values

Bad

The value is bad, the sensor is defective, or 
communication has not been established. The 
value should not be trusted by the receiver.

Uncertain

The quality of the data is unknown. This can be 
caused by errors or a lack of calibration in the 
physical I/O transducer. Blocks can generally be 
configured to treat values of 

Uncertain

 quality 

as either 

Bad

 or 

Good

 with the 

STATUS_OPTS

 

parameter.

Good_NonCascade

The value is good and from a block that does not 
support cascade initialization. This status is also 
used when an alarm is active.

Good_Cascade

The value is good and from a block that supports 
cascade initialization.