Siemens 500 NIM User Manual
Page 59
NIM Primitives
4-6
SIMATIC TIWAY I Series 500 NIM User Manual
Primitive Structure and Operation (continued)
F
I
E
L
D
First Transmitted Bit
Least Significant Bit
Most Significant Bit
Figure 4-2
Binary Weight of the Fields
Normal Primitive operation consists of a request and response sequence
without exceptions. Exceptions are errors found in the interpretation or
execution of a Primitive.
The normal operation of a Primitive is for the initiating (Host) station to
form a request Primitive and address it to a Secondary on the network. If
the request Primitive contains allowable data element types and data
element location ranges, a legal Primitive Code for the specific device–type
addressed, and the correct Length Field value, then the device addressed
will return a response Primitive of the proper format.
An exception procedure is used when the addressed device finds fault with:
•
The value of the Length field
•
A Primitive format
•
A data element type
•
The execution of a Primitive
If a fault is found with the Length field, Primitive format, data element type
or in the execution of the Primitive, the exception reason is returned with
the Exception Primitive, which is Primitive Code 00. The procedure taken
depends on the Primitive used. There are two classes of procedures used for
handling exceptions:
1.
Procedures which are associated with reading data elements
2.
Procedures which are associated with writing data elements
Primitive Operation
and Exceptions
Normal Operation
Procedures
Exception
Procedures