Rockwell Automation 1395 Node Adapter Board - Manual (Page 21)

Chapter 3
Functional Description
3-9
Message Structure
Figure 3.5 illustrates the Data File message structure required by the block
transfer (BTW ) function in the PLC Controller. The BTR Data File
Message does not have to be set up. It will reflect back data depending on
the BTW function. The message is segmented into 16 bit words. The first
four words (header) must be present. The data portion of the message is
only required if the function being executed requires data. Each of the
words in the message are broken into two 8 bit bytes. The following
paragraphs provide a description of the message words.
Words 1 and 2 – used for internal PLC Controller communication
functions. Words 1 and 2 are transparent to the block transfer function
and are always 0.
Word 3 – contains two code numbers which determine the function to
be performed by the Node Adapter Board upon receipt of the message
from the PLC Controller. Table 3.A summarizes the valid command
codes which may appear in the two bytes of Word 3. Note that the Class
and Element codes are reversed. This word is set by the PLC Controller
before the message is sent using the block transfer function. This word
is not changed by the Node Adapter Board, therefore, the board returns
the same data in this word when replying to the PLC Controller.
Figure 3.5
Block Transfer Write Message Header Structure
0
Element Class
Status Length
Data
High Byte Low Byte
0
Word 1
Data
Data
Word 2
Word 3
0 0
Word 4
Word 5
Word 6
Word 7
Message Header
Data (Optional)
Word 4 (high byte) – contains a 8 bit block transfer status byte
(Table 3.B) which is a code number returned from the Node Adapter
Board as a response to the block transfer function. This byte is not used
by the PLC Controller when sending data to the Node Adapter Board
and therefore is set to 0 when performing a block transfer write in the
PLC Controller program.
Word 4 (low byte) – is the total length of the message in bytes
including the number of bytes required for the data portion of the
message. This byte must be calculated in the PLC Controller program
and added to the message before being sent to the Node Adapter Board.