Reference MVI56-DNP
♦
ControlLogix Platform
User Manual
DNP 3.0 Server
Page 112 of 171
ProSoft Technology, Inc.
July 21, 2011
Read Block
READ Blocks transfer information from the module to the ControlLogix
processor. The following table describes the basic block structure of an input
image.
Block Offset
Content
0 Reserved
1
Write block ID
2 to 241
Read data
242 to 248
Spare (Not used)
249
Read block ID
The
Read Block ID
is an index value used to determine the location of where the
data will be placed in the ControlLogix processor read data controller tag array.
Each transfer can move up to 240 words (block offsets 2 to 241) of data. The
value of the Read Block identification code indentifies the type of data contained
in the block, so the sample ladder logic can move it to the correct controller tag
array.
The
Write Block ID
contained in the Read Block tells the ladder logic which block
of data the module is expecting to receive from the ControlLogix processor
during the next backplane transfer. Under normal program operation, the module
sequentially sends read blocks and requests write blocks. For example, if one
block each of binary and analog output data and one block of binary input data,
two blocks of counter data and two blocks of analog input data are used with the
application, the backplane transfer sequence block numbers will be:
R4W0
R16W8
R4W9
R16W12
R4W13
R16W0
R4W8
This sequence will continue until interrupted by other write block numbers sent by
the controller or by a command request from a node on the DNP network or
operator control through the module’s Configuration/Debug port. This sequence
is occasionally interrupted by the read block identification code 100. This block
passes the error/status and error list information from the module to the
processor. Refer to the Error/Status section of this document for the structure
and data contained in a Status Read block.
