ProSoft Technology MVI56-AFC - Manual (Page 162)

MVI56-AFC Backplane Communication MVI56-AFC ♦ ControlLogix Platform
User Manual Liquid and Gas Flow Computer
Page 162 of 316 ProSoft Technology, Inc.
February 25, 2011
11.1.2 Input/Output Transactions
The MVI56-AFC module runs automatically on power up. It receives the process
variables from the ControlLogix controller through the associated output block
array and returns the calculated results back through the Input Block Array.
The data transfer to and from the MVI56-AFC module is implemented through
processor logic, using the concept of Transactions to establish an interface
between the AFC module and the ControlLogix Controller. A transaction is a
transfer of a pre-assembled block of data words between the controller and the
AFC module. The AFC module supports a block multiplexing scheme whereby
you may set up a number of Output Block Arrays (OBAs) with corresponding
Input Block Arrays (IBAs) for two way data transfer. This is done by giving each
OBA a unique non-zero Transaction Identifier number. The AFC module will not
process an OBA unless the Transaction Identifier number of the current OBA is
different from the previous one.
Output (Transaction) Block Array Definition
The 248 integer elements of the OBA that must be formatted in the controller
ladder logic program are defined as follows:
Element # Attribute
0 Sentinel (Transaction Number)
1 Output Block Array Length (≤245)
2
3
4 Space for User Allocated Sub Blocks
. (Total Available Elements = 245)
.
.
.
244
245
246
247 Anchor (Transaction Number)
The AFC module recognizes an OBA as valid only if all three of the following
conditions are true:
1 The values of the sentinel and the anchor are the same (this is called the
"transaction number").
2 The transaction number (sentinel & anchor) is non-zero.
3 The transaction number is different from the preceding one.
AFC Response to an OBA
Each time the AFC module receives an OBA, it processes this array and returns
a corresponding Input Block Array (IBA) with the same transaction number so
that the processor logic can act upon the AFC's response to the data it received
in the OBA. The OBA description follows.