ProSoft Technology MVI56-AFC - Manual (Page 174)

MVI56-AFC Backplane Communication MVI56-AFC ♦ ControlLogix Platform
User Manual Liquid and Gas Flow Computer
Page 174 of 316 ProSoft Technology, Inc.
February 25, 2011
Description
This block fetches any pass-through Modbus write command sent by an external
Modbus host, which is returned to the Processor essentially verbatim (see
following illustration). The AFC module buffers any such command until it is
returned to the Processor via this input function block, at which time the buffer is
made available for the next command.
Because a Modbus write command must write at least one word (bit) and cannot
write more than 125 words (2000 bits), the data length of this function block must
lie between 3 and 127 (register address + number of registers + data).
MB pass-through: Output Function Block MB pass-through: Input Function Block
Element Attribute Element Attribute
0 Block ID - Function Code & Length 0 Block ID - Function Code & Length
1 1 Modbus Register Address
2 2 Number of Registers
3 3 Data Element 1
4 4 Data Element 2
5 5 Data Element 3
… … ……………….
… … ……………….
n n Data Element (n-2)
A Modbus pass-through function block formatted as in the previous example with
data block length of 48 elements (total block size of 49 elements) when executed
returns up to 46 Modbus integer data elements written to the AFC module in
Modbus protocol format. The first data block element contains the number of
Modbus data elements to follow.
Note: If the value in element 2 of the IFB is 0, no Modbus command was pending. If bit 15 in this
element is high, it indicates Modbus data overflow; that is, the SCADA host has written more data
elements than the pass-through allows. In this case, the Pass-Through IFB is filled with as much
data as will fit and the high order bit of "Number of Registers" (element 2 of the IFB) is set
indicating this overflow condition. The number of registers (bits 14-0 of element 2 of the IFB) is
equal to that of the MB message received. On data overflow, the last command stays pending and
may be flushed by setting bit 9 in the Output Function Block ID (skip input to controller).
When the AFC completes processing the OFB, the IFB ID is returned with the
Modbus Message Block as described above. The AFC provides a number of
indicators to allow you to design the Processor logic to take appropriate action
once the controller receives the message in the returned IFB.