ProSoft Technology MVI56-AFC - Manual (Page 169)

MVI56-AFC ♦ ControlLogix Platform MVI56-AFC Backplane Communication
Liquid and Gas Flow Computer User Manual
ProSoft Technology, Inc. Page 169 of 316
February 25, 2011
Fixed and Variable Length Function Blocks
As shown in the Function Block Type table above, some function blocks have
variable lengths (Overall Block Length "User defined") while the others have
lengths that are fixed. Also, some of the fixed-length blocks are specific to
individual meters while the others, and all variable-length blocks, are not. Bits 7
to 0 of the output block ID provide this information.
For a variable-length block, bits 7 to 0 of the output block ID contain the length in
elements of the data portion of the block that follows the block ID itself; that is,
the overall length of the block is one element greater than the number coded into
those bits. For example, a block that performs a Gateway Read of 10 holding
registers from the primary slave will have this Block ID:
In this case, the overall block length is 11 elements (You may specify a longer
block length if you wish, because the number of registers actually read is
specified separately, but the data length must be at least large enough to
accommodate the registers to be transferred; see the detailed function
description for more information).
For a fixed-length meter-specific block, bits 7 to 5 of the output block ID give the
stream number (if applicable) or zero, and bits 4 to 0 give the meter number. For
example, if the Meter Process Variable Block (Function Code 8) is required to
service meter number 3, you must format the function block ID element as shown
in the following diagram:
For a fixed length non-meter-specific block, bits 7 to 0 of the output block ID may
implement options to modify the behavior of the function, depending on the
function.