Rockwell Automation 1772-L8_LW_LWP_LX_LXP,D17726.5.8 User Manual - Manual (Page 271)

Block Transfer
Chapter 18
18-6
Module Address
The module address is stored in BCD with the first digit representing the
rack number, the second digit the I/O group number and the third digit the
slot number. When block transfer is performed, the processor searches the
timer/counter accumulated area of the data table for a match of the module
address. When a match is found, the processor looks 100
8
above that value
to find the address of the block transfer data file.
Block Length
The block length is the number of words that the module will transfer. It
depends on the type of module and the number of channels connected to it.
The number of words requested by the instruction must be a valid number
for the module: i.e. from 1 up to the maximum of 64. The maximum
number depends on the type of module that is performing block transfer.
The block length can also be set at the default value of the module (useful
when programming bi-directional block transfers). For some modules, the
default value allows the module to decide the number of words to be
transferred. See the documentation for the module.
The block length heading of the instruction accepts any value from 00-63,
whether or not valid for a particular module. Enter 00 for the default block
length of 64.
The block length is stored in binary in the byte corresponding to the
module’s address in the output image table.
Equal Block Lengths
When the block lengths are set equal or when the default block length is
specified by the programmer, the following considerations are applicable:
Read and write instructions could and should be enabled in the same
scan (separate but equal input conditions).
Read and write instructions should be enabled in the same scan.
Module decides which operation will be performed first when both
instructions are enabled in the same scan.
Alternate operation will be performed in a subsequent scan.
Transferred data should not be operated upon until the done bit is set.