Rockwell Automation 1772-LS_LSP,D17726.8.6 PROG/OPER MANUAL-MINI PLC-2/05 - Manual (Page 163)

Block Transfer
Chapter 14
14-8
Enable/Done Bit
The read and write bits are the enable bits for block transfer modules.
Either one (or both for a bidirectional transfer) is set on in the program
scan when the rung containing the block transfer instruction is true.
The done bit is set on in the I/O scan that the words are transferred,
provided that the transfer was initiated and successfully completed. The
done bit remains on for only one scan.
A block transfer is requested in each program scan that the read and/or
write bit remains on. The read and/or write bits are turned off when the
rung containing the instruction goes false.
RunĆTime Errors
Misuse and/or inadvertent changes of instruction data can cause run-time
errors when:
The module address is given a non-existent I/O rack number.
A read transfer overruns the file into a processor work area or into user
program by an inadvertent change of the block length code.
Purpose: Block transfer reads data from an I/O module into the
processor’s input image table in one I/O scan.
Programmed as an output instruction.
Block length depends on the type of module you are using.
Request for transfer is made in the program scan.
I/O scan is interrupted for the transfer.
Done bit remains on for one scan after a valid transfer.
Instruction requires two words of the data table.
Figure 14.5 shows an example rung containing a block transfer read
instruction and the data table areas used by the instruction.
Block Transfer Read