Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 173)

ASCII Module
PLC-2 Family Processors
Appendix A
AĆ5
Two consecutive data addresses must be used in bidirectional block
transfer. Both contain the I/O rack address of the ASCII module.
A boundary word containing zeros should be entered in the data table
following the last block transfer data address. When the processor sees
this boundary word, it will terminate the block transfer search routine so
subsequent data table values cannot be interpreted as rack, module group,
and slot numbers associated with block transfer data addresses.
File Addresses
The block transfer read and write instructions each require a file. The file
of the read instruction receives data transferred from the module. The file
of the write instruction temporarily holds data to be sent to the module.
Each file address is stored in the preset area of the data table, 100
8
above
the corresponding data address in the accumulated area. You enter the file
address in the instruction after you enter the instruction. The files
themselves can be located elsewhere in the data table.
Enable and Done Bits
The read enable bit is bit 07 or 17 of the module’s output image table byte
depending on whether the block transfer module is in a lower or upper
slot, respectively. The write enable bit is bit 06 or 16 of this byte. These
bits are entered automatically in the instruction when you enter the
module address.
The done bit has the identical bit number as the enable bit but the done bit
is set in the module’s input image table word. The done bit is set in the
I/O scan that the transfer is made, provided that the transfer was
successfully completed.
The done bit remains set for one program scan.
Example Instructions
Example bidirectional block transfer instructions and their associated data
table map are shown in Figure A.3.
The time for a block transfer read or write operation for PLC-2 family
processors depends on the system, scan time(s), the number of words to
be transferred, the I/O configuration, and the number of enabled block
Block Transfer Timing