Rockwell Automation 1771-QA Stepper Positioning Assembly User Manual - Manual (Page 50)

3–28 Programming and Operation
Publication 1771-UM002A–EN–P – May 2000
Secondly, the block length can be changed alternately in accordance
with the block lengths of the enabled instructions. The block length
of the last enabled write block transfer instruction having a
common module address will govern the number of words
transferred.
File Addresses
Two files are required for bidirectional block transfer: one to receive
data transferred from the module, the other temporarily holding data
to be sent to the module. The addresses of these files (in BCD) are
located in two consecutive storage locations in the preset area of the
data table 100
8
above the corresponding consecutive data addresses
in the accumulated area. 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. In a bidirectional block transfer where the block lengths are
unequal, one operation must not be enabled until the other is
completed as determined by the done bit. The exception is when the
block length is set to the default value for both the read and write
operation. Then the PC processor will automatically inhibit the
alternate operation until the first is completed.
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 3.17. The block lengths are set
to the default value.