Rockwell Automation 1771-QA Stepper Positioning Assembly User Manual - Manual (Page 52)

3–30 Programming and Operation
Publication 1771-UM002A–EN–P – May 2000
Programming Considerations
This section describes how block transfer concepts can be applied to
programming the stepper positioning assembly. It is assumed that an
Industrial Terminal (cat. no. 1770-T3) will be used and that the
programmable controller is either an Allen-Bradley Mini-PLC-2/15
or PLC-2/30.
Programming Strategy
Move commands and moveset blocks containing data of different
block lengths can be transferred to the stepper controller module by
programming one unconditional write block transfer instruction with
a write block transfer file. Data blocks can be moved alternately into
the write block transfer file and immediately transferred to the
stepper controller module during the next block transfer.
File-to-file-move instructions are used to transfer data blocks from
their storage locations in the data table to the write block transfer
file. When a transfer is needed, program logic enables the file-to-file
move instruction. The block transfer is automatic because it is
unconditionally programmed.
Status data from the stepper controller module is transferred using
one unconditional read block transfer instruction. The data is
transferred into a buffer file where it is held temporarily until the PC
processor can verify that the transfer and the data are valid. Valid
data can be moved to a final storage file where it can be manipulated
by program logic. Invalid data is not moved from the buffer file but
over-written by the next block transfer read operation. Examination
of the read block transfer done bit is the condition for moving valid
data into its final storage file. Following a transfer, the stepper
controller module will set itself for the alternate read or write
operation when the block lengths are set to the default value.
Block Length
The block length of read or write block transfer instructions must be
set to default value, 00, causing the default mode of the stepper
controller module to automatically perform block transfer
handshaking. The stepper controller module will toggle from a write
operation to a read operation and vice-versa.
Important: Do not confuse block transfer handshaking (which
verifies a successful transfer of data) with other
recommended handshaking that examines the status bits
in the axis status word. Status bits verify that a
command and/or data have been received by the stepper
controller module and indicate when the command can
be disabled.