Rockwell Automation 1771-QC , D17716.5.25 SER/B SERVO POS - Manual (Page 33)

Positioning with Allen-Bradley PC
Chapter 4
4Ć9
You can monitor the in-position signal of each axis through the status
block. When all axes are in position, you can send a start command to
each axis through the command block.
Alternatively, you can monitor the in-position signal of each axis through
the hardware done output terminal of the 1771-ES expander. When all
axes are in position, you can send a start command to each axis through
the hardware start input terminal of the 1771-ES expander.
Using the hardware start and done signals is faster than using block
transfer for the status and command blocks. Furthermore, if the axis
synchronization includes multiple servo positioning assemblies, precise
synchronization cannot occur through block transfer because two block
transfers cannot occur simultaneously.
Continuous Moves
For continuous moves with the next move in the same direction, axis
synchronization requires precise programming of feedrates, acceleration
rates, and deceleration rates. You must program the move blocks so that
each axis takes the same amount of time for corresponding moves.
Furthermore, you must plan the moves to be long enough to adhere to the
following constraints:
Each move must take longer than the time it takes to transfer a move
block from the 1771-M3 controller to the 1771-ES expander. This time
is a function of the number of axes as follows:
No. of Axes Time
1 20ms
2 25ms
3 30ms
If the number of moves requires additional moveset blocks, the last two
moves of each preceding moveset block must not be too short. They
must take a long enough time for the following moveset block to be
transferred from the data table. (Refer to chapter 8 for details about
block transfer timing.)