Rockwell Automation 8520 9/Series CNC Lathe - Manual (Page 536)

Tool Tip Radius Compensation (TTRC)
Function
Chapter 21
21-52
During normal program execution, the control is constantly scanning ahead
several blocks to set up the necessary motions to correctly execute the
current block. This is called Block Look-Ahead.
The 9/Series control has 21 set-up buffers. Different features require the
use of some of these setup buffers. One is always used for the currently
executing block. TTRC requires at least 3 of these buffers. Any remaining
setup buffers are used for block look-ahead, with one buffer used for each
block.
At times (especially during TTRC) the control may not have enough
look-ahead blocks to correctly execute the current block. When this
happens, the control automatically starts disabling the block retrace
feature.
The block retrace feature uses one setup buffer for every re-traceable
block. The number of re-traceable blocks is set in AMP by your system
installer (a maximum of 15 is possible).
If necessary, the control decreases the number of available re-traceable
blocks until either there are sufficient setup buffers available to
successfully execute the current program, or until there are no more block
retrace blocks left. The control displays a message on line 2 of the CRT if
it has to eliminate some of the block retrace blocks.
Avoid using too many buffers for block retrace. The larger the number of
look-ahead blocks that the control has available to set up future part
program motion requests, the more efficiently the control executes
programs. We recommend that you keep the number of setup buffers
available to the block retrace feature as low as possible.
Error detection for TTRC blocks can be separated into 3 categories:
Backwards motion detection
Circular departure too small
Interference
21.6.8
Block Look-Ahead
21.7
Error Detection