Siemens SINUMERIK 840D sl Function Manual page 191

Axis assignments
Link axes may only be connected to NCUs that have the same IPO cycle as the link cycle.
Example: Axis X9 on the fast NCU-U in the diagram above cannot be interpolated as a link
axis by NCU-A or NCU-B. However, axis Z4 can be interpolated by NCU-A, NCU-B and
NCU-U.
Setpoint delay
From NCU-U's point of view, it is interpolating with the local X axis and the two link axes C
and Z when performing eccentric machining. To achieve a correct contour, it is necessary to
compensate for the time delay which occurs when the setpoints are transferred to the link
axes and for the differences in clock cycles by delaying the setpoints to the local X axis. This
can be done by setting machine data:
MD18720 $MN_MM_SERVO_FIFO_SIZE.
Use the following formula for assigning MM_SERVO_FIFO_SIZE:
MM_SERVO_FIFO_SIZE = 2 * link cycle / IPO cycle +1
The formula applies for NCU-U and NCU-A/NCU-B as illustrated in the diagram "Schematic
example of a configuration with several NCUs and eccentric machining unit" (Section
"Diagram of general solution").
Examples:
NCU-U:
Link cycle = 16 msec
IPO cycle n = 2 msec
The cycle ratio is therefore 8; the value of the formula 2 * 8 + 1 = 17
NCU-A/NCU-B:
Link cycle = 16 msec
IPO cycle m = 16 msec
The cycle ratio is therefore 1; the value of the formula 2 * 1 + 1 = 3
In some cases, incorrect settings cause an alarm to be output in machine data:
MD18720 $MN_MM_SERVO_FIFO_SIZE
Extended Functions
Function Manual, 03/2009, 6FC5397-1BP10-4BA0
Several Operator Panels on Several NCUs, Distributed Systems (B3)
NCU link with different interpolation cycles
191