FIFO Parameters and the FIFO

Parameterisation of the FIFO Group

The FIFO group is parameterised by a number of data items. These must be set by the user:

• FIFO dimension N = number of FIFO axes,
• FIFO length Z = number of FIFO lines,
• FIFO cycle time T_f (the timebase of the FIFO entries) = cycle time for which the FIFO data was generated.
• FIFO-Override

The FIFO is a table, [Z][N]T, with N columns and Z lines. These lines are cyclically refilled after they have been used. At position T[T][A] the FIFO contains the set position value for axis A at time T × T_f (T_f FIFO cycle time).

FIFO Timing and Size

The timing of the data generation in the PLC (the PLC cycle time), of the ADS transfer via PLC (the PLC cycle time and the size of the PLC's internal data buffer), the usage of the FIFO contents (SAF cycle time) and the length of the FIFO, N must be set up and adapted to one another in such a way that

• the FIFO does not run dry,
• the FIFO (as a rule) can accept the data offered by the PLC.

FIFO Cycle Time

The statement execution time constant (known as the SAF cycle time, T_saf) does not have to be identical to the time constant for which the FIFO was designed or generated (the FIFO cycle time, T_f).

T_f > T_saf:

If the FIFO cycle time is bigger than the SAF cycle time, then a fine interpolation is in operation, so that the necessary intermediate values for the position control may in some cases be linearly interpolated a number of times from the same pair of neighbouring reference points in the FIFO. The acceleration and the speed have correspondingly coarse resolutions, and should not be used in unfiltered form as input magnitudes.

T_f < T_saf:

If on the other hand the FIFO cycle time is smaller than the SAF cycle time, then coarse interpolation is in action. In such cases, a portion of the FIFO information remains unused.