Reference Manual

PMAC 2 Software Reference
PMAC I-Variable Specification 163
I1019 SSI Channel 2 Word Length Control {Geo PMAC only}
Range
0 – 3
Units
None
Default
3
Remarks
I1019 specifies the word length in bits from the second SSI-encoder interface on a Geo
PMAC. The following table lists the possible values of I1019 and the word lengths they
cause the Geo PMAC to request:
I1019 Word Length
0 12 bits
1 16 bits
2 20 bits
3 24 bits
I1019 is only used if the Geo PMAC’s Feedback Option 2 for absolute position feedback is
ordered.
I1020 Lookahead Length {Option 6L firmware only}
Range
0 – 65,535
Units
I13 segmentation periods
Default
0
Remarks
I1020 controls the enabling of the lookahead buffering function for the coordinate system
that has a defined lookahead buffer, and if enabled, determines how far ahead the buffer
will look ahead.
If I1020 is set to 0 (the default), the buffered lookahead function is not used, even if a
lookahead buffer has been defined.
If I1020 is set to 1, points are stored in the lookahead buffer as they are calculated, but no
lookahead velocity or acceleration-limiting calculations are done. The stored points can
then be used to back up along the path as necessary.
If I1020 is set to a value greater than 1, PMAC will look I1020 segments ahead on
LINEAR and CIRCLE mode moves, provided that the PMAC is in segmentation mode
(I13 > 0) and a lookahead buffer has been defined. The lookahead algorithm can extend
the time for each segment in the buffer as needed to keep velocities under the Ix16 limits
and the accelerations under the Ix17 limits.
For proper lookahead control, I1020 must be set to a value large enough so that PMAC
looks ahead far enough that it can create a controlled stop from the maximum speed within
the acceleration limit. This required stopping time for a motor can be expressed as:
17Ix
16Ix
max
A
max
V
StopTime ==
All motors in the coordinate system should be evaluated to see which motor has the
longest stopping time. This motor’s stopping time will be used to compute I1020.
The average speed during this stopping time is V
max
/2, so as the moves enter the lookahead
algorithm at V
max
(the worst case), the required time to look ahead is StopTime/2.
Therefore, the required number of segments always corrected in the lookahead buffer can