Reference Manual

PMAC 2 Software Reference
214 PMAC On-Line Command Specification
M-variable assignments that can be stored in the buffer
Remarks
This command establishes a lookahead buffer for the addressed coordinate system. It
reserves memory to buffer both motion equations and “synchronous M-variable” output
commands for the lookahead function.
PMAC can only have one lookahead buffer at a time. The coordinate system that is
addressed when the lookahead buffer is defined is the only coordinate system that can
execute the special lookahead function.
Segment Buffer Size: The first constant value in the command determines the number of
motion segments that can be stored in the lookahead buffer. Each motion segment takes I13
milliseconds at the motion program speeds and acceleration times (the velocity and
acceleration limits may extend these times).
However, it is the variable I1020 for the coordinate system that determines how many motion
segments the coordinate system will actually look ahead in operation.
The lookahead buffer should be sized large enough to store all of the lookahead segments
calculated, which means that this constant value must be greater than or equal to I1020. If
backup capability is desired, the buffer must be sized to be large enough to contain the
desired lookahead distance plus the desired backup distance.
The method for calculating the number of segments that must be stored ahead is explained in
the I1020 specification and in the PMAC User’s Guide section on Lookahead. The
fundamental equation is:
13I*2
1
*
x
17Ix
16Ix
max*
3
4
1020I






=
If backup capability is desired, the buffer must be able to store an additional number of
segments for the entire distance to be covered in reversal. This number of segments can be
calculated as:
(
)
(
)
()
)segsec/m(SegTime*sec/units
max
V
secsec/m1000*unitsBackupDist
tsBackSegmen =
The total number of segments to reserve for the buffer is simply the sum of the forward and
back segments you wish to be able to hold:
tsBackSegmen1020IntsTotalSegme
+
=
Memory Requirements: For each segment PMAC is told to reserve space for in the lookahead
buffer, PMAC will reserve (2x+4) 48-bit words of memory from the main buffer memory
space, where x is the number of motors in the coordinate system at the time that the buffer is
defined. For example, if there are 5 motors in the coordinate system, a command to reserve
space for 50 segments will reserve 50*(2*5 + 4) = 700 words of memory.
Once a lookahead buffer has been defined for a coordinate system, motors cannot be added
to, or removed from, the coordinate system without upsetting the structure of the lookahead
buffer. Attempting to do this will result in a “run-time” error on the next lookahead move.
If it is desired to add a motor to the coordinate system, or remove one, the lookahead buffer
must first be erased with the DELETE LOOKAHEAD command, then re-defined after the
change to the coordinate system has been made.
Output Buffer Size: The second constant value in the command determines the number of