Reference Manual

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PMAC 2 Software Reference

PMAC I-Variable Specification 83

If Ix27 is set to a value greater than zero, for a programmed axis move in Absolute (ABS)

mode, the motor will take the shortest path around the circular range defined by Ix27 to get

to the destination point. No absolute move will be greater than half a revolution in this

mode.

If Ix27 is set to a negative number, an alternate rollover mode for the rotary axis assigned

to the motor is activated that uses the sign of the commanded destination in absolute mode

to specify the direction of motion to that destination. In this mode, all absolute-mode

moves are less than one revolution (with the size of the revolution specified by the

magnitude of Ix27), but can be greater than one-half revolution.

The sign of the commanded absolute destination in this mode is also part of the destination

value. So a command of A-90 in this mode is a command to move to –90 degrees (=

+270 degrees) in the negative direction. For commands to move in the positive direction,

the + sign is not required, but it is permitted (e.g. to command a move to 90 degrees in the

positive direction, either A90 or A+90 can be used).

PMAC cannot store the difference between a +0 and a –0 destination command, so a

command with a tiny non-zero magnitude must be used (e.g. A+0.0000001 and A-

0.0000001). This increment can be small enough not to have any effect on the final

destination.

If using commands from a similar mode in which only the magnitude, and not the sign, of

the value specifies the destination position, then the destination values for negative-

direction moves must be modified so that the magnitude is 360 degrees minus the

magnitude in the other mode. For example, if the command were C-120, specifying a

move to (+)120 degrees in the negative direction, the command would have to be modified

for PMAC to C-240, which specifies a move to –240 degrees (= +120 degrees) in the

negative direction. Commands for positive-direction moves do not have to be modified.

Axis moves in Incremental (INC) mode are not affected by rollover. When Ix27 is set to

0, there is no rollover. Rollover should not be attempted for axes other than A, B, or C.

Jog moves are not affected by rollover. Reported motor position is not affected by

rollover. (To obtain motor position information “rolled over” to within one motor

revolution, use the modulo (remainder) operator, either in PMAC or in the host computer:

e.g. P4=(M462/(I408*32))%I427).

Example

Motor #4 drives a rotary table with 36,000 counts per revolution. It is defined to the A-

axis with #4->100A (A is in units of degrees). I427 is set to 36000. With motor #4 at

zero counts (A-axis at zero degrees), an A270 move in a program is executed in Absolute

mode. Instead of moving the motor from 0 to 27,000 counts, which it would have done

with I427=0, PMAC moves the motor from 0 to -9,000 counts, or -90 degrees, which is

equivalent to +270 degrees on the rotary table.

Motor #5 drives a positioning spindle with an 8192-line-per-rev (32,768-count-per-rev)

encoder on the motor and a 10-to-1 gear reduction to the load. It is defined to the C-axis

with #5->910.222222222C (C is in units of degrees). I527 is set to 327,680. An

absolute-mode move of C-355 is commanded. PMAC moves Motor 5 in the negative

direction less than one revolution to +5 degrees (= -355 degrees).