Reference Manual

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PMAC 2 Software Reference

PMAC I-Variable Specification 133

I902 Servo Clock Frequency Control {PMAC2 only}

Range

0 .. 15

Units

Servo Clock Frequency = PHASE Clock Frequency / (I902+1)

Default

3 — SERVO Clock Frequency = 9.0346 kHz / (3+1) = 2.2587 kHz

(with default values of I900 and I901)

Remarks

I902, in conjunction with I901 and I900, determines the frequency of the SERVO clock on

PMAC2 (except for PMAC2 Ultralites, which use I992, I997, and I998 for this). Each

cycle of the SERVO clock, PMAC2 updates the commanded position for each activated

motor, and executes the servo algorithm to compute the command output to the amplifier.

Specifically, I902 controls how many times the SERVO clock frequency is divided down

from the PHASE clock, whose frequency is set by I900 and I901. The SERVO clock

frequency is equal to the PHASE clock frequency divided by (I902+1). I902 has a range

of 0 to 15, so the frequency division can be by a factor of 1 to 16. The equation for I902

is:

)kHz(ServoFreq

)kHz(PhaseFreq

902I −=

The ratio of PHASE Clock Freq. to SERVO Clock Freq. must be an integer.

Note: If jumper E1 is ON, PMAC2 gets its SERVO clock signal externally from a serial-

port input, and I902 is not used.

For execution of trajectories at the proper speed, I10 must be set properly to tell the

trajectory generation software what the SERVO clock cycle time is. The formula for I10

is:

)kHz(ServoFreq

608,388,8

10I =

In terms of the variables that determine the SERVO clock frequency on a (non-Ultralite)

PMAC2 board, the formula for I10 is:

()()()

1902I1901I3900I*2

640

10I +++=

At the default servo clock frequency, I10 should be set to 3,713,707 in order that PMAC’s

interpolation routines use the proper servo update time.

Note:

If the servo clock frequency is set too high, lower priority tasks

such as communications can be starved for time. If the background

tasks are completely starved, the watchdog timer will trip, shutting

down the board. If a normal reset of the board does not re-establish

a state where the watchdog timer has not tripped and

communications works well, it will be necessary to re-initialize the

board by powering up with the E3 re-initialization jumper on. This

restores default settings, so communication is possible, and I900

and I901 can be set to supportable values.

Example

With a 6.67 kHz PHASE Clock frequency established by I900 and I901, and a desired 3.33

kHz SERVO Clock frequency:

I902 = (6.67 / 3.33) - 1 = 2 - 1 = 1