User's Manual

PMAC User Manual
222 Synchronizing PMAC to External Events
How It Works
Time-base control works by lying to the commanded position update equations that occur every servo
cycle about the amount of elapsed time since the last servo cycle. (Variable I10 contains the actual
amount of time.)
Note:
The actual time between servo cycles does not change, nor do the dynamics of the
servo loops. It is only the rate of the commanded trajectories that change with the
external frequency, and since all of the trajectories in the coordinate system change
together, the path through space does not.
Instructions for Using an External Time-Base Signal
Using an external time-base signal requires several steps to set up. However, once the setup is complete,
the time-base control is automatic. The steps in the set-up are detailed below.
Step 1: Signal Decoding
The signal is input to the PMAC at one of the incremental encoder inputs (Channels A and B). The signal
must be either a quadrature signal (as out of an encoder) or a pulse and direction signal (pulse into A,
direction into B). For the encoder inputs used (one of the Encoders 1 to 16), Encoder I-variable 0 (I900
for Encoder 1, I905 for Encoder 2, etc.) controls the decode method, and defines what a count is. For
instance, with a quadrature signal into Encoder 4 lines, I915 = 3 or 7 defines four counts per encoder
cycle, whereas I915 = 2 or 6 defines only two counts per encoder cycle. The difference between 3 and 7,
or 2 and 6 is for which sense of the signal does the decoder count up.
Make sure that to count up in the direction that the master signal is going. Counting down would imply a
negative time-base, which PMAC cannot handle.
Analog Source for Frequency
PMAC has a single on-board voltage-to-frequency (V-to-F) converter that allows a voltage level input to
the Wiper line of the JPAN connector (Pin 20 of J2) to control the time base. The input is 0 to +10V
analog signal that is converted to a nominal 0 to 250 KHz frequency (25KHz/V). Jumpers E72 and E73
ON connect this signal to the Encoder 4 decoder-counter (there is no choice about which encoder). Make
sure jumper E24 connects pins 1 and 2 (the default). I-variable I915, which controls decoding this signal,
should be set to 4 (pulse and direction, counting up on this signal).
From this point on, the time-base control can be treated just as if it came from an external frequency
source. Note that the default conversion table is set up to handle time-base information from this encoder
counter. Refer to the diagram under Control-Panel I/O in the Connecting PMAC to the Machine section
of this manual.
Step 2: Interpolation
Once decoded and counted, the value from the signal is brought into the encoder conversion table once
per servo cycle, exactly as a position feedback signal would be. Using the 1/T conversion method here is
recommended, because this method gives a very good sub-count interpolation of the signal (using timers
associated with the counter) that significantly enhances the smoothness of the time base information.
Make sure that the conversion table is set up to process the counter from the input signal this way. The
encoder conversion table is set up at the factory to do 1/T conversion on encoder counters 1 through 8.
See the description of the encoder conversion table for more details.