Reference Manual

Turbo PMAC/PMAC2 Software Reference
Turbo PMAC Global I-Variables 227
When the hardware-1/T functionality is enabled, the IC computes a new fractional-count position estimate
based on timers every SCLK (encoder sample clock) cycle. This permits the fractional count data to be
used for hardware capture and compare functions, enhancing their resolution. The sub-count position-
capture data can be used automatically in Turbo PMAC triggered-move functions if bit 12 of Ixx24 is set
to 1. This is particularly useful when the IC is used on an Acc-51 high-resolution analog-encoder
interpolator board. However, it replaces the timer registers at the first two “Y” addresses for the channel
with fractional count position data, so the traditional software-1/T method of the conversion table cannot
work if this is enabled.
If the hardware-1/T functionality is enabled and to be able to use 1/T interpolation in the servo loop, use
the hardware-1/T extension method ($C method digit with the mode switch bit set to 1) in the encoder
conversion table.
PMAC-Style Servo IC Setup I-Variables
I7mn0 Servo IC m Channel n Encoder/Timer Decode Control
Range: 0 - 15
Units: None
Default: 7
I7mn0 controls how the input signal for Encoder n on PMAC-style Servo IC m is decoded into counts.
As such, this defines the sign and magnitude of a count. The following settings may be used to decode an
input signal.
 I7mn0 = 0: Pulse and direction CW
 I7mn0 = 1: x1 quadrature decode CW
 I7mn0 = 2: x2 quadrature decode CW
 I7mn0 = 3: x4 quadrature decode CW
 I7mn0 = 4: Pulse and direction CCW
 I7mn0 = 5: x1 quadrature decode CCW
 I7mn0 = 6: x2 quadrature decode CCW
 I7mn0 = 7: x4 quadrature decode CCW
In any of the quadrature decode modes, PMAC is expecting two input waveforms on CHAn and CHBn,
each with approximately 50% duty cycle, and approximately one-quarter of a cycle out of phase with
each other. Times-one (x1) decode provides one count per cycle; x2 provides two counts per cycle; and
x4 provides four counts per cycle. The vast majority of users select x4 decode to get maximum
resolution.
The clockwise (CW) and counterclockwise (CCW) options simply control which direction counts up. If
the wrong direction sense is received, simply change to the other option (e.g. from 7 to 3 or vice versa).
WARNING:
Changing the direction sense of the encoder decode for a motor that is servoing
properly will result in unstable positive feedback and a dangerous runaway
condition in the absence of other changes (for motors not commutated by PMAC
from the same encoder). The output polarity must be changed as well to re-
establish polarity match for stable negative feedback.
In the pulse-and-direction decode modes, PMAC is expecting the pulse train on CHAn, and the direction
(sign) signal on CHBn. If the signal is unidirectional, the CHBn input can be tied high (to +5V) or low
(to GND), or, if set up by E18-E21, E24-E27 for single-ended (non-differential) input, left to float high.
Any spare encoder counters may be used as fast and accurate timers by setting this parameter in the 8 to
15 range. In this range, any input signal is ignored.